libpq C libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library functions that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries. libpq is also the underlying engine for several other PostgreSQL application interfaces, including libpq++ (C++), libpgtcl (Tcl), Perl, and ECPG. So some aspects of libpq's behavior will be important to you if you use one of those packages. Some short programs are included at the end of this chapter () to show how to write programs that use libpq. There are also several complete examples of libpq applications in the directory src/test/examples in the source code distribution. Client programs that use libpq must include the header file libpq-fe.hlibpq-fe.h and must link with the libpq library. The following functions deal with making a connection to a PostgreSQL backend server. An 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 PGconnPGconn object which is obtained from the function PQconnectdb or PQsetdbLogin. Note that these functions will always return a non-null object pointer, unless perhaps there is too little memory even to allocate the PGconn object. The PQstatus function should be called to check whether a connection was successfully made before queries are sent via the connection object. PQconnectdbPQconnectdb Makes a new connection to the database server. PGconn *PQconnectdb(const char *conninfo); This function opens a new database connection using the parameters taken from the string conninfo. Unlike PQsetdbLogin below, the parameter set can be extended without changing the function signature, so use of this function (or its nonblocking analogues PQconnectStart and PQconnectPoll) is preferred for new application programming. The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace. Each parameter setting is in the form keyword = value. (To write an empty value or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., \' and \\.) Spaces around the equal sign are optional. The currently recognized parameter keywords are: host Name of host to connect to.host name 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 /tmp.Unix domain socket hostaddr Numeric IP address of host to connect to. This should be in the standard IPv4 address format, e.g., 172.28.40.9. If your machine supports IPv6, you can also use those addresses. TCP/IP communication is always used when a nonempty string is specified for this parameter. Using hostaddr instead of host allows the application to avoid a host name look-up, which may be important in applications with time constraints. However, Kerberos authentication requires the host name. The following therefore applies: If host is specified without hostaddr, a host name lookup occurs. If hostaddr is specified without host, the value for hostaddr gives the remote address. When Kerberos is used, a reverse name query occurs to obtain the host name for Kerberos. If both host and hostaddr are specified, the value for hostaddr gives the remote address; the value for host is ignored, unless Kerberos is used, in which case that value is used for Kerberos authentication. (Note that authentication is likely to fail if libpq is passed a host name that is not the name of the machine at hostaddr.) Also, host rather than hostaddr is used to identify the connection in $HOME/.pgpass. Without either a host name or host address, libpq will connect using a local Unix domain socket. port Port number to connect to at the server host, or socket file name extension for Unix-domain connections.port dbname The database name. Defaults to be the same as the user name. user PostgreSQL user name to connect as. password Password to be used if the server demands password authentication. connect_timeout Maximum wait for connection, in seconds (write as a decimal integer string). Zero or not specified means wait indefinitely. It is not recommended to use a timeout of less than 2 seconds. options Command-line options to be sent to the server. tty Ignored (formerly, this specified where to send server debug output). sslmode This option determines whether or with what priority an SSL connection will be negotiated with the server. There are four modes: disable will attempt only an unencrypted SSL connection; allow will negotiate, trying first a non-SSL connection, then if that fails, trying an SSL connection; prefer (the default) will negotiate, trying first an SSL connection, then if that fails, trying a regular non-SSL connection; require will try only an SSL connection. If PostgreSQL is compiled without SSL support, using option require will cause an error, and options allow and prefer will be tolerated but libpq will be unable to negotiate an SSL connection.SSLwith libpq requiressl This option is deprecated in favor of the sslmode setting. If set to 1, an SSL connection to the server is required (this is equivalent to sslmode require). libpq will then refuse to connect if the server does not accept an SSL connection. If set to 0 (default), libpq will negotiate the connection type with the server (equivalent to sslmode prefer). This option is only available if PostgreSQL is compiled with SSL support. service Service name to use for additional parameters. It specifies a service name in pg_service.conf that holds additional connection parameters. This allows applications to specify only a service name so connection parameters can be centrally maintained. See PREFIX/share/pg_service.conf.sample for information on how to set up the file. If any parameter is unspecified, then the corresponding environment variable (see ) is checked. If the environment variable is not set either, then built-in defaults are used. PQsetdbLoginPQsetdbLogin Makes a new connection to the database server. PGconn *PQsetdbLogin(const char *pghost, const char *pgport, const char *pgoptions, const char *pgtty, const char *dbName, const char *login, const char *pwd); This is the predecessor of PQconnectdb with a fixed set of parameters. It has the same functionality except that the missing parameters will always take on default values. Write NULL or an empty string for any one of the fixed parameters that is to be defaulted. PQsetdbPQsetdb Makes a new connection to the database server. PGconn *PQsetdb(char *pghost, char *pgport, char *pgoptions, char *pgtty, char *dbName); This is a macro that calls PQsetdbLogin with null pointers for the login and pwd parameters. It is provided for backward compatibility with very old programs. PQconnectStartPQconnectStart PQconnectPollPQconnectPoll nonblocking connection Make a connection to the database server in a nonblocking manner. PGconn *PQconnectStart(const char *conninfo); PostgresPollingStatusType PQconnectPoll(PGconn *conn); These two functions are used to open a connection to a database server such that your application's thread of execution is not blocked on remote I/O whilst doing so. The point of this approach is that the waits for I/O to complete can occur in the application's main loop, rather than down inside PQconnectdb(), and so the application can manage this operation in parallel with other activities. The database connection is made using the parameters taken from the string conninfo, passed to PQconnectStart. This string is in the same format as described above for PQconnectdb. Neither PQconnectStart nor PQconnectPoll will block, so long as a number of restrictions are met: The hostaddr and host parameters are used appropriately to ensure that name and reverse name queries are not made. See the documentation of these parameters under PQconnectdb above for details. If you call PQtrace, ensure that the stream object into which you trace will not block. You ensure that the socket is in the appropriate state before calling PQconnectPoll, as described below. To begin a nonblocking connection request, call conn = PQconnectStart("connection_info_string"). If conn is null, then libpq has been unable to allocate a new PGconn structure. Otherwise, a valid PGconn pointer is returned (though not yet representing a valid connection to the database). On return from PQconnectStart, call status = PQstatus(conn). If status equals CONNECTION_BAD, PQconnectStart has failed. If PQconnectStart succeeds, the next stage is to poll libpq so that it may proceed with the connection sequence. Use PQsocket(conn) to obtain the descriptor of the socket underlying the database connection. Loop thus: If PQconnectPoll(conn) last returned PGRES_POLLING_READING, wait until the socket is ready to read (as indicated by select(), poll(), or similar system function). Then call PQconnectPoll(conn) again. Conversely, if PQconnectPoll(conn) last returned PGRES_POLLING_WRITING, wait until the socket is ready to write, then call PQconnectPoll(conn) again. If you have yet to call PQconnectPoll, i.e., just after the call to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. Continue this loop until PQconnectPoll(conn) returns PGRES_POLLING_FAILED, indicating the connection procedure has failed, or PGRES_POLLING_OK, indicating the connection has been successfully made. At any time during connection, the status of the connection may be checked by calling PQstatus. If this gives CONNECTION_BAD, then the connection procedure has failed; if it gives CONNECTION_OK, then the connection is ready. Both of these states are equally detectable from the return value of 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: CONNECTION_STARTED Waiting for connection to be made. CONNECTION_MADE Connection OK; waiting to send. CONNECTION_AWAITING_RESPONSE Waiting for a response from the server. CONNECTION_AUTH_OK Received authentication; waiting for backend start-up to finish. CONNECTION_SSL_STARTUP Negotiating SSL encryption. CONNECTION_SETENV Negotiating environment-driven parameter settings. Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these appearing in a particular order, or at all, or on the status always being one of these documented values. An application might do something like this: switch(PQstatus(conn)) { case CONNECTION_STARTED: feedback = "Connecting..."; break; case CONNECTION_MADE: feedback = "Connected to server..."; break; . . . default: feedback = "Connecting..."; } The connect_timeout connection parameter is ignored when using PQconnectPoll; it is the application's responsibility to decide whether an excessive amount of time has elapsed. Otherwise, PQconnectStart followed by a PQconnectPoll loop is equivalent to PQconnectdb. Note that if PQconnectStart returns a non-null pointer, you must call PQfinish when you are finished with it, in order to dispose of the structure and any associated memory blocks. This must be done even if the connection attempt fails or is abandoned. PQconndefaultsPQconndefaults Returns the default connection options. PQconninfoOption *PQconndefaults(void); typedef struct { char *keyword; /* The keyword of the option */ char *envvar; /* Fallback environment variable name */ char *compiled; /* Fallback compiled in default value */ char *val; /* Option's current value, or NULL */ 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 "D" Debug option - don't show by default */ int dispsize; /* Field size in characters for dialog */ } PQconninfoOption; Returns a connection options array. This may be used to determine all possible PQconnectdb options and their current default values. The return value points to an array of PQconninfoOption structures, which ends with an entry having a null keyword pointer. Note that the current default values (val fields) will depend on environment variables and other context. Callers must treat the connection options data as read-only. After processing the options array, free it by passing it to PQconninfoFree. If this is not