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<chapter id="libpq">
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<title><application>libpq</application> - C Library</title>
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<indexterm zone="libpq">
<primary>libpq</primary>
</indexterm>
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<para>
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<application>libpq</application> is the <acronym>C</acronym>
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application programmer's interface to
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<productname>PostgreSQL</productname>. <application>libpq</application> is a set
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of library functions that allow client programs to pass queries to the
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<productname>PostgreSQL</productname> backend server and to receive the
results of these queries. <application>libpq</application> is also the
underlying engine for several other <productname>PostgreSQL</productname>
application interfaces, including <application>libpq++</application> (C++),
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<application>libpgtcl</application> (Tcl), <productname>Perl</productname>, and
<application>ECPG</application>. So some aspects of <application>libpq</>'s behavior will be
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important to you if you use one of those packages.
</para>
<para>
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Three short programs are included at the end of this chapter (<xref linkend="libpq-example">) to show how
to write programs that use <application>libpq</application>. There are also several
complete examples of <application>libpq</application> applications in the
directory <filename>src/test/examples</filename> in the source code distribution.
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</para>
<para>
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Client programs that use <application>libpq</application> must include the
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header file <filename>libpq-fe.h</filename> and must link with the
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<application>libpq</application> library.
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</para>
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<sect1 id="libpq-connect">
<title>Database Connection Functions</title>
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<para>
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The following functions deal with making a connection to a
<productname>PostgreSQL</productname> backend server. An
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application program can have several backend connections open at
one time. (One reason to do that is to access more than one
database.) Each connection is represented by a
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<structname>PGconn</> object which is obtained from the function
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<function>PQconnectdb</> or <function>PQsetdbLogin</>. Note that
these functions will always return a non-null object pointer,
unless perhaps there is too little memory even to allocate the
<structname>PGconn</> object. The <function>PQstatus</> function
should be called to check whether a connection was successfully
made before queries are sent via the connection object.
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<variablelist>
<varlistentry>
<term><function>PQconnectdb</function></term>
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<listitem>
<para>
Makes a new connection to the database server.
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<synopsis>
PGconn *PQconnectdb(const char *conninfo);
</synopsis>
</para>
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<para>
This function opens a new database connection using the parameters taken
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from the string <literal>conninfo</literal>. Unlike <function>PQsetdbLogin</> below,
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the parameter set can be extended without changing the function signature,
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so use either of this function or the nonblocking analogues <function>PQconnectStart</>
and <function>PQconnectPoll</function> is preferred for new application programming.
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</para>
<para>
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The passed string
can be empty to use all default parameters, or it can contain one or more
parameter settings separated by whitespace.
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Each parameter setting is in the form <literal>keyword = value</literal>.
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(To write an empty value or a value containing
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spaces, surround it with single quotes, e.g.,
<literal>keyword = 'a value'</literal>.
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Single quotes and backslashes within the value must be escaped with a
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backslash, i.e., <literal>\'</literal> and <literal>\\</literal>.)
Spaces around the equal sign are optional.
</para>
<para>
The currently recognized parameter key words are:
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<variablelist>
<varlistentry>
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<term><literal>host</literal></term>
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<listitem>
<para>
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Name of host to connect to.
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If this begins with a slash, it specifies Unix-domain communication
rather than TCP/IP communication; the value is the name of the
directory in which the socket file is stored.
The default is to connect to a Unix-domain socket in
<filename>/tmp</filename>.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>hostaddr</literal></term>
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<listitem>
<para>
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IP address of host to connect to. This should be in the
standard IPv4 address format, e.g., <literal>172.28.40.9</>. If
your machine supports IPv6, you can also use those addresses. If
a nonzero-length string is specified, TCP/IP communication is
used.
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</para>
<para>
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Using <literal>hostaddr</> instead of <literal>host</> allows the application to avoid a host
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name look-up, which may be important in applications with time
constraints. However, Kerberos authentication requires the host
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name. The following therefore applies: If <literal>host</> is specified without
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<literal>hostaddr</>, a host name lookup is forced. If <literal>hostaddr</> is specified without
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<literal>host</>, the value for <literal>hostaddr</> gives the remote address; if Kerberos is
used, this causes a reverse name query. If both <literal>host</> and <literal>hostaddr</> are
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specified, the value for <literal>hostaddr</> gives the remote address; the value
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for <literal>host</> is ignored, unless Kerberos is used, in which case that value
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is used for Kerberos authentication. Note that authentication is likely
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to fail if <application>libpq</application> is passed a host name that is not the name of the
machine at <literal>hostaddr</>.
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</para>
<para>
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Without either a host name or host address, <application>libpq</application> will connect using a
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local Unix domain socket.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>port</literal></term>
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<listitem>
<para>
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Port number to connect to at the server host,
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or socket file name extension for Unix-domain connections.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>dbname</literal></term>
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<listitem>
<para>
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The database name.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>user</literal></term>
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<listitem>
<para>
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User name to connect as.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>password</literal></term>
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<listitem>
<para>
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Password to be used if the server demands password authentication.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
<term><literal>connect_timeout</literal></term>
<listitem>
<para>
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Time space in seconds given to connection function. Zero or not set means infinite.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>options</literal></term>
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<listitem>
<para>
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Configuration options to be sent to the server.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
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<term><literal>tty</literal></term>
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<listitem>
<para>
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A file or <acronym>TTY</acronym> for optional debug output from the server.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
<term><literal>requiressl</literal></term>
<listitem>
<para>
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If set to 1, an <acronym>SSL</acronym> connection to the server is required.
<application>libpq</> will then refuse to connect if the server does not
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accept an <acronym>SSL</acronym> connection.
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If set to 0 (default), <application>libpq</> will negotiate the connection type with server.
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This option is only available if
<productname>PostgreSQL</> is compiled with SSL support.
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</para>
</listitem>
</varlistentry>
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<varlistentry>
<term><literal>service</literal></term>
<listitem>
<para>
Service name to use for additional parameters. It specifies a service
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name in <filename>pg_service.conf</filename> that holds additional connection parameters.
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This allows applications to specify only a service name so connection parameters
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can be centrally maintained. See
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<filename><replaceable>PREFIX</>/share/pg_service.conf.sample</> for
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information on how to set up the file.
</para>
</listitem>
</varlistentry>
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</variablelist>
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If any parameter is unspecified, then the corresponding
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environment variable (see <xref linkend="libpq-envars">)
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is checked. If the environment variable is not set either,
then hardwired defaults are used.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQsetdbLogin</function></term>
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<listitem>
<para>
Makes a new connection to the database server.
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<synopsis>
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PGconn *PQsetdbLogin(const char *pghost,
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const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
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const char *pwd);
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</synopsis>
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</para>
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<para>
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This is the predecessor of <function>PQconnectdb</function> with a fixed number
of parameters but the same functionality.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQsetdb</function></term>
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<listitem>
<para>
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Makes a new connection to the database server.
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<synopsis>
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PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
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char *dbName);
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</synopsis>
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</para>
<para>
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This is a macro that calls <function>PQsetdbLogin</function> with null pointers
for the <parameter>login</> and <parameter>pwd</> parameters. It is provided primarily
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for backward compatibility with old programs.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQconnectStart</function></term>
<term><function>PQconnectPoll</function></term>
<listitem>
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<para>
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<indexterm><primary>nonblocking connection</primary></indexterm>
Make a connection to the database server in a nonblocking manner.
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<synopsis>
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PGconn *PQconnectStart(const char *conninfo);
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</synopsis>
<synopsis>
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PostgresPollingStatusType PQconnectPoll(PGconn *conn);
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</synopsis>
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</para>
<para>
These two functions are used to open a connection to a database server such
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that your application's thread of execution is not blocked on remote I/O
whilst doing so.
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</para>
<para>
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The database connection is made using the parameters taken from the string
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<literal>conninfo</literal>, passed to <function>PQconnectStart</function>. This string is in
the same format as described above for <function>PQconnectdb</function>.
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</para>
<para>
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Neither <function>PQconnectStart</function> nor <function>PQconnectPoll</function> will block, as long as a number of
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restrictions are met:
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<itemizedlist>
<listitem>
<para>
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The <literal>hostaddr</> and <literal>host</> parameters are used appropriately to ensure that
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name and reverse name queries are not made. See the documentation of
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these parameters under <function>PQconnectdb</function> above for details.
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</para>
</listitem>
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<listitem>
<para>
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If you call <function>PQtrace</function>, ensure that the stream object into which you trace
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will not block.
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</para>
</listitem>
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<listitem>
<para>
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You ensure for yourself that the socket is in the appropriate state
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before calling <function>PQconnectPoll</function>, as described below.
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</para>
</listitem>
</itemizedlist>
</para>
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<para>
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To begin a nonblocking connection request, call <literal>conn = PQconnectStart("<replaceable>connection_info_string</>")</literal>.
If <varname>conn</varname> is null, then <application>libpq</> has been unable to allocate a new <structname>PGconn</>
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structure. Otherwise, a valid <structname>PGconn</> pointer is returned (though not yet
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representing a valid connection to the database). On return from
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<function>PQconnectStart</function>, call <literal>status = PQstatus(conn)</literal>. If <varname>status</varname> equals
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<symbol>CONNECTION_BAD</symbol>, <function>PQconnectStart</function> has failed.
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</para>
<para>
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If <function>PQconnectStart</> succeeds, the next stage is to poll <application>libpq</> so that it may
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proceed with the connection sequence. Loop thus: If <function>PQconnectPoll(conn)</function> last returned
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<symbol>PGRES_POLLING_READING</symbol>, perform a <function>select()</> for reading on the socket determined using <function>PQsocket(conn)</function>. If
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it last returned <symbol>PGRES_POLLING_WRITING</symbol>, perform a <function>select()</> for writing on
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that same socket. If you have yet to call <function>PQconnectPoll</function>, i.e., after the call
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to <function>PQconnectStart</function>, behave as if it last returned <symbol>PGRES_POLLING_WRITING</symbol>. If
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<function>select()</> shows that the socket is ready, consider it <quote>active</quote>. If it has
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been decided that this connection is <quote>active</quote>, call <function>PQconnectPoll(conn)</function>
again. If this call returns <symbol>PGRES_POLLING_FAILED</symbol>, the connection procedure
has failed. If this call returns <symbol>PGRES_POLLING_OK</symbol>, the connection has been
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successfully made.
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</para>
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<para>
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Note that the use of <function>select()</function> to ensure that the socket is ready is merely
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a (likely) example; those with other facilities available, such as a
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<function>poll()</function> call, may of course use that instead.
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</para>
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<para>
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At any time during connection, the status of the connection may be
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checked, by calling <function>PQstatus</>. If this gives <symbol>CONNECTION_BAD</>, then the
connection procedure has failed; if it gives <function>CONNECTION_OK</>, then the
connection is ready. Both of these states are equally detectable
from the return value of <function>PQconnectPoll</>, described above. Other states may also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and may be useful
to provide feedback to the user for example. These statuses are:
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<variablelist>
<varlistentry>
<term><symbol>CONNECTION_STARTED</symbol></term>
<listitem>
<para>
Waiting for connection to be made.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><symbol>CONNECTION_MADE</symbol></term>
<listitem>
<para>
Connection OK; waiting to send.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
<listitem>
<para>
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Waiting for a response from the server.
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</para>
</listitem>
</varlistentry>
<varlistentry>
<term><symbol>CONNECTION_AUTH_OK</symbol></term>
<listitem>
<para>
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Received authentication; waiting for connection start-up to continue.