done, a small amount of memory is leaked for each call to PQconndefaults. PQfinishPQfinish Closes the connection to the server. Also frees memory used by the PGconn object. void PQfinish(PGconn *conn); Note that even if the server connection attempt fails (as indicated by PQstatus), the application should call PQfinish to free the memory used by the PGconn object. The PGconn pointer must not be used again after PQfinish has been called. PQresetPQreset Resets the communication channel to the server. void PQreset(PGconn *conn); This function will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. PQresetStartPQresetStart PQresetPollPQresetPoll Reset the communication channel to the server, in a nonblocking manner. int PQresetStart(PGconn *conn); PostgresPollingStatusType PQresetPoll(PGconn *conn); These functions will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. They differ from PQreset (above) in that they act in a nonblocking manner. These functions suffer from the same restrictions as PQconnectStart and PQconnectPoll. To initiate a connection reset, call PQresetStart. If it returns 0, the reset has failed. If it returns 1, poll the reset using PQresetPoll in exactly the same way as you would create the connection using PQconnectPoll. These functions may be used to interrogate the status of an existing database connection object. libpq-fe.h libpq-int.h libpq application programmers should be careful to maintain the PGconn abstraction. Use the accessor functions described below to get at the contents of PGconn. Avoid directly referencing the fields of the PGconn structure because they are subject to change in the future. (Beginning in PostgreSQL release 6.4, the definition of the struct behind PGconn is not even provided in libpq-fe.h. If you have old code that accesses PGconn fields directly, you can keep using it by including libpq-int.h too, but you are encouraged to fix the code soon.) The following functions return parameter values established at connection. These values are fixed for the life of the PGconn object. PQdbPQdb Returns the database name of the connection. char *PQdb(const PGconn *conn); PQuserPQuser Returns the user name of the connection. char *PQuser(const PGconn *conn); PQpassPQpass Returns the password of the connection. char *PQpass(const PGconn *conn); PQhostPQhost Returns the server host name of the connection. char *PQhost(const PGconn *conn); PQportPQport Returns the port of the connection. char *PQport(const PGconn *conn); PQttyPQtty Returns the debug TTY of the connection. (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backwards compatibility.) char *PQtty(const PGconn *conn); PQoptionsPQoptions Returns the command-line options passed in the connection request. char *PQoptions(const PGconn *conn); The following functions return status data that can change as operations are executed on the PGconn object. PQstatusPQstatus Returns the status of the connection. ConnStatusType PQstatus(const PGconn *conn); The status can be one of a number of values. However, only two of these are seen outside of an asynchronous connection procedure: CONNECTION_OK and CONNECTION_BAD. A good connection to the database has the status CONNECTION_OK. A failed connection attempt is signaled by status CONNECTION_BAD. Ordinarily, an OK status will remain so until PQfinish, but a communications failure might result in the status changing to CONNECTION_BAD prematurely. In that case the application could try to recover by calling PQreset. See the entry for PQconnectStart and PQconnectPoll with regards to other status codes that might be seen. PQtransactionStatusPQtransactionStatus Returns the current in-transaction status of the server. PGTransactionStatusType PQtransactionStatus(const PGconn *conn); The status can be PQTRANS_IDLE (currently idle), PQTRANS_ACTIVE (a command is in progress), PQTRANS_INTRANS (idle, in a valid transaction block), or PQTRANS_INERROR (idle, in a failed transaction block). PQTRANS_UNKNOWN is reported if the connection is bad. PQTRANS_ACTIVE is reported only when a query has been sent to the server and not yet completed. PQtransactionStatus will give incorrect results when using a PostgreSQL 7.3 server that has AUTOCOMMIT set to OFF. The server-side autocommit feature has been deprecated and does not exist in later server versions. PQparameterStatusPQparameterStatus Looks up a current parameter setting of the server. const char *PQparameterStatus(const PGconn *conn, const char *paramName); Certain parameter values are reported by the server automatically at connection startup or whenever their values change. PQparameterStatus can be used to interrogate these settings. It returns the current value of a parameter if known, or NULL if the parameter is not known. Parameters reported as of the current release include server_version (cannot change after startup); server_encoding (also not presently changeable after start); client_encoding, is_superuser, and DateStyle. Pre-3.0-protocol servers do not report parameter settings, but libpq includes logic to obtain values for server_version, server_encoding, and client_encoding. Applications are encouraged to use PQparameterStatus rather than ad-hoc code to determine these values. (Beware however that on a pre-3.0 connection, changing client_encoding via SET after connection startup will not be reflected by PQparameterStatus.) PQprotocolVersionPQprotocolVersion Interrogates the frontend/backend protocol being used. int PQprotocolVersion(const PGconn *conn); Applications may wish to use this to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). This will not change after connection startup is complete, but it could theoretically change during a reset. The 3.0 protocol will normally be used when communicating with PostgreSQL 7.4 or later servers; pre-7.4 servers support only protocol 