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</para>
</listitem>
</varlistentry>
<varlistentry>
<term><symbol>CONNECTION_SETENV</symbol></term>
<listitem>
<para>
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Negotiating environment (part of the connection start-up).
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</para>
</listitem>
</varlistentry>
</variablelist>
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Note that, although these constants will remain (in order to maintain
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compatibility), an application should never rely upon these appearing in a
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particular order, or at all, or on the status always being one of these
documented values. An application may do something like this:
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<programlisting>
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switch(PQstatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
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.
.
.
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default:
feedback = "Connecting...";
}
1999-12-06 17:38:25 +01:00
</programlisting>
</para>
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1999-12-06 17:38:25 +01:00
<para>
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Note that if <function>PQconnectStart</function> returns a non-null pointer, you must call
2001-09-10 23:58:47 +02:00
<function>PQfinish</function> when you are finished with it, in order to dispose of
1999-11-30 04:08:19 +01:00
the structure and any associated memory blocks. This must be done even if a
2001-09-10 23:58:47 +02:00
call to <function>PQconnectStart</function> or <function>PQconnectPoll</function> failed.
1999-12-06 17:38:25 +01:00
</para>
2001-09-10 23:58:47 +02:00
1999-12-06 17:38:25 +01:00
<para>
2001-09-10 23:58:47 +02:00
<function>PQconnectPoll</function> will currently block if
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<application>libpq</> is compiled with SSL support. This restriction may be removed in the future.
1999-12-06 17:38:25 +01:00
</para>
2001-09-10 23:58:47 +02:00
2000-01-18 07:09:24 +01:00
<para>
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Finally, these functions leave the socket in a nonblocking state as if
2000-01-18 07:09:24 +01:00
<function>PQsetnonblocking</function> had been called.
</para>
2003-03-13 02:30:29 +01:00
</listitem>
</varlistentry>
1999-11-30 04:08:19 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQconndefaults</function></term>
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<listitem>
<para>
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Returns the default connection options.
1998-10-10 19:12:18 +02:00
<synopsis>
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PQconninfoOption *PQconndefaults(void);
1998-04-04 18:35:22 +02:00
2003-03-13 02:30:29 +01:00
typedef struct
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{
char *keyword; /* The keyword of the option */
char *envvar; /* Fallback environment variable name */
char *compiled; /* Fallback compiled in default value */
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char *val; /* Option's current value, or NULL */
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char *label; /* Label for field in connect dialog */
char *dispchar; /* Character to display for this field
in a connect dialog. Values are:
"" Display entered value as is
"*" Password field - hide value
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"D" Debug option - don't show by default */
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int dispsize; /* Field size in characters for dialog */
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} PQconninfoOption;
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
converts an escaped string representation of binary data into binary
data --- the reverse of <function>PQescapeBytea</function>.
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Returns a connection options array. This may
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be used to determine all possible <function>PQconnectdb</function> options and their
1999-11-11 22:52:28 +01:00
current default values. The return value points to an array of
2003-03-13 02:30:29 +01:00
<structname>PQconninfoOption</structname> structures, which ends with an entry having a null
key-word pointer. Note that the current default values (<structfield>val</structfield> fields)
1999-11-11 22:52:28 +01:00
will depend on environment variables and other context.
Callers must treat the connection options data as read-only.
1999-12-06 17:38:25 +01:00
</para>
2001-09-10 23:58:47 +02:00
1999-12-06 17:38:25 +01:00
<para>
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After processing the options array, free it by passing it to
2001-09-10 23:58:47 +02:00
<function>PQconninfoFree</function>. If this is not done, a small amount of memory
is leaked for each call to <function>PQconndefaults</function>.
2000-03-11 04:09:28 +01:00
</para>
2001-09-10 23:58:47 +02:00
2000-03-11 04:09:28 +01:00
<para>
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In <productname>PostgreSQL</productname> versions before 7.0, <function>PQconndefaults</function> returned a pointer
2000-03-11 04:09:28 +01:00
to a static array, rather than a dynamically allocated array. That
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was not thread-safe, so the behavior has been changed.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
1999-11-11 22:52:28 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQfinish</function></term>
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<listitem>
<para>
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Closes the connection to the server. Also frees
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memory used by the <structname>PGconn</structname> object.
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<synopsis>
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void PQfinish(PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
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</para>
<para>
Note that even if the server connection attempt fails (as
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indicated by <function>PQstatus</function>), the application should call <function>PQfinish</function>
to free the memory used by the <structname>PGconn</structname> object.
The <structname>PGconn</> pointer should not be used after <function>PQfinish</function> has been called.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
1999-11-11 22:52:28 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQreset</function></term>
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<listitem>
<para>
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Resets the communication channel to the server.
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<synopsis>
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void PQreset(PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
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</para>
<para>
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This function will close the connection
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to the server and attempt to reestablish a new
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connection to the same server, using all the same
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parameters previously used. This may be useful for
error recovery if a working connection is lost.
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</para>
</listitem>
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</varlistentry>
1998-04-04 18:35:22 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQresetStart</function></term>
<term><function>PQresetPoll</function></term>
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<listitem>
<para>
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Reset the communication channel to the server, in a nonblocking manner.
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<synopsis>
int PQresetStart(PGconn *conn);
</synopsis>
<synopsis>
PostgresPollingStatusType PQresetPoll(PGconn *conn);
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
These functions will close the connection to the server and attempt to
2002-01-07 03:29:15 +01:00
reestablish a new connection to the same server, using all the same
1999-11-30 04:08:19 +01:00
parameters previously used. This may be useful for error recovery if a
2001-09-10 23:58:47 +02:00
working connection is lost. They differ from <function>PQreset</function> (above) in that they
2001-11-12 20:19:39 +01:00
act in a nonblocking manner. These functions suffer from the same
2001-09-10 23:58:47 +02:00
restrictions as <function>PQconnectStart</> and <function>PQconnectPoll</>.
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</para>
<para>
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To initiate a connection reset, call <function>PQresetStart</function>. If it returns 0, the reset has failed. If it returns 1,
2001-09-10 23:58:47 +02:00
poll the reset using <function>PQresetPoll</function> in exactly the same way as you would
create the connection using <function>PQconnectPoll</function>.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1999-11-30 04:08:19 +01:00
2003-03-13 02:30:29 +01:00
</variablelist>
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</para>
1998-10-05 06:11:47 +02:00
1999-12-06 17:38:25 +01:00
<para>
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<indexterm><primary>libpq-fe.h</></>
<indexterm><primary>libpq-int.h</></>
2001-09-10 23:58:47 +02:00
<application>libpq</application> application programmers should be careful to
maintain the <structname>PGconn</structname> abstraction. Use the accessor functions below to get
at the contents of <structname>PGconn</structname>. Avoid directly referencing the fields of the
<structname>PGconn</> structure because they are subject to change in the future.
2002-01-07 03:29:15 +01:00
(Beginning in <productname>PostgreSQL</productname> release 6.4, the
2003-03-13 02:30:29 +01:00
definition of the <type>struct</type> behind <structname>PGconn</> is not even provided in <filename>libpq-fe.h</filename>.
2001-09-10 23:58:47 +02:00
If you have old code that accesses <structname>PGconn</structname> fields directly, you can keep using it
1999-11-11 22:52:28 +01:00
by including <filename>libpq-int.h</filename> too, but you are encouraged to fix the code
1998-10-05 06:11:47 +02:00
soon.)
2001-11-12 20:19:39 +01:00
2003-03-13 02:30:29 +01:00
<variablelist>
<varlistentry>
<term><function>PQdb</function></term>
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<listitem>
<para>
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Returns the database name of the connection.
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<synopsis>
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char *PQdb(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
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</para>
<para>
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<function>PQdb</> and the next several functions return the values established
at connection. These values are fixed for the life of the <structname>PGconn</>
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object.
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</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQuser</function></term>
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<listitem>
<para>
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Returns the user name of the connection.
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<synopsis>
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char *PQuser(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQpass</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
1998-10-05 06:11:47 +02:00
Returns the password of the connection.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
char *PQpass(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-10-05 06:11:47 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQhost</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
1998-10-05 06:11:47 +02:00
Returns the server host name of the connection.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
char *PQhost(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQport</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
1998-10-05 06:11:47 +02:00
Returns the port of the connection.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
char *PQport(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQtty</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2003-03-13 02:30:29 +01:00
Returns the debug <acronym>TTY</acronym> of the connection.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
char *PQtty(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQoptions</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2003-03-13 02:30:29 +01:00
Returns the configuration options passed in the connection request.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
char *PQoptions(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQstatus</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
1998-10-05 06:11:47 +02:00
Returns the status of the connection.
1998-10-10 19:12:18 +02:00
<synopsis>
2003-03-13 02:30:29 +01:00
ConnStatusType PQstatus(const PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
<para>
The status can be one of a number of values.
However, only two of these are
2003-03-13 02:30:29 +01:00
seen outside of an asynchronous connection procedure:
<literal>CONNECTION_OK</literal> and
1999-12-06 17:38:25 +01:00
<literal>CONNECTION_BAD</literal>. A good
2002-01-07 03:29:15 +01:00
connection to the database has the status <literal>CONNECTION_OK</literal>.
1999-12-06 17:38:25 +01:00
A failed connection
attempt is signaled by status
<literal>CONNECTION_BAD</literal>.
Ordinarily, an OK status will remain so until
<function>PQfinish</function>, but a
communications failure might result in the status changing to
<literal>CONNECTION_BAD</literal> prematurely.
In that case the application
could try to recover by calling <function>PQreset</function>.
</para>
<para>
2001-09-10 23:58:47 +02:00
See the entry for <function>PQconnectStart</> and <function>PQconnectPoll</> with regards
1999-12-06 17:38:25 +01:00
to other status codes
that might be seen.
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1999-12-06 17:38:25 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQerrorMessage</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2001-11-12 20:19:39 +01:00
<indexterm><primary>error message</></>
1999-12-06 17:38:25 +01:00
Returns the error message most recently generated by
an operation on the connection.
2003-03-13 02:30:29 +01:00
<synopsis>
2000-02-08 00:10:11 +01:00
char *PQerrorMessage(const PGconn* conn);
2003-03-13 02:30:29 +01:00
</synopsis>
1999-12-06 17:38:25 +01:00
</para>
<para>
2003-03-13 02:30:29 +01:00
Nearly all <application>libpq</> functions will set a message for
1999-12-06 17:38:25 +01:00
<function>PQerrorMessage</function> if they fail.
2003-03-13 02:30:29 +01:00
Note that by <application>libpq</application> convention, a nonempty
<function>PQerrorMessage</function> result will
1999-12-06 17:38:25 +01:00
include a trailing newline.
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1999-12-06 17:38:25 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQsocket</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2003-03-13 02:30:29 +01:00
Obtains the file descriptor number of the connection socket to
the server. A valid descriptor will be greater than or equal
to 0; a result of -1 indicates that no server connection is
currently open.
<synopsis>
int PQsocket(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>PQbackendPID</function></term>
<listitem>
<para>
Returns the process <acronym>ID</acronym> of the backend server process
handling this connection.
<synopsis>
1999-11-11 22:52:28 +01:00
int PQbackendPID(const PGconn *conn);
2003-03-13 02:30:29 +01:00
</synopsis>
</para>
<para>
1999-12-06 17:38:25 +01:00
The backend <acronym>PID</acronym> is useful for debugging
2003-02-19 04:59:02 +01:00
purposes and for comparison to <command>NOTIFY</command>
messages (which include the <acronym>PID</acronym> of the
2003-03-13 02:30:29 +01:00
notifying backend process). Note that the
<acronym>PID</acronym> belongs to a process executing on the
database server host, not the local host!