2.0. (Protocol 1.0 is obsolete and not supported by libpq.) PQerrorMessagePQerrorMessage error message Returns the error message most recently generated by an operation on the connection. char *PQerrorMessage(const PGconn* conn); Nearly all libpq functions will set a message for PQerrorMessage if they fail. Note that by libpq convention, a nonempty PQerrorMessage result will include a trailing newline. PQsocketPQsocket 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. (This will not change during normal operation, but could change during connection setup or reset.) int PQsocket(const PGconn *conn); PQbackendPIDPQbackendPID Returns the process ID (PID)PIDdetermining PID of server processin libpq of the backend server process handling this connection. int PQbackendPID(const PGconn *conn); The backend PID is useful for debugging purposes and for comparison to NOTIFY messages (which include the PID of the notifying backend process). Note that the PID belongs to a process executing on the database server host, not the local host! PQgetsslPQgetssl SSLin libpq Returns the SSL structure used in the connection, or null if SSL is not in use. SSL *PQgetssl(const PGconn *conn); This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure. You must define USE_SSL in order to get the prototype for this function. Doing this will also automatically include ssl.h from OpenSSL. Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands. PQexecPQexec Submits a command to the server and waits for the result. PGresult *PQexec(PGconn *conn, const char *command); Returns a PGresult pointer or possibly a null pointer. A non-null pointer will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the server. If a null pointer is returned, it should be treated like a PGRES_FATAL_ERROR result. Use PQerrorMessage to get more information about the error. It is allowed to include multiple SQL commands (separated by semicolons) in the command string. Multiple queries sent in a single PQexec call are processed in a single transaction, unless there are explicit BEGIN/COMMIT commands included in the query string to divide it into multiple transactions. Note however that the returned PGresult structure describes only the result of the last command executed from the string. Should one of the commands fail, processing of the string stops with it and the returned PGresult describes the error condition. PQexecParamsPQexecParams Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text. PGresult *PQexecParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); PQexecParams is like PQexec, but offers additional functionality: parameter values can be specified separately from the command string proper, and query results can be requested in either text or binary format. PQexecParams is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0. If parameters are used, they are referred to in the command string as $1, $2, etc. nParams is the number of parameters supplied; it is the length of the arrays paramTypes[], paramValues[], paramLengths[], and paramFormats[]. (The array pointers may be NULL when nParams is zero.) paramTypes[] specifies, by OID, the datatypes to be assigned to the parameter symbols. If paramTypes is NULL, or any particular element in the array is zero, the backend assigns a datatype to the parameter symbol in the same way it would do for an untyped literal string. paramValues[] specifies the actual values of the parameters. A NULL pointer in this array means the corresponding parameter is NULL; otherwise the pointer points to a zero-terminated text string (for text format) or binary data in the format expected by the backend (for binary format). paramLengths[] specifies the actual data lengths of binary-format parameters. It is ignored for NULL parameters and text-format parameters. The array pointer may be NULL when there are no binary parameters. paramFormats[] specifies whether parameters are text (put a zero in the array) or binary (put a one in the array). If the array pointer is NULL then all parameters are presumed to be text. resultFormat is zero to obtain results in text format, or one to obtain results in binary format. (There is not currently a provision to obtain different result columns in different formats, although that is possible in the underlying protocol.) The primary advantage of PQexecParams over PQexec is that parameter values may be separated from the command string, thus avoiding the need for tedious and error-prone quoting and escaping. Unlike PQexec, PQexecParams allows at most one SQL command in the given string. (There can be semicolons in it, but not more than one nonempty command.) This is a limitation of the underlying protocol, but has some usefulness as an extra defense against SQL-injection attacks. PQexecPreparedPQexecPrepared Sends a request to execute a prepared statement with given parameters, and waits for the result. PGresult *PQexecPrepared(PGconn *conn, const char *stmtName, int nParams, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); PQexecPrepared is like PQexecParams, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. This feature allows commands that will be used repeatedly to be parsed and planned just once, rather than each time they are executed. PQexecPrepared is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0. The parameters are identical to PQexecParams, except that the name of a prepared statement is given instead of a query string, and the paramTypes[] parameter is not present (it is not needed since the prepared statement's parameter types were determined when it was created). Presently, prepared statements for use with PQexecPrepared must be set up by executing an SQL PREPARE command, which is typically sent with PQexec (though any of libpq's query-submission functions may be used). A lower-level interface for preparing statements may be offered in a future release. The PGresultPGresult structure encapsulates the result returned by the server. libpq application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future. PQresultStatusPQresultStatus Returns the result status of the command. ExecStatusType PQresultStatus(const PGresult *res); PQresultStatus can return one of the following values: PGRES_EMPTY_QUERY The string sent to the server was empty. PGRES_COMMAND_OK Successful completion of a command returning no data. PGRES_TUPLES_OK Successful completion of a command returning data (such as a SELECT or SHOW). PGRES_COPY_OUT Copy Out (from server) data transfer started. PGRES_COPY_IN Copy In (to server) data transfer started. PGRES_BAD_RESPONSE The server's response was not understood. PGRES_NONFATAL_ERROR A nonfatal error (a notice or warning) occurred. PGRES_FATAL_ERROR A fatal error occurred. If the result status is PGRES_TUPLES_OK, then the functions described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT, UPDATE, etc.). A response of PGRES_EMPTY_QUERY may indicate a bug in the client software. A result of status PGRES_NONFATAL_ERROR will never be returned directly by PQexec or other query execution functions; results of this kind are instead passed to the notice processor (see ). PQresStatusPQresStatus Converts the enumerated type returned by PQresultStatus into a string constant describing the status code. char *PQresStatus(ExecStatusType status); PQresultErrorMessagePQresultErrorMessage Returns the error message associated with the command, or an empty string if there was no error. char *PQresultErrorMessage(const PGresult *res); If there was an error, the returned string will include a trailing newline. Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will return the same string as PQresultErrorMessage (on the result). However, a PGresult will retain its error message until destroyed, whereas the connection's error message will change when subsequent operations are done. Use PQresultErrorMessage when you want to know the status associated with a particular PGresult; use PQerrorMessage when you want to know the status from the latest operation on the connection. PQresultErrorFieldPQresultErrorField Returns an individual field of an error report. char *PQresultErrorField(const PGresult *res, int fieldcode); fieldcode is an error field identifier; see the symbols listed below. NULL is returned if the PGresult is not an error or warning result, or does not include the specified field. Field values will normally not include a trailing newline. The following field codes are available: PG_DIAG_SEVERITY The severity; the field contents are ERROR, FATAL, or PANIC (in an error message), or WARNING, NOTICE, DEBUG, INFO, or LOG (in a notice message), or a localized translation of one of these. Always present. PG_DIAG_SQLSTATE The SQLSTATE code for the error (a 5-character string following SQL spec conventions). Not localizable. Always present. PG_DIAG_MESSAGE_PRIMARY The primary human-readable error message (typically one line). Always present. PG_DIAG_MESSAGE_DETAIL Detail: an optional secondary error message carrying more detail about the problem. May run to multiple lines. PG_DIAG_MESSAGE_HINT Hint: an optional suggestion what to do about the problem. This is intended to differ from detail in that it offers advice (potentially inappropriate) rather than hard facts. May run to multiple lines. PG_DIAG_STATEMENT_POSITION A string containing a decimal integer indicating an error cursor position as an index into the original statement string. The first character has index 1, and positions are measured in characters not bytes. PG_DIAG_CONTEXT An indication of the context in which the error occurred. Presently this includes a call stack traceback of active PL functions. The trace is one entry per line, most recent first. PG_DIAG_SOURCE_FILE The file name of the source-code location where the error was reported. PG_DIAG_SOURCE_LINE The line number of the source-code location where the error was reported. PG_DIAG_SOURCE_FUNCTION The name of the source-code function reporting the error. The client is responsible for formatting displayed information to meet its needs; in particular it should break long lines as needed. Newline characters appearing in the error message fields should be treated as paragraph breaks, not line breaks. Errors generated internally by libpq will have severity and primary message, but typically no other fields. Errors returned by a pre-3.0-protocol server will include severity and primary message, and sometimes a detail message, but no other fields. Note that error fields are only available from PGresult objects, not PGconn objects; there is no PQerrorField function. PQclearPQclear Frees the storage associated with a PGresult. Every command result should be freed via PQclear when it is no longer needed. void PQclear(PQresult *res); You can keep a PGresult object around for as long as you need it; it does not go away when you issue a new command, nor even if you close the connection. To get rid of it, you must call PQclear. Failure to do this will result in memory leaks in your application. PQmakeEmptyPGresultPQmakeEmptyPGresult Constructs an empty PGresult object with the given status. PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status); This is libpq's internal function to allocate and initialize an empty PGresult object. It is exported because some applications find it useful to generate result objects (particularly objects with error status) themselves. If conn is not null and status indicates an error, the current error message of the specified connection is copied into the PGresult. Note that PQclear should eventually be called on the object, just as with a PGresult returned by libpq itself. These functions are used to extract information