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
2000-08-29 06:15:43 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQgetssl</function></term>
2000-08-29 06:15:43 +02:00
<listitem>
<para>
2001-11-12 20:19:39 +01:00
<indexterm><primary>SSL</></>
2003-03-13 02:30:29 +01:00
Returns the SSL structure used in the connection, or null
2000-08-29 06:15:43 +02:00
if SSL is not in use.
2003-03-13 02:30:29 +01:00
<synopsis>
2000-08-29 06:15:43 +02:00
SSL *PQgetssl(const PGconn *conn);
2003-03-13 02:30:29 +01:00
</synopsis>
</para>
<para>
2000-08-29 06:15:43 +02:00
This structure can be used to verify encryption levels, check
2003-03-13 02:30:29 +01:00
server certificates, and more. Refer to the <productname>OpenSSL</> documentation
2000-10-16 05:25:17 +02:00
for information about this structure.
</para>
<para>
2003-03-13 02:30:29 +01:00
You must define <symbol>USE_SSL</symbol> in order to get the
2000-10-16 05:25:17 +02:00
prototype for this function. Doing this will also
2001-09-10 23:58:47 +02:00
automatically include <filename>ssl.h</filename> from <productname>OpenSSL</productname>.
2000-10-16 05:25:17 +02:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
2000-10-16 05:25:17 +02:00
2003-03-13 02:30:29 +01:00
</variablelist>
1999-12-06 17:38:25 +01:00
</para>
</sect1>
2000-09-29 22:21:34 +02:00
<sect1 id="libpq-exec">
2002-01-07 03:29:15 +01:00
<title>Command Execution Functions</title>
1999-12-06 17:38:25 +01:00
<para>
1998-10-05 06:11:47 +02:00
Once a connection to a database server has been successfully
established, the functions described here are used to perform
SQL queries and commands.
2001-05-01 12:48:57 +02:00
</para>
2001-04-30 19:38:00 +02:00
<sect2 id="libpq-exec-main">
2003-03-13 02:30:29 +01:00
<title>Main Functions</title>
<variablelist>
<varlistentry>
<term><function>PQexec</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2003-03-13 02:30:29 +01:00
Submits a command to the server
and waits for the result.
1998-10-10 19:12:18 +02:00
<synopsis>
1998-03-01 09:16:16 +01:00
PGresult *PQexec(PGconn *conn,
2003-03-13 02:30:29 +01:00
const char *command);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
Returns a <structname>PGresult</structname> pointer or possibly a null pointer.
A non-null pointer will generally be returned except in
1998-10-05 06:11:47 +02:00
out-of-memory conditions or serious errors such as inability
2003-03-13 02:30:29 +01:00
to send the command to the server.
If a null pointer is returned, it
2001-09-10 23:58:47 +02:00
should be treated like a <symbol>PGRES_FATAL_ERROR</symbol> result. Use
<function>PQerrorMessage</function> to get more information about the error.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
</variablelist>
1998-10-05 06:11:47 +02:00
1999-12-06 17:38:25 +01:00
<para>
2002-01-07 03:29:15 +01:00
The <function>PGresult</function> structure encapsulates the result
2003-03-13 02:30:29 +01:00
returned by the server.
<application>libpq</application> application programmers should be careful to
2001-09-10 23:58:47 +02:00
maintain the <structname>PGresult</structname> abstraction. Use the accessor functions below to get
at the contents of <structname>PGresult</structname>. Avoid directly referencing the fields of the
<structname>PGresult</structname> structure because they are subject to change in the future.
2003-03-24 19:33:52 +01:00
If <quote>autocommit</quote> is on, multiple queries sent in a single
function call are processed in a single transaction.
2001-05-01 12:48:57 +02:00
</para>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<variablelist>
<varlistentry>
<term><function>PQresultStatus</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
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Returns the result status of the command.
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<synopsis>
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ExecStatusType PQresultStatus(const PGresult *res);
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</synopsis>
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</para>
<para>
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<function>PQresultStatus</function> can return one of the following values:
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<variablelist>
<varlistentry>
<term><literal>PGRES_EMPTY_QUERY</literal></term>
<listitem>
<para>The string sent to the server was empty.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_COMMAND_OK</literal></term>
<listitem>
<para>Successful completion of a command returning no data.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_TUPLES_OK</literal></term>
<listitem>
<para>The query successfully executed.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_COPY_OUT</literal></term>
<listitem>
<para>Copy Out (from server) data transfer started.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_COPY_IN</literal></term>
<listitem>
<para>Copy In (to server) data transfer started.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_BAD_RESPONSE</literal></term>
<listitem>
<para>The server's response was not understood.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_NONFATAL_ERROR</literal></term>
<listitem>
<para>A nonfatal error occurred.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PGRES_FATAL_ERROR</literal></term>
<listitem>
<para>A fatal error occurred.</para>
</listitem>
</varlistentry>
</variablelist>
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2003-02-19 04:59:02 +01:00
If the result status is <literal>PGRES_TUPLES_OK</literal>, then the
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functions described below can be used to retrieve the rows returned by
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the query. Note that a <command>SELECT</command> command that happens
to retrieve zero rows still shows <literal>PGRES_TUPLES_OK</literal>.
<literal>PGRES_COMMAND_OK</literal> is for commands that can never
return rows (<command>INSERT</command>, <command>UPDATE</command>,
etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> often
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exposes a bug in the client software.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQresStatus</function></term>
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<listitem>
<para>
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Converts the enumerated type returned by <function>PQresultStatus</> into
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a string constant describing the status code.
<synopsis>
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char *PQresStatus(ExecStatusType status);
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</synopsis>
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</para>
</listitem>
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</varlistentry>
1999-05-21 02:36:46 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQresultErrorMessage</function></term>
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<listitem>
<para>
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Returns the error message associated with the command, or an empty string
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if there was no error.
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<synopsis>
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char *PQresultErrorMessage(const PGresult *res);
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</synopsis>
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</para>
<para>
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Immediately following a <function>PQexec</function> or <function>PQgetResult</function>
call, <function>PQerrorMessage</function> (on the connection) will return the same
string as <function>PQresultErrorMessage</function> (on the result). However, a
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<structname>PGresult</structname> will retain its error message
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until destroyed, whereas the connection's error message will change when
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subsequent operations are done. Use <function>PQresultErrorMessage</function> when you want to
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know the status associated with a particular <structname>PGresult</structname>; use <function>PQerrorMessage</function>
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when you want to know the status from the latest operation on the connection.
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</para>
</listitem>
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</varlistentry>
1998-03-01 09:16:16 +01:00
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<varlistentry>
<term><function>PQclear</function></term>
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<listitem>
<para>
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Frees the storage associated with a <structname>PGresult</structname>.
Every command result should be freed via <function>PQclear</function> when
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it is no longer needed.
<synopsis>
void PQclear(PQresult *res);
</synopsis>
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</para>
<para>
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You can keep a <structname>PGresult</structname> object around for as long as you
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need it; it does not go away when you issue a new command,
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nor even if you close the connection. To get rid of it,
you must call <function>PQclear</function>. Failure to do this will
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result in memory leaks in your client application.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQmakeEmptyPGresult</function></term>
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<listitem>
<para>
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Constructs an empty <structname>PGresult</structname> object with the given status.
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<synopsis>
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
</synopsis>
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</para>
<para>
This is <application>libpq</>'s internal function to allocate and initialize an empty
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<structname>PGresult</structname> object. It is exported because some applications find it
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useful to generate result objects (particularly objects with error
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status) themselves. If <parameter>conn</parameter> is not null and <parameter>status</> indicates an error,
the current error message of the specified connection is copied into the <structname>PGresult</structname>.
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Note that <function>PQclear</function> should eventually be called on the object, just
as with a <structname>PGresult</structname> returned by <application>libpq</application> itself.
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</para>
</listitem>
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</varlistentry>
</variablelist>
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</sect2>
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<sect2 id="libpq-exec-escape-string">
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<title>Escaping Strings for Inclusion in SQL Commands</title>
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<indexterm zone="libpq-exec-escape-string"><primary>escaping strings</></>
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<para>
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<function>PQescapeString</function> escapes a string for use within an SQL commmand.
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<synopsis>
size_t PQescapeString (char *to, const char *from, size_t length);
</synopsis>
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</para>
<para>
If you want to use strings that have been received
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from a source that is not trustworthy (for example, because a random user
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entered them), you should not directly include them in SQL
commands for security reasons. Instead, you have to escape certain
characters that are otherwise interpreted specially by the SQL parser.
<function>PQescapeString</> performs this operation.
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</para>
<para>
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The
parameter <parameter>from</> points to the first character of the string that
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is to be escaped, and the <parameter>length</> parameter counts the
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number of characters in this string. (A terminating zero byte is
neither necessary nor counted.) <parameter>to</> shall point to a
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buffer that is able to hold at least one more character than twice
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the value of <parameter>length</>, otherwise the behavior is
undefined. A call to <function>PQescapeString</> writes an escaped
version of the <parameter>from</> string to the <parameter>to</>
buffer, replacing special characters so that they cannot cause any
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harm, and adding a terminating zero byte. The single quotes that
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must surround <productname>PostgreSQL</> string literals are not part of the result
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string.
</para>
<para>
<function>PQescapeString</> returns the number of characters written
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to <parameter>to</>, not including the terminating zero byte.
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Behavior is undefined when the <parameter>to</> and <parameter>from</>
strings overlap.
</para>
2001-09-08 18:46:34 +02:00
</sect2>
2001-09-08 00:02:32 +02:00
2001-11-20 21:28:31 +01:00
<sect2 id="libpq-exec-escape-bytea">
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<title>Escaping Binary Strings for Inclusion in SQL Commands</title>
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<indexterm zone="libpq-exec-escape-bytea">
<primary>escaping binary strings</primary>
</indexterm>
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<variablelist>
<varlistentry>
<term><function>PQescapeBytea</function></term>
<listitem>
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<para>
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Escapes binary data for use within an SQL command with the type <type>bytea</type>.
<synopsis>
unsigned char *PQescapeBytea(const unsigned char *from,
size_t from_length,
size_t *to_length);
</synopsis>
</para>
<para>
Certain byte values <emphasis>must</emphasis> be escaped (but all
byte values <emphasis>may</emphasis> be escaped) when used as part
of a <type>bytea</type> literal in an <acronym>SQL</acronym>
statement. In general, to escape a byte, it is converted into the
three digit octal number equal to the octet value, and preceded by
two backslashes. The single quote (<literal>'</>) and backslash
(<literal>\</>) characters have special alternative escape
sequences. See the &cite-user; for more
information. <function>PQescapeBytea</function> performs this
operation, escaping only the minimally required bytes.
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</para>
<para>
The <parameter>from</parameter> parameter points to the first
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byte of the string that is to be escaped, and the
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<parameter>from_length</parameter> parameter reflects the number of
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bytes in this binary string. (A terminating zero byte is
neither necessary nor counted.) The <parameter>to_length</parameter>
parameter points to a variable that will hold the resultant
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escaped string length. The result string length includes the terminating
zero byte of the result.
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</para>
<para>
<function>PQescapeBytea</> returns an escaped version of the
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<parameter>from</parameter> parameter binary string in memory allocated with <function>malloc()</>.
The return string has all special characters replaced
so that they can be properly processed by the PostgreSQL string literal
parser, and the <type>bytea</type> input function. A terminating zero
byte is also added. The single quotes that must surround
PostgreSQL string literals are not part of the result string.
2001-11-20 21:28:31 +01:00
</para>
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</listitem>
</varlistentry>
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<varlistentry>
<term><function>PQunescapeBytea</function></term>
<listitem>
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<para>
Converts an escaped string representation of binary data into binary
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data --- the reverse of <function>PQescapeBytea</function>.
<synopsis>
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
</synopsis>
</para>
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<para>
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The <parameter>from</parameter> parameter points to an escaped string
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such as might be returned by <function>PQgetvalue</function> when applied to a
<type>bytea</type> column. <function>PQunescapeBytea</function> converts
this string representation into its binary representation.
It returns a pointer to a buffer allocated with <function>malloc()</function>, or null on error, and puts the size
of the buffer in <parameter>to_length</parameter>.
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</para>
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</listitem>
</varlistentry>
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<varlistentry>
<term><function>PQfreemem</function></term>
<listitem>
<para>
Frees memory allocated by <application>libpq</>
<synopsis>
void PQfreemem(void *ptr);
</synopsis>
</para>
<para>
Frees memory allocated by <application>libpq</>, particularly
<function>PQescapeBytea</function> and <function>PQunescapeBytea</function>.
It is needed by Win32, which can not free memory across
DLL's, unless multithreaded DLL's (/MD in VC6) are used.
</para>
</listitem>
</varlistentry>
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</variablelist>
2002-03-05 00:59:14 +01:00
2001-11-20 21:28:31 +01:00
</sect2>
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<sect2 id="libpq-exec-select-info">
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<title>Retrieving Query Result Information</title>
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2003-03-13 02:30:29 +01:00
<variablelist>
<varlistentry>
<term><function>PQntuples</function></term>
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<listitem>
<para>
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Returns the number of rows (tuples)
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in the query result.
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<synopsis>
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int PQntuples(const PGresult *res);
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</synopsis>
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</para>
</listitem>
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</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQnfields</function></term>
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<listitem>
<para>
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Returns the number of columns (fields)
in each row of the query result.
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<synopsis>
1999-11-11 22:52:28 +01:00
int PQnfields(const PGresult *res);
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</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQfname</function></term>
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<listitem>
<para>
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Returns the column name associated with the given column number.
Column numbers start at 0.
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<synopsis>
2000-02-08 00:10:11 +01:00
char *PQfname(const PGresult *res,
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int column_number);
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</synopsis>
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQfnumber</function></term>
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<listitem>
<para>
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Returns the column number
associated with the given column name.
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<synopsis>
1999-11-11 22:52:28 +01:00
int PQfnumber(const PGresult *res,
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const char *column_name);
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</synopsis>
1999-12-06 17:38:25 +01:00
</para>
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1999-12-06 17:38:25 +01:00
<para>
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-1 is returned if the given name does not match any column.
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</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQftype</function></term>
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<listitem>
<para>
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Returns the column data type associated with the
given column number. The integer returned is the
internal OID number of the type. Column numbers start
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at 0.
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<synopsis>
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Oid PQftype(const PGresult *res,
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int column_number);
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</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
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You can query the system table <literal>pg_type</literal> to obtain
2001-09-10 23:58:47 +02:00
the name and properties of the various data types. The <acronym>OID</acronym>s
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of the built-in data types are defined in the file <filename>src/include/catalog/pg_type.h</filename>
1999-11-11 22:52:28 +01:00
in the source tree.
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</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQfmod</function></term>
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<listitem>
<para>
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Returns the type-specific modification data of the column
associated with the given column number.
Column numbers start at 0.
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<synopsis>
int PQfmod(const PGresult *res,
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int column_number);
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</synopsis>
</para>
</listitem>
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</varlistentry>
2001-04-30 19:38:00 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQfsize</function></term>
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<listitem>
<para>
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Returns the size in bytes of the column
associated with the given column number.
Column numbers start at 0.
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<synopsis>
1999-11-11 22:52:28 +01:00
int PQfsize(const PGresult *res,
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int column_number);
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</synopsis>
1999-12-06 17:38:25 +01:00
</para>
2001-04-30 19:38:00 +02:00
2003-03-13 02:30:29 +01:00
<para>
<function>PQfsize</> returns the space allocated for this column in a database
row, in other words the size of the server's binary representation
of the data type. -1 is returned if the column has a variable size.
</para>
1999-12-06 17:38:25 +01:00
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-05-07 01:51:16 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQbinaryTuples</function></term>
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<listitem>
<para>
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Returns 1 if the <structname>PGresult</> contains binary row data
and 0 if it contains text data.
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<synopsis>
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int PQbinaryTuples(const PGresult *res);
1998-10-10 19:12:18 +02:00
</synopsis>
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</para>
<para>
Currently, binary row data can only be returned by a query that
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extracts data from a binary cursor.
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</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
</variablelist>
2001-04-30 19:38:00 +02:00
</sect2>
<sect2 id="libpq-exec-select-values">
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<title>Retrieving Query Result Values</title>
1998-03-01 09:16:16 +01:00
2003-03-13 02:30:29 +01:00
<variablelist>
<varlistentry>
<term><function>PQgetvalue</function></term>
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<listitem>
<para>
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Returns a single column value of one row
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of a <structname>PGresult</structname>.
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Row and colums indices start at 0.
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<synopsis>
2000-02-08 00:10:11 +01:00
char* PQgetvalue(const PGresult *res,
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int row_number,
int column_number);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
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For most queries, the value returned by <function>PQgetvalue</function>
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is a null-terminated character string representation
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of the column value. But if <function>PQbinaryTuples</function> returns 1,
1999-11-11 22:52:28 +01:00
the value returned by <function>PQgetvalue</function> is the binary
representation of the
type in the internal format of the backend server
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(but not including the size word, if the column is variable-length).
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It is then the programmer's responsibility to cast and
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convert the data to the correct C type.
</para>
<para>
The pointer
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returned by <function>PQgetvalue</function> points to storage that is
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part of the <structname>PGresult</structname> structure. One should not modify the data it points to,
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and one must explicitly
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copy the data into other storage if it is to
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be used past the lifetime of the <structname>PGresult</structname> structure itself.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQgetisnull</function></term>
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<listitem>
<para>
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Tests a column for a null value.
Row and column numbers start at 0.
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<synopsis>
int PQgetisnull(const PGresult *res,
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int row_number,
int column_number);
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</synopsis>
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</para>
<para>
This function returns 1 if the column is null and 0 if
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it contains a non-null value. (Note that <function>PQgetvalue</function>
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will return an empty string, not a null pointer, for a null
column.)
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</para>
</listitem>
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</varlistentry>
2001-04-30 19:38:00 +02:00
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<varlistentry>
<term><function>PQgetlength</function></term>
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<listitem>
<para>
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Returns the length of a column value in bytes.
Row and column numbers start at 0.
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<synopsis>
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int PQgetlength(const PGresult *res,
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int row_number,
int column_number);
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</synopsis>
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</para>
<para>
This is the actual data length for the particular data value, that is, the
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size of the object pointed to by <function>PQgetvalue</function>. Note that for character-represented
values, this size has little to do with the binary size reported by <function>PQfsize</function>.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQprint</function></term>
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<listitem>
<para>
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Prints out all the rows and, optionally, the
column names to the specified output stream.
<synopsis>
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void PQprint(FILE* fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
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typedef struct {
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pqbool header; /* print output field headings and row count */
pqbool align; /* fill align the fields */
pqbool standard; /* old brain dead format */
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pqbool html3; /* output HTML tables */
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pqbool expanded; /* expand tables */
pqbool pager; /* use pager for output if needed */
char *fieldSep; /* field separator */
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char *tableOpt; /* attributes for HTML table element */
char *caption; /* HTML table caption */
char **fieldName; /* null-terminated array of replacement field names */
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} PQprintOpt;
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</synopsis>
</para>
<para>
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This function was formerly used by <application>psql</application>
to print query results, but this is no longer the case and this
function is no longer actively supported.
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</para>
</listitem>
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</varlistentry>
</variablelist>
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</sect2>
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<sect2 id="libpq-exec-nonselect">
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<title>Retrieving Result Information for Other Commands</title>
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<variablelist>
<varlistentry>
<term><function>PQcmdStatus</function></term>
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<listitem>
<para>
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Returns the command status string from the SQL command that
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generated the <structname>PGresult</structname>.
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<synopsis>
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char * PQcmdStatus(PGresult *res);
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</synopsis>
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQcmdTuples</function></term>
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<listitem>
<para>
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Returns the number of rows affected by the SQL command.
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<synopsis>
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char * PQcmdTuples(PGresult *res);
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</synopsis>
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</para>
<para>
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If the <acronym>SQL</acronym> command that generated the
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<structname>PGresult</structname> was <command>INSERT</>, <command>UPDATE</>, or <command>DELETE</command>, this returns a
string containing the number of rows affected. If the
command was anything else, it returns the empty string.
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</para>
</listitem>
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</varlistentry>
1998-04-04 18:35:22 +02:00
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<varlistentry>
<term><function>PQoidValue</function></term>
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<listitem>
<para>
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Returns the OID of the inserted row, if the
<acronym>SQL</acronym> command was an <command>INSERT</command>
that inserted exactly one row into a table that has OIDs.
Otherwise, returns <literal>InvalidOid</literal>.
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<synopsis>
Oid PQoidValue(const PGresult *res);
</synopsis>
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</para>
<para>
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The type <type>Oid</type> and the constant
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<literal>InvalidOid</literal> will be defined if you include
the <application>libpq</application> header file. They will
both be some integer type.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
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</varlistentry>
1999-11-11 22:52:28 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQoidStatus</function></term>
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<listitem>
<para>
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Returns a string with the OID of the inserted row, if the
<acronym>SQL</acronym> command was an
<command>INSERT</command>. (The string will be
<literal>0</> if the <command>INSERT</command> did not
insert exactly one row, or if the target table does not have
OIDs.) If the command was not an <command>INSERT</command>,
returns an empty string.
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<synopsis>
2000-02-08 00:10:11 +01:00
char * PQoidStatus(const PGresult *res);
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</synopsis>
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</para>
<para>
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This function is deprecated in favor of <function>PQoidValue</function>
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and is not thread-safe.
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</para>
</listitem>
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</varlistentry>
</variablelist>
2001-05-01 00:21:22 +02:00
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</sect2>
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</sect1>
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2000-09-29 22:21:34 +02:00
<sect1 id="libpq-async">
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<title>Asynchronous Command Processing</title>
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2001-11-12 20:19:39 +01:00
<indexterm zone="libpq-async"><primary>nonblocking connection</></>
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<para>
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The <function>PQexec</function> function is adequate for submitting commands in
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normal, synchronous
applications. It has a couple of deficiencies, however, that can be of importance to some users:
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1999-12-06 17:38:25 +01:00
<itemizedlist>
<listitem>
<para>
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<function>PQexec</function> waits for the command to be completed. The application may have other
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work to do (such as maintaining a user interface), in which case it won't
want to block waiting for the response.
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</para>
</listitem>
<listitem>
<para>
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Since the execution of the client application is suspended while it
waits for the result, it is hard for the application to decide that it
would like to try to cancel the ongoing command. (It can be done from
a signal handler, but not otherwise.)
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</para>
</listitem>
<listitem>
<para>
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<function>PQexec</function> can return only one <structname>PGresult</structname> structure. If the submitted command
2001-09-10 23:58:47 +02:00
string contains multiple <acronym>SQL</acronym> commands, all but the last <structname>PGresult</structname> are
1999-11-11 22:52:28 +01:00
discarded by <function>PQexec</function>.