from a PGresult object that represents a successful query result (that is, one that has status PGRES_TUPLES_OK). For objects with other status values they will act as though the result has zero rows and zero columns. PQntuplesPQntuples Returns the number of rows (tuples) in the query result. int PQntuples(const PGresult *res); PQnfieldsPQnfields Returns the number of columns (fields) in each row of the query result. int PQnfields(const PGresult *res); PQfnamePQfname Returns the column name associated with the given column number. Column numbers start at 0. char *PQfname(const PGresult *res, int column_number); NULL is returned if the column number is out of range. PQfnumberPQfnumber Returns the column number associated with the given column name. int PQfnumber(const PGresult *res, const char *column_name); -1 is returned if the given name does not match any column. PQftablePQftable Returns the OID of the table from which the given column was fetched. Column numbers start at 0. Oid PQftable(const PGresult *res, int column_number); InvalidOid is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol. You can query the system table pg_class to determine exactly which table is referenced. The type Oid and the constant InvalidOid will be defined when you include the libpq header file. They will both be some integer type. PQftablecolPQftablecol Returns the column number (within its table) of the column making up the specified query result column. Result column numbers start at 0. int PQftablecol(const PGresult *res, int column_number); Zero is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol. PQfformatPQfformat Returns the format code indicating the format of the given column. Column numbers start at 0. int PQfformat(const PGresult *res, int column_number); Format code zero indicates textual data representation, while format code one indicates binary representation. (Other codes are reserved for future definition.) PQftypePQftype Returns the data type associated with the given column number. The integer returned is the internal OID number of the type. Column numbers start at 0. Oid PQftype(const PGresult *res, int column_number); You can query the system table pg_type to obtain the names and properties of the various data types. The OIDs of the built-in data types are defined in the file src/include/catalog/pg_type.h in the source tree. PQfmodPQfmod Returns the type modifier of the column associated with the given column number. Column numbers start at 0. int PQfmod(const PGresult *res, int column_number); The interpretation of modifier values is type-specific; they typically indicate precision or size limits. The value -1 is used to indicate no information available. Most data types do not use modifiers, in which case the value is always -1. PQfsizePQfsize Returns the size in bytes of the column associated with the given column number. Column numbers start at 0. int PQfsize(const PGresult *res, int column_number); PQfsize returns the space allocated for this column in a database row, in other words the size of the server's internal representation of the data type. (Accordingly, it is not really very useful to clients.) A negative value indicates the data type is variable-length. PQbinaryTuplesPQbinaryTuples Returns 1 if the PGresult contains binary data and 0 if it contains text data. int PQbinaryTuples(const PGresult *res); This function is deprecated (except for its use in connection with COPY), because it is possible for a single PGresult to contain text data in some columns and binary data in others. PQfformat() is preferred. PQbinaryTuples returns 1 only if all columns of the result are binary (format 1). PQgetvaluePQgetvalue Returns a single field value of one row of a PGresult. Row and column numbers start at 0. char* PQgetvalue(const PGresult *res, int row_number, int column_number); For data in text format, the value returned by PQgetvalue is a null-terminated character string representation of the field value. For data in binary format, the value is in the binary representation determined by the datatype's typsend and typreceive functions. (The value is actually followed by a zero byte in this case too, but that is not ordinarily useful, since the value is likely to contain embedded nulls.) An empty string is returned if the field value is NULL. See PQgetisnull to distinguish NULLs from empty-string values. The pointer returned by PQgetvalue points to storage that is part of the PGresult structure. One should not modify the data it points to, and one must explicitly copy the data into other storage if it is to be used past the lifetime of the PGresult structure itself. PQgetisnullPQgetisnull null valuein libpq Tests a field for a null value. Row and column numbers start at 0. int PQgetisnull(const PGresult *res, int row_number, int column_number); This function returns 1 if the field is null and 0 if it contains a non-null value. (Note that PQgetvalue will return an empty string, not a null pointer, for a null field.) PQgetlengthPQgetlength Returns the actual length of a field value in bytes. Row and column numbers start at 0. int PQgetlength(const PGresult *res, int row_number, int column_number); This is the actual data length for the particular data value, that is, the size of the object pointed to by PQgetvalue. For text data format this is the same as strlen(). For binary format this is essential information. Note that one should not rely on PQfsize to obtain the actual data length. PQprintPQprint Prints out all the rows and, optionally, the column names to the specified output stream. void PQprint(FILE* fout, /* output stream */ const PGresult *res, const PQprintOpt *po); typedef struct { pqbool header; /* print output field headings and row count */ pqbool align; /* fill align the fields */ pqbool standard; /* old brain dead format */ pqbool html3; /* output HTML tables */ pqbool expanded; /* expand tables */ pqbool pager; /* use pager for output if needed */ char *fieldSep; /* field separator */ char *tableOpt; /* attributes for HTML table element */ char *caption; /* HTML table caption */ char **fieldName; /* null-terminated array of replacement field names */ } PQprintOpt; This function was formerly used by psql to print query results, but this is no longer the case. Note that it assumes all the data is in text format. These functions are used to extract information from PGresult objects that are not SELECT results. PQcmdStatusPQcmdStatus Returns the command status tag from the SQL command that generated the PGresult. char * PQcmdStatus(PGresult *res); Commonly this is just the name of the command, but it may include additional data such as the number of rows processed. PQcmdTuplesPQcmdTuples Returns the number of rows affected by the SQL command. char * PQcmdTuples(PGresult *res); If the SQL command that generated the PGresult was INSERT, UPDATE, DELETE, MOVE, or FETCH, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string. PQoidValuePQoidValue Returns the OIDOIDin libpq of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs. Otherwise, returns InvalidOid. Oid PQoidValue(const PGresult *res); PQoidStatusPQoidStatus Returns a string with the OID of the inserted row, if the SQL command was an INSERT. (The string will be 0 if the INSERT did not insert exactly one row, or if the target table does not have OIDs.) If the command was not an INSERT, returns an empty string. char * PQoidStatus(const PGresult *res); This function is deprecated in favor of PQoidValue. It is not thread-safe. PQescapeString escaping strings PQescapeString escapes a string for use within an SQL commmand. This is useful when inserting data values as literal constants in SQL commands. Certain characters (such as quotes and backslashes) must be escaped to prevent them from being interpreted specially by the SQL parser. PQescapeString performs this operation. It is especially important to do proper escaping when handling strings that were received from an untrustworthy source. Otherwise there is a security risk: you are vulnerable to SQL injection attacks wherein unwanted SQL commands are fed to your database. Note that it is not necessary nor correct to do escaping when a data value is passed as a separate parameter in PQexecParams or its sibling routines. size_t PQescapeString (char *to, const char *from, size_t length); The parameter from points to the first character of the string that is to be escaped, and the length parameter gives the number of characters in this string. (A terminating zero byte is neither necessary nor counted.) to shall point to a buffer that is able to hold at least one more character than twice the value of length, otherwise the behavior is undefined. A call to PQescapeString writes an escaped version of the from string to the to buffer, replacing special characters so that they cannot cause any harm, and adding a terminating zero byte. The single quotes that must surround PostgreSQL string literals are not included in the result string; they should be provided in the SQL command that the result is inserted into. PQescapeString returns the number of characters written to to, not including the terminating zero byte. Behavior is undefined if the to and from strings overlap. bytea in libpq PQescapeByteaPQescapeBytea Escapes binary data for use within an SQL command with the type bytea. As with PQescapeString, this is only used when inserting data directly into an SQL command string. unsigned char *PQescapeBytea(const unsigned char *from, size_t from_length, size_t *to_length); Certain byte values must be escaped (but all byte values may be escaped) when used as part of a bytea literal in an SQL 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 (') and backslash (\) characters have special alternative escape sequences. See for more information. PQescapeBytea performs this operation, escaping only the minimally required bytes. The from parameter points to the first byte of the string that is to be escaped, and the from_length parameter gives the number of bytes in this binary string. (A terminating zero byte is neither necessary nor counted.) The to_length parameter points to a variable that will hold the resultant escaped string length. The result string length includes the terminating zero byte of the result. PQescapeBytea returns an escaped version of the from parameter binary string in memory allocated with malloc(). This memory must be freed using PQfreemem() when the result is no longer needed. The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser, and the bytea 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. PQunescapeByteaPQunescapeBytea Converts an escaped string representation of binary data into binary data --- the reverse of PQescapeBytea. This is needed when retrieving bytea data in text format, but not when retrieving it in binary format. unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length); The from parameter points to an escaped string such as might be returned by PQgetvalue when applied to a bytea column. PQunescapeBytea converts this string representation into its binary representation. It returns a pointer to a buffer allocated with malloc(), or null on error, and puts the size of the buffer in to_length. The result must be freed using PQfreemem() when it is no longer needed. PQfreememPQfreemem Frees memory allocated by libpq. void PQfreemem(void *ptr); Frees memory allocated by libpq, particularly PQescapeBytea, PQunescapeBytea, and PQnotifies. It is needed by Win32, which can not free memory across DLL's, unless multithreaded DLL's (/MD in VC6) are used. On other platforms it is the same as free(). nonblocking connection The PQexec function is adequate for submitting commands in normal, synchronous applications. It has a couple of deficiencies, however, that can be of importance to some users: PQexec waits for the command to be completed. The application may have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response. 