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</para>
</listitem>
</itemizedlist>
</para>
1998-05-07 01:51:16 +02:00
1999-12-06 17:38:25 +01:00
<para>
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Applications that do not like these limitations can instead use the
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underlying functions that <function>PQexec</function> is built from:
<function>PQsendQuery</function> and <function>PQgetResult</function>.
2000-01-18 07:09:24 +01:00
</para>
<para>
Older programs that used this functionality as well as
<function>PQputline</function> and <function>PQputnbytes</function>
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could block waiting to send data to the server. To
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address that issue, the function <function>PQsetnonblocking</function>
was added.
Old applications can neglect to use <function>PQsetnonblocking</function>
2003-03-13 02:30:29 +01:00
and get the old potentially blocking behavior. Newer programs can use
2001-11-12 20:19:39 +01:00
<function>PQsetnonblocking</function> to achieve a completely nonblocking
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connection to the server.
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2003-03-13 02:30:29 +01:00
<variablelist>
<varlistentry>
<term><function>PQsetnonblocking</function></term>
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<listitem>
<para>
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Sets the nonblocking status of the connection.
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<synopsis>
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int PQsetnonblocking(PGconn *conn, int arg);
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</synopsis>
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</para>
<para>
Sets the state of the connection to nonblocking if <parameter>arg</parameter> is 1 and
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blocking if <parameter>arg</parameter> is 0. Returns 0 if OK, -1 if error.
2000-12-28 01:16:11 +01:00
</para>
<para>
In the nonblocking state, calls to
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<function>PQputline</function>, <function>PQputnbytes</function>,
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<function>PQsendQuery</function>, and <function>PQendcopy</function>
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will not block but instead return an error if they need to be called
again.
</para>
<para>
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When a database connection has been set to nonblocking mode and
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<function>PQexec</function> is called, it will temporarily set the state
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of the connection to blocking until the <function>PQexec</function> call
2000-01-18 07:09:24 +01:00
completes.
</para>
<para>
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More of <application>libpq</application> is expected to be made safe for
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the nonblocking mode in the future.
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</para>
</listitem>
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</varlistentry>
2000-01-18 07:09:24 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQisnonblocking</function></term>
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<listitem>
<para>
Returns the blocking status of the database connection.
<synopsis>
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int PQisnonblocking(const PGconn *conn);
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</synopsis>
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</para>
<para>
Returns 1 if the connection is set to nonblocking mode and
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0 if blocking.
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</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
2000-01-18 07:09:24 +01:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQsendQuery</function></term>
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<listitem>
<para>
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Submits a command to the server without
2002-01-07 03:29:15 +01:00
waiting for the result(s). 1 is returned if the command was
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successfully dispatched and 0 if not (in which case, use
2002-01-07 03:29:15 +01:00
<function>PQerrorMessage</> to get more information about the failure).
1998-10-10 19:12:18 +02:00
<synopsis>
1998-05-07 01:51:16 +02:00
int PQsendQuery(PGconn *conn,
2003-03-13 02:30:29 +01:00
const char *command);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
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After successfully calling <function>PQsendQuery</function>, call
<function>PQgetResult</function> one or more
2002-01-07 03:29:15 +01:00
times to obtain the results. <function>PQsendQuery</function> may not be called
2003-03-13 02:30:29 +01:00
again (on the same connection) until <function>PQgetResult</function> has returned a null pointer,
2002-01-07 03:29:15 +01:00
indicating that the command is done.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-05-07 01:51:16 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQgetResult</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
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Waits for the next result from a prior <function>PQsendQuery</function>,
and return it. A null pointer is returned when the command is complete
1998-05-07 01:51:16 +02:00
and there will be no more results.
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<synopsis>
1998-05-07 01:51:16 +02:00
PGresult *PQgetResult(PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
<function>PQgetResult</function> must be called repeatedly until it returns a null pointer,
2002-01-07 03:29:15 +01:00
indicating that the command is done. (If called when no command is
2003-03-13 02:30:29 +01:00
active, <function>PQgetResult</function> will just return a null pointer at once.)
Each non-null result from <function>PQgetResult</function> should be processed using
2002-09-21 20:32:54 +02:00
the same <structname>PGresult</> accessor functions previously described.
1999-11-11 22:52:28 +01:00
Don't forget to free each result object with <function>PQclear</function> when done with it.
2003-03-13 02:30:29 +01:00
Note that <function>PQgetResult</function> will block only if a command is active and the
1999-11-11 22:52:28 +01:00
necessary response data has not yet been read by <function>PQconsumeInput</function>.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
</variablelist>
1999-12-06 17:38:25 +01:00
</para>
1998-05-07 01:51:16 +02:00
1999-12-06 17:38:25 +01:00
<para>
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Using <function>PQsendQuery</function> and <function>PQgetResult</function>
solves one of <function>PQexec</function>'s problems:
2002-01-07 03:29:15 +01:00
If a command string contains multiple <acronym>SQL</acronym> commands, the results of those
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commands can be obtained individually. (This allows a simple form of
2003-03-13 02:30:29 +01:00
overlapped processing, by the way: the client can be handling the
results of one command while the server is still working on later
2002-01-07 03:29:15 +01:00
queries in the same command string.) However, calling <function>PQgetResult</function> will
2003-03-13 02:30:29 +01:00
still cause the client to block until the server completes the
1999-11-11 22:52:28 +01:00
next <acronym>SQL</acronym> command. This can be avoided by proper use of three more
1998-05-07 01:51:16 +02:00
functions:
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<variablelist>
<varlistentry>
<term><function>PQconsumeInput</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
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If input is available from the server, consume it.
1998-10-10 19:12:18 +02:00
<synopsis>
1998-10-05 06:11:47 +02:00
int PQconsumeInput(PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
2001-09-13 17:55:24 +02:00
<function>PQconsumeInput</function> normally returns 1 indicating <quote>no error</quote>,
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but returns 0 if there was some kind of trouble (in which case
2003-03-13 02:30:29 +01:00
<function>PQerrorMessage</function> can be used). Note that the result does not say
1999-11-11 22:52:28 +01:00
whether any input data was actually collected. After calling
<function>PQconsumeInput</function>, the application may check
<function>PQisBusy</function> and/or <function>PQnotifies</function> to see if
their state has changed.
1999-12-06 17:38:25 +01:00
</para>
<para>
1999-11-11 22:52:28 +01:00
<function>PQconsumeInput</function> may be called even if the application is not
prepared to deal with a result or notification just yet. The
2003-03-13 02:30:29 +01:00
function will read available data and save it in a buffer, thereby
2002-01-07 03:29:15 +01:00
causing a <function>select()</function> read-ready indication to go away. The
1999-11-11 22:52:28 +01:00
application can thus use <function>PQconsumeInput</function> to clear the
2002-01-07 03:29:15 +01:00
<function>select()</function> condition immediately, and then examine the results at leisure.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-05-07 01:51:16 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQisBusy</function></term>
1999-12-06 17:38:25 +01:00
<listitem>
<para>
2003-03-13 02:30:29 +01:00
Returns 1 if a command is busy, that is, <function>PQgetResult</function> would block
1999-11-11 22:52:28 +01:00
waiting for input. A 0 return indicates that <function>PQgetResult</function> can
be called with assurance of not blocking.
1998-10-10 19:12:18 +02:00
<synopsis>
1998-05-07 01:51:16 +02:00
int PQisBusy(PGconn *conn);
1998-10-10 19:12:18 +02:00
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
<function>PQisBusy</function> will not itself attempt to read data from the server;
1999-11-11 22:52:28 +01:00
therefore <function>PQconsumeInput</function> must be invoked first, or the busy
state will never end.
1999-12-06 17:38:25 +01:00
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
1998-05-07 01:51:16 +02:00
2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQflush</function></term>
2000-01-18 07:09:24 +01:00
<listitem>
<para>
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Attempts to flush any data queued to the server,
2002-01-07 03:29:15 +01:00
returns 0 if successful (or if the send queue is empty) or <symbol>EOF</symbol> if it failed for
2000-01-18 07:09:24 +01:00
some reason.
<synopsis>
int PQflush(PGconn *conn);
</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
2001-11-12 20:19:39 +01:00
<function>PQflush</function> needs to be called on a nonblocking connection
2002-01-07 03:29:15 +01:00
before calling <function>select()</function> to determine if a response has
2000-01-18 07:09:24 +01:00
arrived. If 0 is returned it ensures that there is no data queued to the
2003-03-13 02:30:29 +01:00
server that has not actually been sent. Only applications that have used
2000-01-18 07:09:24 +01:00
<function>PQsetnonblocking</function> have a need for this.
</para>
</listitem>
2003-03-13 02:30:29 +01:00
</varlistentry>
</variablelist>
1999-12-06 17:38:25 +01:00
</para>
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<para>
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A typical application using these functions will have a main loop that uses
<function>select()</function> to wait for all the conditions that it must
respond to. One of the conditions will be input available from the server,
which in terms of <function>select()</function> means readable data on the file
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descriptor identified by <function>PQsocket</function>.
When the main loop detects input ready, it should call
<function>PQconsumeInput</function> to read the input. It can then call
<function>PQisBusy</function>, followed by <function>PQgetResult</function>
if <function>PQisBusy</function> returns false (0). It can also call
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<function>PQnotifies</function> to detect <command>NOTIFY</> messages (see <xref linkend="libpq-notify">).
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</para>
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<para>
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Nonblocking connections (that have used <function>PQsetnonblocking</function>)
should not use <function>select()</function> until <function>PQflush</function>
has returned 0 indicating that there is no buffered data waiting to be sent
to the server.
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</para>
<para>
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A client that uses <function>PQsendQuery</function>/<function>PQgetResult</function>
can also attempt to cancel a command that is still being processed by the server.
<variablelist>
<varlistentry>
<term><function>PQrequestCancel</function></term>
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<listitem>
<para>
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Requests that the server abandon
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processing of the current command.
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<synopsis>
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int PQrequestCancel(PGconn *conn);
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</synopsis>
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</para>
<para>
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The return value is 1 if the cancel request was successfully
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dispatched and 0 if not. (If not, <function>PQerrorMessage</function> tells why not.)
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Successful dispatch is no guarantee that the request will have any
effect, however. Regardless of the return value of <function>PQrequestCancel</function>,
the application must continue with the normal result-reading
sequence using <function>PQgetResult</function>. If the cancellation
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is effective, the current command will terminate early and return
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an error result. If the cancellation fails (say, because the
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server was already done processing the command), then there will
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be no visible result at all.
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</para>
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<para>
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Note that if the current command is part of a transaction block, cancellation
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will abort the whole transaction.
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</para>
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<para>
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<function>PQrequestCancel</function> can safely be invoked from a signal handler.
So, it is also possible to use it in conjunction with plain
<function>PQexec</function>, if the decision to cancel can be made in a signal
handler. For example, <application>psql</application> invokes
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<function>PQrequestCancel</function> from a <symbol>SIGINT</> signal handler, thus allowing
interactive cancellation of commands that it issues through <function>PQexec</function>.
</para>
</listitem>
</varlistentry>
</variablelist>
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</para>
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1999-12-06 17:38:25 +01:00
</sect1>
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<sect1 id="libpq-fastpath">
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<title>The Fast-Path Interface</title>
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<para>
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<productname>PostgreSQL</productname> provides a fast-path interface to send
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function calls to the server. This is a trapdoor into system internals and
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can be a potential security hole. Most users will not need this feature.
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</para>
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<para>
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The function <function>PQfn</function> requests execution of a server
function via the fast-path interface:
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<synopsis>
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PGresult* PQfn(PGconn* conn,
int fnid,
int *result_buf,
int *result_len,
int result_is_int,
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const PQArgBlock *args,
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int nargs);
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1998-03-01 09:16:16 +01:00
typedef struct {
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int len;
int isint;
union {
int *ptr;
int integer;
} u;
} PQArgBlock;
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</synopsis>
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</para>
<para>
The <parameter>fnid</> argument is the OID of the function to be
executed.
<parameter>result_buf</parameter> is the buffer in which
to place the return value. The caller must have allocated
sufficient space to store the return value. (There is no check!)
The actual result length will be returned in the integer pointed
to by <parameter>result_len</parameter>. If a 4-byte integer result is expected, set
<parameter>result_is_int</parameter> to 1, otherwise set it to 0. (Setting <parameter>result_is_int</parameter> to 1
tells <application>libpq</> to byte-swap the value if necessary, so that it is
delivered as a proper <type>int</type> value for the client machine. When
<parameter>result_is_int</> is 0, the byte string sent by the server is returned
unmodified.)
<parameter>args</> and <parameter>nargs</> specify the arguments to be passed to the function.
</para>
<para>
<function>PQfn</function> always returns a valid <structname>PGresult</structname> pointer. The result status
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should be checked before the result is used. The
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caller is responsible for freeing the <structname>PGresult</structname> with
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<function>PQclear</function> when it is no longer needed.
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</para>
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1999-12-06 17:38:25 +01:00
</sect1>
1998-03-01 09:16:16 +01:00
2000-09-29 22:21:34 +02:00
<sect1 id="libpq-notify">
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<title>Asynchronous Notification</title>
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<indexterm zone="libpq-notify"><primary>NOTIFY</primary></indexterm>
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<para>
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<productname>PostgreSQL</productname> offers asynchronous notification via the
<command>LISTEN</command> and <command>NOTIFY</command> commands. A server-side session registers its interest in a particular
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notification condition with the <command>LISTEN</command> command (and can stop listening
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with the <command>UNLISTEN</command> command). All sessions listening on a
particular condition will be notified asynchronously when a <command>NOTIFY</command> command with that
condition name is executed by any session. No additional information is
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passed from the notifier to the listener. Thus, typically, any actual data
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that needs to be communicated is transferred through a database table.
Commonly, the condition name is the same as the associated table, but it is
not necessary for there to be any associated table.
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</para>
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<para>
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<application>libpq</application> applications submit <command>LISTEN</command> and <command>UNLISTEN</command>
commands as ordinary SQL command. The arrival of <command>NOTIFY</command>
messages can subsequently be detected by calling <function>PQnotifies</function>.
</para>
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<para>
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The function <function>PQnotifies</function>
returns the next notification from a list of unhandled
notification messages received from the server. It returns a null pointer if
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there are no pending notifications. Once a notification is
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returned from <function>PQnotifies</>, it is considered handled and will be
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removed from the list of notifications.
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<synopsis>
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PGnotify* PQnotifies(PGconn *conn);
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typedef struct pgNotify {
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char *relname; /* notification name */
int be_pid; /* process ID of server process */
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} PGnotify;
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</synopsis>
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After processing a <structname>PGnotify</structname> object returned by <function>PQnotifies</function>,
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be sure to free it with <function>free()</function> to avoid a memory leak.
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</para>
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1999-12-06 17:38:25 +01:00
<note>
<para>
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In <productname>PostgreSQL</productname> 6.4 and later,
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the <literal>be_pid</literal> is that of the notifying backend process,
whereas in earlier versions it was always the <acronym>PID</acronym> of your own backend process.
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</para>
</note>
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<para>
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<xref linkend="libpq-example-2"> gives a sample program that illustrates the use
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of asynchronous notification.
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</para>
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<para>
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<function>PQnotifies()</function> does not actually read data from the server; it just
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returns messages previously absorbed by another <application>libpq</application>
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function. In prior releases of <application>libpq</application>, the only way
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to ensure timely receipt of <command>NOTIFY</> messages was to constantly submit commands,
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even empty ones, and then check <function>PQnotifies()</function> after each
<function>PQexec()</function>. While this still works, it is
deprecated as a waste of processing power.
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</para>
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1999-12-06 17:38:25 +01:00
<para>
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A better way to check for <command>NOTIFY</>
messages when you have no useful commands to execute is to call
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<function>PQconsumeInput()</function>, then check
<function>PQnotifies()</function>.
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You can use <function>select()</function> to wait for data to
arrive from the server, thereby using no <acronym>CPU</acronym> power unless there is something
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to do. (See <function>PQsocket()</function> to obtain the file descriptor
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number to use with <function>select()</function>.)
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Note that this will work OK whether you submit commands with
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<function>PQsendQuery</function>/<function>PQgetResult</function> or simply
use <function>PQexec</function>. You should, however, remember to
check <function>PQnotifies()</function> after each
<function>PQgetResult</function> or <function>PQexec</function>, to see
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if any notifications came in during the processing of the command.
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</para>
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1999-12-06 17:38:25 +01:00
</sect1>
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<sect1 id="libpq-copy">
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<title>Functions Associated with the <command>COPY</command> Command</title>
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<indexterm zone="libpq-copy">
<primary>COPY</primary>
<secondary>with libpq</secondary>
</indexterm>
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<para>
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The <command>COPY</command> command in <productname>PostgreSQL</productname> has options to read from
or write to the network connection used by <application>libpq</application>.
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Therefore, functions are necessary to access this network
connection directly so applications may take advantage of this capability.
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</para>
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<para>
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These functions should be executed only after obtaining a result
status of <literal>PGRES_COPY_OUT</literal> or
<literal>PGRES_COPY_IN</literal> from <function>PQexec</function> or
<function>PQgetResult</function>.
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</para>
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<variablelist>
<varlistentry>
<term><function>PQgetline</function></term>
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<listitem>
<para>
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Reads a newline-terminated line of characters
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(transmitted by the server) into a buffer
string of size <parameter>length</>.
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<synopsis>
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int PQgetline(PGconn *conn,
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char *buffer,
int length);
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</synopsis>
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</para>
<para>
This function copies up to <parameter>length</>-1 characters
into the buffer and converts
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the terminating newline into a zero byte.
<function>PQgetline</function> returns <symbol>EOF</symbol> at the end of input, 0 if the
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entire line has been read, and 1 if the buffer is full but the
terminating newline has not yet been read.
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</para>
<para>
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Note that the application must check to see if a
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new line consists of the two characters <literal>\.</literal>,
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which indicates that the server has finished sending
the results of the <command>COPY</command> command.
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If the application might
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receive lines that are more than <parameter>length</>-1 characters long,
care is needed to be sure it recognizes the <literal>\.</literal> line correctly
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(and does not, for example, mistake the end of a long data line
for a terminator line).
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The code in the file
<filename>src/bin/psql/copy.c</filename>
contains example functions that correctly handle the <command>COPY</command> protocol.
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQgetlineAsync</function></term>
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<listitem>
<para>
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Reads a newline-terminated line of characters
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(transmitted by the server) into a buffer
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without blocking.
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<synopsis>
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int PQgetlineAsync(PGconn *conn,
char *buffer,
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int length);
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</synopsis>
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</para>
<para>
This function is similar to <function>PQgetline</function>, but it can be used
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by applications
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that must read <command>COPY</command> data asynchronously, that is, without blocking.
Having issued the <command>COPY</command> command and gotten a <literal>PGRES_COPY_OUT</literal>
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response, the
application should call <function>PQconsumeInput</function> and
<function>PQgetlineAsync</function> until the
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end-of-data signal is detected.
</para>
<para>
Unlike <function>PQgetline</function>, this function takes
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responsibility for detecting end-of-data.
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On each call, <function>PQgetlineAsync</function> will return data if a complete newline-
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terminated data line is available in <application>libpq</>'s input buffer, or if the
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incoming data line is too long to fit in the buffer offered by the caller.
Otherwise, no data is returned until the rest of the line arrives.
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The function returns -1 if the end-of-copy-data marker has been recognized,
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or 0 if no data is available, or a positive number giving the number of
bytes of data returned. If -1 is returned, the caller must next call
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<function>PQendcopy</function>, and then return to normal processing.
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</para>
<para>
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The data returned will not extend beyond a newline character. If possible
a whole line will be returned at one time. But if the buffer offered by
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the caller is too small to hold a line sent by the server, then a partial
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data line will be returned. This can be detected by testing whether the
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last returned byte is <literal>\n</literal> or not.
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The returned string is not null-terminated. (If you want to add a
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terminating null, be sure to pass a <parameter>length</parameter> one smaller than the room
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actually available.)
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</para>
</listitem>
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</varlistentry>
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2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQputline</function></term>
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<listitem>
<para>
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Sends a null-terminated string to the server.
Returns 0 if OK and <symbol>EOF</symbol> if unable to send the string.
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<synopsis>
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int PQputline(PGconn *conn,
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const char *string);
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</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
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Note the application must explicitly send the two
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characters <literal>\.</literal> on a final line to indicate to
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the server that it has finished sending its data.
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</para>
</listitem>
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</varlistentry>
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2003-03-13 02:30:29 +01:00
<varlistentry>
<term><function>PQputnbytes</function></term>
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<listitem>
<para>
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Sends a non-null-terminated string to the server.
Returns 0 if OK and <symbol>EOF</symbol> if unable to send the string.
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<synopsis>
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int PQputnbytes(PGconn *conn,
const char *buffer,
int nbytes);
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</synopsis>
2003-03-13 02:30:29 +01:00
</para>
<para>
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This is exactly like <function>PQputline</function>, except that the data buffer need
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not be null-terminated since the number of bytes to send is
specified directly.
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</para>
</listitem>
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</varlistentry>
1998-03-01 09:16:16 +01:00
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<varlistentry>
<term><function>PQendcopy</function></term>
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<listitem>
<para>
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Synchronizes with the server.
<synopsis>
int PQendcopy(PGconn *conn);
</synopsis>
This function waits until
the server has finished the copying. It should
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either be issued when the last string has been
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sent to the server using <function>PQputline</function> or when the
last string has been received from the server
using <function>PGgetline</function>. It must be issued or the server
may get <quote>out of sync</quote> with the client. Upon
return from this function, the server is ready to
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receive the next SQL command.
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The return value is 0 on successful completion,
nonzero otherwise.
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</para>
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1999-12-06 17:38:25 +01:00
<para>
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When using <function>PQgetResult</function>, the application should respond to
a <literal>PGRES_COPY_OUT</literal> result by executing <function>PQgetline</function>
repeatedly, followed by <function>PQendcopy</function> after the terminator line is seen.
It should then return to the <function>PQgetResult</function> loop until
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<function>PQgetResult</function> returns a null pointer. Similarly a <literal>PGRES_COPY_IN</literal>
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result is processed by a series of <function>PQputline</function> calls followed by
<function>PQendcopy</function>, then return to the <function>PQgetResult</function> loop.
This arrangement will ensure that
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a <command>COPY</command> command embedded in a series of <acronym>SQL</acronym> commands
1998-05-07 01:51:16 +02:00
will be executed correctly.
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</para>
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<para>
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Older applications are likely to submit a <command>COPY</command>
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via <function>PQexec</function> and assume that the transaction is done after
<function>PQendcopy</function>.
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This will work correctly only if the <command>COPY</command> is the only
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<acronym>SQL</acronym> command in the command string.
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</para>
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</listitem>
</varlistentry>
</variablelist>
<para>
An example:
<programlisting>
PQexec(conn, "CREATE TABLE foo (a integer, b varchar(16), d double precision);");
PQexec(conn, "COPY foo FROM STDIN;");
PQputline(conn, "3\thello world\t4.5\n");
PQputline(conn, "4\tgoodbye world\t7.11\n");
...
PQputline(conn, "\\.\n");
PQendcopy(conn);
</programlisting>
</para>
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</sect1>
1998-03-01 09:16:16 +01:00
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<sect1 id="libpq-trace">
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<title>Tracing Functions</title>
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<variablelist>
<varlistentry>
<term><function>PQtrace</function></term>
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<listitem>
<para>
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Enables tracing of the client/server communication to a debugging file stream.
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<synopsis>
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void PQtrace(PGconn *conn
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FILE *stream);
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</synopsis>
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</para>
</listitem>
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</varlistentry>
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<varlistentry>
<term><function>PQuntrace</function></term>
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<listitem>
<para>
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Disables tracing started by <function>PQtrace</function>.
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<synopsis>
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void PQuntrace(PGconn *conn);
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</synopsis>
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</para>
</listitem>
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</varlistentry>
</variablelist>
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</sect1>
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<sect1 id="libpq-control">
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<title>Notice Processing</title>
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<para>
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The function <function>PQsetNoticeProcessor</function>
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<indexterm><primary>notice processor</></>
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controls the reporting of notice and warning messages generated by the server.
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<synopsis>
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typedef void (*PQnoticeProcessor) (void *arg, const char *message);
PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
PQnoticeProcessor proc,
void *arg);
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</synopsis>
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</para>
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<para>
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By default, <application>libpq</application> prints notice messages
from the server, as well as a few error messages that it generates by
itself, on <filename>stderr</filename>.
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This behavior can be overridden by supplying a callback function that
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does something else with the messages, a so-called notice processor.
The callback function is passed
the text of the message (which includes a trailing newline), plus
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a void pointer that is the same one passed to
<function>PQsetNoticeProcessor</function>.
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(This pointer can be used to access application-specific state if needed.)
The default notice processor is simply
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<programlisting>
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static void
defaultNoticeProcessor(void * arg, const char * message)
{
fprintf(stderr, "%s", message);
}
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</programlisting>
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To use a special notice processor, call
<function>PQsetNoticeProcessor</function> just after
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creation of a new <structname>PGconn</> object.
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</para>
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<para>
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The return value is the pointer to the previous notice processor.
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If you supply a null callback function pointer, no action is taken,
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but the current pointer is returned.
</para>
<para>
Once you have set a notice processor, you should expect that that function
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could be called as long as either the <structname>PGconn</> object or <structname>PGresult</> objects
made from it exist. At creation of a <structname>PGresult</>, the <structname>PGconn</>'s current
notice processor pointer is copied into the <structname>PGresult</> for possible use by
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functions like <function>PQgetvalue</function>.
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</para>
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1999-12-06 17:38:25 +01:00
</sect1>
1998-03-01 09:16:16 +01:00
1999-12-06 17:38:25 +01:00
<sect1 id="libpq-envars">
<title>Environment Variables</title>
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<indexterm zone="libpq-envars">
<primary>environment variables</primary>
</indexterm>
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<para>
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The following environment variables can be used to select default
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connection parameter values, which will be used by
<function>PQconnectdb</>, <function>PQsetdbLogin</> and
<function>PQsetdb</> if no value is directly specified by the calling
code. These are useful to avoid hard-coding database connection
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information into simple client applications, for example.
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<itemizedlist>
<listitem>
<para>
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<indexterm>
<primary><envar>PGHOST</envar></primary>
</indexterm>
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<envar>PGHOST</envar> sets the default server name.
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If this begins with a slash, it specifies Unix-domain communication
rather than TCP/IP communication; the value is the name of the
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directory in which the socket file is stored (default <filename>/tmp</filename>).
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</para>
</listitem>
<listitem>
<para>
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<indexterm>
<primary><envar>PGPORT</envar></primary>
</indexterm>
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<envar>PGPORT</envar> sets the default TCP port number or Unix-domain
socket file extension for communicating with the
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<productname>PostgreSQL</productname> server.
UUNET is looking into offering PostgreSQL as a part of a managed web
hosting product, on both shared and dedicated machines. We currently
offer Oracle and MySQL, and it would be a nice middle-ground.
However, as shipped, PostgreSQL lacks the following features we need
that MySQL has:
1. The ability to listen only on a particular IP address. Each
hosting customer has their own IP address, on which all of their
servers (http, ftp, real media, etc.) run.
2. The ability to place the Unix-domain socket in a mode 700 directory.
This allows us to automatically create an empty database, with an
empty DBA password, for new or upgrading customers without having
to interactively set a DBA password and communicate it to (or from)
the customer. This in turn cuts down our install and upgrade times.
3. The ability to connect to the Unix-domain socket from within a
change-rooted environment. We run CGI programs chrooted to the
user's home directory, which is another reason why we need to be
able to specify where the Unix-domain socket is, instead of /tmp.
4. The ability to, if run as root, open a pid file in /var/run as
root, and then setuid to the desired user. (mysqld -u can almost
do this; I had to patch it, too).
The patch below fixes problem 1-3. I plan to address #4, also, but
haven't done so yet. These diffs are big enough that they should give
the PG development team something to think about in the meantime :-)
Also, I'm about to leave for 2 weeks' vacation, so I thought I'd get
out what I have, which works (for the problems it tackles), now.
With these changes, we can set up and run PostgreSQL with scripts the
same way we can with apache or proftpd or mysql.
In summary, this patch makes the following enhancements:
1. Adds an environment variable PGUNIXSOCKET, analogous to MYSQL_UNIX_PORT,
and command line options -k --unix-socket to the relevant programs.
2. Adds a -h option to postmaster to set the hostname or IP address to
listen on instead of the default INADDR_ANY.
3. Extends some library interfaces to support the above.
4. Fixes a few memory leaks in PQconnectdb().
The default behavior is unchanged from stock 7.0.2; if you don't use
any of these new features, they don't change the operation.
David J. MacKenzie
2000-11-13 16:18:15 +01:00
</para>
</listitem>
<listitem>
<para>
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<indexterm>
<primary><envar>PGDATABASE</envar></primary>
</indexterm>
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<envar>PGDATABASE</envar> sets the default
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<productname>PostgreSQL</productname> database name.
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</para>
</listitem>
<listitem>
<para>
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<indexterm>
<primary><envar>PGUSER</envar></primary>
</indexterm>
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<envar>PGUSER</envar>
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sets the user name used to connect to the database.
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</para>
</listitem>
<listitem>
<para>
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<indexterm>
<primary><envar>PGPASSWORD</envar></primary>
</indexterm>
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<envar>PGPASSWORD</envar>
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sets the password used if the server demands password
authentication. This environment variable is deprecated for security
reasons; consider migrating to use the <filename>$HOME/.pgpass</>
file (see <xref linkend="libpq-pgpass">).
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</para>
</listitem>
<listitem>
<para>
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<envar>PGREALM</envar> sets the Kerberos realm to use with
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<productname>PostgreSQL</productname>, if it is different from the local realm.
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If <envar>PGREALM</envar> is set, <application>libpq</application>
1999-11-11 22:52:28 +01:00
applications will attempt authentication with servers for this realm and use
separate ticket files to avoid conflicts with local
ticket files. This environment variable is only
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used if Kerberos authentication is selected by the server.
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</para>
</listitem>
<listitem>
<para>
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<envar>PGOPTIONS</envar> sets additional run-time options for
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the <productname>PostgreSQL</productname> server.
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</para>
</listitem>
<listitem>
<para>
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<envar>PGTTY</envar> sets the file or <acronym>TTY</> on which debugging
messages from the server are displayed.
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</para>
</listitem>
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<listitem>
<para>
<envar>PGREQUIRESSL</envar> sets whether or not the connection must be
made over <acronym>SSL</acronym>. If set to
<quote>1</quote>, <application>libpq</>
will refuse to connect if the server does not accept
an <acronym>SSL</acronym> connection.
This option is only available if
<productname>PostgreSQL</> is compiled with SSL support.
</para>
</listitem>
<listitem>
<para>
<envar>PGCONNECT_TIMEOUT</envar> sets the maximum number of seconds
that <application>libpq</application> will wait when attempting to
connect to the <productname>PostgreSQL</productname> server. This
option should be set to at least 2 seconds.
</para>
</listitem>
1999-12-06 17:38:25 +01:00
</itemizedlist>
</para>
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<para>
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The following environment variables can be used to specify default
behavior for every <productname>PostgreSQL</productname> session.
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<itemizedlist>
<listitem>
<para>
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<envar>PGDATESTYLE</envar>
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sets the default style of date/time representation.
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(Equivalent to <literal>SET datestyle TO ...</literal>.)
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</para>
</listitem>
<listitem>
<para>
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<envar>PGTZ</envar>
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sets the default time zone.
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(Equivalent to <literal>SET timezone TO ...</literal>.)
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</para>
</listitem>
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<listitem>
<para>
<envar>PGCLIENTENCODING</envar>
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sets the default client character set encoding.
(Equivalent to <literal>SET client_encoding TO ...</literal>.)
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</para>
</listitem>
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<listitem>
<para>
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<envar>PGGEQO</envar>
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sets the default mode for the genetic query optimizer.
(Equivalent to <literal>SET geqo TO ...</literal>.)
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</para>
</listitem>
</itemizedlist>
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Refer to the <acronym>SQL</acronym> command <command>SET</command>
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for information on correct values for these environment variables.
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</para>
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1999-12-06 17:38:25 +01:00
</sect1>
1998-10-05 06:11:47 +02:00
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<sect1 id="libpq-pgpass">
<title>The Password File</title>
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<indexterm zone="libpq-pgpass">
<primary>password file</primary>
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</indexterm>
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<indexterm zone="libpq-pgpass">
<primary>.pgpass</primary>
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</indexterm>
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<para>
The file <filename>.pgpass</filename> in a user's home directory is a file
that can contain passwords to be used if the connection requires a
password (and no password has been specified otherwise).
This file should have lines of the following format:
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<synopsis>
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<replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
2002-11-11 21:14:04 +01:00
</synopsis>
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Each of these fields may be a literal name or <literal>*</literal>, which
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matches anything. The first matching entry will be used, so put more-specific
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entries first. When an entry contain <literal>:</literal> or
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<literal>\</literal>, it must be escaped with <literal>\</literal>.
2002-09-06 00:05:50 +02:00
</para>
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<para>
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The permissions on <filename>.pgpass</filename> must disallow any
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access to world or group; achieve this by the command
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<command>chmod 0600 ~/.pgpass</command>.
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If the permissions are less strict than this, the file will be ignored.
2002-11-11 21:14:04 +01:00
</para>
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</sect1>
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<sect1 id="libpq-threading">
<title>Threading Behavior</title>
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<indexterm zone="libpq-threading">
<primary>threads</primary>
<secondary>with libpq</secondary>
</indexterm>
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<para>
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<application>libpq</application> is thread-safe as of
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<productname>PostgreSQL</productname> 7.0, so long as no two threads
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attempt to manipulate the same <structname>PGconn</> object at the same
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time. In particular, you cannot issue concurrent commands from different
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threads through the same connection object. (If you need to run
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concurrent commands, start up multiple connections.)
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</para>
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<para>
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<structname>PGresult</> objects are read-only after creation, and so can be passed around
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freely between threads.
</para>
<para>
The deprecated functions <function>PQoidStatus</function> and
<function>fe_setauthsvc</function> are not thread-safe and should not be
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used in multithread programs. <function>PQoidStatus</function> can be
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replaced by <function>PQoidValue</function>. There is no good reason to
call <function>fe_setauthsvc</function> at all.
</para>
2002-01-18 21:39:04 +01:00
<para>
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<application>libpq</application> applications that use the <literal>crypt</literal>
authentication method rely on the <literal>crypt()</literal> operating
system function, which is often not thread-safe. It is better to use the
<literal>md5</literal> method, which is thread-safe on all
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platforms.
</para>
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</sect1>
1998-03-01 09:16:16 +01:00
2001-08-28 16:20:28 +02:00
<sect1 id="libpq-build">
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<title>Building <application>libpq</application> Programs</title>
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<para>
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To build (i.e., compile and link) your <application>libpq</application> programs you need to
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do all of the following things:
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<itemizedlist>
<listitem>
<para>
Include the <filename>libpq-fe.h</filename> header file:
<programlisting>
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#include <libpq-fe.h>
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</programlisting>
If you failed to do that then you will normally get error
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messages from your compiler similar to
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<screen>
foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
</screen>
</para>
</listitem>
<listitem>
<para>
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Point your compiler to the directory where the <productname>PostgreSQL</> header
2001-08-28 16:20:28 +02:00
files were installed, by supplying the
<literal>-I<replaceable>directory</replaceable></literal> option
to your compiler. (In some cases the compiler will look into
the directory in question by default, so you can omit this
option.) For instance, your compile command line could look
like:
<programlisting>
cc -c -I/usr/local/pgsql/include testprog.c
</programlisting>
If you are using makefiles then add the option to the
<varname>CPPFLAGS</varname> variable:
<programlisting>
CPPFLAGS += -I/usr/local/pgsql/include
</programlisting>
</para>
<para>
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<indexterm><primary>pg_config</></>
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If there is any chance that your program might be compiled by
other users then you should not hardcode the directory location
like that. Instead, you can run the utility
<command>pg_config</command> to find out where the header files
are on the local system:
<screen>
<prompt>$</prompt> pg_config --includedir
<computeroutput>/usr/local/include</computeroutput>
</screen>
</para>
<para>
Failure to specify the correct option to the compiler will
result in an error message such as
<screen>
testlibpq.c:8:22: libpq-fe.h: No such file or directory
</screen>
</para>
</listitem>
<listitem>
<para>
When linking the final program, specify the option
2001-09-10 23:58:47 +02:00
<literal>-lpq</literal> so that the <application>libpq</application> library gets pulled
2001-08-28 16:20:28 +02:00
in, as well as the option
<literal>-L<replaceable>directory</replaceable></literal> to
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point the compiler to the directory where the <application>libpq</application> library resides. (Again, the
2001-08-28 16:20:28 +02:00
compiler will search some directories by default.) For maximum
portability, put the <option>-L</option> option before the
<option>-lpq</option> option. For example:
<programlisting>
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
</programlisting>
</para>
<para>
You can find out the library directory using
<command>pg_config</command> as well:
<screen>
<prompt>$</prompt> pg_config --libdir
<computeroutput>/usr/local/pgsql/lib</computeroutput>
</screen>
</para>
<para>
Error messages that point to problems in this area could look
like the following.
<screen>
testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
</screen>
This means you forgot <option>-lpq</option>.
<screen>
/usr/bin/ld: cannot find -lpq
</screen>
2003-03-13 02:30:29 +01:00
This means you forgot the <option>-L</option> option or did not specify
the right directory.
2001-08-28 16:20:28 +02:00
</para>
</listitem>
</itemizedlist>
</para>
<para>
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<indexterm><primary>libpq-int.h</></>
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If your codes references the header file
<filename>libpq-int.h</filename> and you refuse to fix your code to
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not use it, starting in <productname>PostgreSQL</> 7.2, this file will be found in
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<filename><replaceable>includedir</replaceable>/postgresql/internal/libpq-int.h</filename>,
so you need to add the appropriate <option>-I</option> option to
your compiler command line.
</para>
</sect1>
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<sect1 id="libpq-example">
<title>Example Programs</title>
<example id="libpq-example-1">
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<title><application>libpq</application> Example Program 1</title>
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<programlisting>
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/*
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* testlibpq.c
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*
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* Test the C version of libpq, the <productname>PostgreSQL</> frontend
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* library.
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*/
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#include <stdio.h>
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#include <libpq-fe.h>
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void
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exit_nicely(PGconn *conn)
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{
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PQfinish(conn);
exit(1);
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}
main()
{
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char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
/* FILE *debug; */
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* debugging tty for the backend server */
dbName = "template1";
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
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fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
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exit_nicely(conn);
}
/* debug = fopen("/tmp/trace.out","w"); */
/* PQtrace(conn, debug); */
/* start a transaction block */
res = PQexec(conn, "BEGIN");
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if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
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{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
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PQclear(res);
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/*
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* fetch rows from the pg_database, the system catalog of
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* databases
*/
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res = PQexec(conn, "DECLARE mycursor CURSOR FOR SELECT * FROM pg_database");
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if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
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{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
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PQclear(res);
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res = PQexec(conn, "FETCH ALL in mycursor");
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if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
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{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
PQclear(res);
exit_nicely(conn);
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}
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/* first, print out the attribute names */
nFields = PQnfields(res);
for (i = 0; i < nFields; i++)
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printf("%-15s", PQfname(res, i));
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printf("\n\n");
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/* next, print out the rows */
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for (i = 0; i < PQntuples(res); i++)
{
for (j = 0; j < nFields; j++)
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printf("%-15s", PQgetvalue(res, i, j));
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printf("\n");
}
PQclear(res);
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/* close the cursor */
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
/* commit the transaction */
res = PQexec(conn, "COMMIT");
PQclear(res);
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/* close the connection to the database and cleanup */
PQfinish(conn);
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/* fclose(debug); */
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return 0;
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}
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</programlisting>
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</example>
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2000-10-03 21:16:17 +02:00
<example id="libpq-example-2">
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<title><application>libpq</application> Example Program 2</title>
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<programlisting>
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/*
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* testlibpq2.c
* Test of the asynchronous notification interface
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*
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* Start this program, then from psql in another window do
* NOTIFY TBL2;
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*
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* Or, if you want to get fancy, try this:
* Populate a database with the following:
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*
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* CREATE TABLE TBL1 (i int4);
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*
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* CREATE TABLE TBL2 (i int4);
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*
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* CREATE RULE r1 AS ON INSERT TO TBL1 DO
* (INSERT INTO TBL2 values (new.i); NOTIFY TBL2);
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*
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* and do
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*
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* INSERT INTO TBL1 values (10);
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*
*/
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#include <stdio.h>
#include "libpq-fe.h"
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void
exit_nicely(PGconn *conn)
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{
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PQfinish(conn);
exit(1);
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}
main()
{
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char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
PGconn *conn;
PGresult *res;
PGnotify *notify;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* debugging tty for the backend server */
dbName = getenv("USER"); /* change this to the name of your test
* database */
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
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fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
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exit_nicely(conn);
}
res = PQexec(conn, "LISTEN TBL2");
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if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
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{
fprintf(stderr, "LISTEN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
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*/
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PQclear(res);
while (1)
{
/*
* wait a little bit between checks; waiting with select()
* would be more efficient.
*/
sleep(1);
/* collect any asynchronous backend messages */
PQconsumeInput(conn);
/* check for asynchronous notify messages */
while ((notify = PQnotifies(conn)) != NULL)
{
fprintf(stderr,
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"ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
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notify->relname, notify->be_pid);
free(notify);
}
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}
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/* close the connection to the database and cleanup */
PQfinish(conn);
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return 0;
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}
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</programlisting>
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</example>
1998-03-01 09:16:16 +01:00
2000-10-03 21:16:17 +02:00
<example id="libpq-example-3">
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<title><application>libpq</application> Example Program 3</>
1998-03-01 09:16:16 +01:00
1999-12-06 17:38:25 +01:00
<programlisting>
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/*
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* testlibpq3.c Test the C version of Libpq, the <productname>PostgreSQL</> frontend
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* library. tests the binary cursor interface
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*
*
*
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* populate a database by doing the following:
*
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* CREATE TABLE test1 (i int4, d real, p polygon);
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*
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* INSERT INTO test1 values (1, 3.567, polygon '(3.0, 4.0, 1.0, 2.0)');
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*
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* INSERT INTO test1 values (2, 89.05, polygon '(4.0, 3.0, 2.0, 1.0)');
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*
* the expected output is:
*
* tuple 0: got i = (4 bytes) 1, d = (4 bytes) 3.567000, p = (4
* bytes) 2 points boundbox = (hi=3.000000/4.000000, lo =
* 1.000000,2.000000) tuple 1: got i = (4 bytes) 2, d = (4 bytes)
* 89.050003, p = (4 bytes) 2 points boundbox =
* (hi=4.000000/3.000000, lo = 2.000000,1.000000)
*
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*
*/
#include <stdio.h>
#include "libpq-fe.h"
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#include "utils/geo_decls.h" /* for the POLYGON type */
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1998-10-15 07:47:04 +02:00
void
exit_nicely(PGconn *conn)
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{
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PQfinish(conn);
exit(1);
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}
main()
{
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char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
int i_fnum,
d_fnum,
p_fnum;
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* debugging tty for the backend server */
dbName = getenv("USER"); /* change this to the name of your test
* database */
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
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fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
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exit_nicely(conn);
}
/* start a transaction block */
res = PQexec(conn, "BEGIN");
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if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
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{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
/*
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* fetch rows from the pg_database, the system catalog of
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* databases
*/
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res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR SELECT * FROM test1");
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if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
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{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in mycursor");
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if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
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{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
PQclear(res);
exit_nicely(conn);
}
i_fnum = PQfnumber(res, "i");
d_fnum = PQfnumber(res, "d");
p_fnum = PQfnumber(res, "p");
for (i = 0; i < 3; i++)
{
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printf("type[%d] = %d, size[%d] = %d\n",
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i, PQftype(res, i),
i, PQfsize(res, i));
}
for (i = 0; i < PQntuples(res); i++)
{
int *ival;
float *dval;
int plen;
POLYGON *pval;
/* we hard-wire this to the 3 fields we know about */
ival = (int *) PQgetvalue(res, i, i_fnum);
dval = (float *) PQgetvalue(res, i, d_fnum);
plen = PQgetlength(res, i, p_fnum);
/*
* plen doesn't include the length field so need to
* increment by VARHDSZ
*/
pval = (POLYGON *) malloc(plen + VARHDRSZ);
pval->size = plen;
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memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen);
printf("tuple %d: got\n", i);
printf(" i = (%d bytes) %d,\n",
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PQgetlength(res, i, i_fnum), *ival);
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printf(" d = (%d bytes) %f,\n",
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PQgetlength(res, i, d_fnum), *dval);
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printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
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PQgetlength(res, i, d_fnum),
pval->npts,
pval->boundbox.xh,
pval->boundbox.yh,
pval->boundbox.xl,
pval->boundbox.yl);
}
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PQclear(res);
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/* close the cursor */
res = PQexec(conn, "CLOSE mycursor");
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PQclear(res);
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/* commit the transaction */
res = PQexec(conn, "COMMIT");
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PQclear(res);
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/* close the connection to the database and cleanup */
PQfinish(conn);
1998-10-10 19:12:18 +02:00
2000-04-25 18:39:07 +02:00
return 0;
1998-10-10 19:12:18 +02:00
}
1999-12-06 17:38:25 +01:00
</programlisting>
2000-10-03 21:16:17 +02:00
</example>
1999-12-06 17:38:25 +01:00
2000-10-03 21:16:17 +02:00
</sect1>
1999-12-06 17:38:25 +01:00
</chapter>
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