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.) PQexec can return only one PGresult structure. If the submitted command string contains multiple SQL commands, all but the last PGresult are discarded by PQexec. Applications that do not like these limitations can instead use the underlying functions that PQexec is built from: PQsendQuery and PQgetResult. There are also PQsendQueryParams and PQsendQueryPrepared, which can be used with PQgetResult to duplicate the functionality of PQexecParams and PQexecPrepared respectively. PQsendQueryPQsendQuery Submits a command to the server without waiting for the result(s). 1 is returned if the command was successfully dispatched and 0 if not (in which case, use PQerrorMessage to get more information about the failure). int PQsendQuery(PGconn *conn, const char *command); After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the results. PQsendQuery may not be called again (on the same connection) until PQgetResult has returned a null pointer, indicating that the command is done. PQsendQueryParamsPQsendQueryParams Submits a command and separate parameters to the server without waiting for the result(s). int PQsendQueryParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); This is equivalent to PQsendQuery except that query parameters can be specified separately from the query string. The function's parameters are handled identically to PQexecParams. Like PQexecParams, it will not work on 2.0-protocol connections, and it allows only one command in the query string. PQsendQueryPreparedPQsendQueryPrepared Sends a request to execute a prepared statement with given parameters, without waiting for the result(s). int PQsendQueryPrepared(PGconn *conn, const char *stmtName, int nParams, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); This is similar to PQsendQueryParams, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. The function's parameters are handled identically to PQexecPrepared. Like PQexecPrepared, it will not work on 2.0-protocol connections. PQgetResultPQgetResult Waits for the next result from a prior PQsendQuery, PQsendQueryParams, or PQsendQueryPrepared call, and returns it. A null pointer is returned when the command is complete and there will be no more results. PGresult *PQgetResult(PGconn *conn); PQgetResult must be called repeatedly until it returns a null pointer, indicating that the command is done. (If called when no command is active, PQgetResult will just return a null pointer at once.) Each non-null result from PQgetResult should be processed using the same PGresult accessor functions previously described. Don't forget to free each result object with PQclear when done with it. Note that PQgetResult will block only if a command is active and the necessary response data has not yet been read by PQconsumeInput. Using PQsendQuery and PQgetResult solves one of PQexec's problems: If a command string contains multiple SQL commands, the results of those commands can be obtained individually. (This allows a simple form of overlapped processing, by the way: the client can be handling the results of one command while the server is still working on later queries in the same command string.) However, calling PQgetResult will still cause the client to block until the server completes the next SQL command. This can be avoided by proper use of two more functions: PQconsumeInputPQconsumeInput If input is available from the server, consume it. int PQconsumeInput(PGconn *conn); PQconsumeInput normally returns 1 indicating no error, but returns 0 if there was some kind of trouble (in which case PQerrorMessage can be consulted). Note that the result does not say whether any input data was actually collected. After calling PQconsumeInput, the application may check PQisBusy and/or PQnotifies to see if their state has changed. PQconsumeInput may be called even if the application is not prepared to deal with a result or notification just yet. The function will read available data and save it in a buffer, thereby causing a select() read-ready indication to go away. The application can thus use PQconsumeInput to clear the select() condition immediately, and then examine the results at leisure. PQisBusyPQisBusy Returns 1 if a command is busy, that is, PQgetResult would block waiting for input. A 0 return indicates that PQgetResult can be called with assurance of not blocking. int PQisBusy(PGconn *conn); PQisBusy will not itself attempt to read data from the server; therefore PQconsumeInput must be invoked first, or the busy state will never end. A typical application using these functions will have a main loop that uses select() or poll() 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 select() means readable data on the file descriptor identified by PQsocket. When the main loop detects input ready, it should call PQconsumeInput to read the input. It can then call PQisBusy, followed by PQgetResult if PQisBusy returns false (0). It can also call PQnotifies to detect NOTIFY messages (see ). A client that uses PQsendQuery/PQgetResult can also attempt to cancel a command that is still being processed by the server.cancelingSQL command PQrequestCancelPQrequestCancel Requests that the server abandon processing of the current command. int PQrequestCancel(PGconn *conn); The return value is 1 if the cancel request was successfully dispatched and 0 if not. (If not, PQerrorMessage tells why not.) Successful dispatch is no guarantee that the request will have any effect, however. Regardless of the return value of PQrequestCancel, the application must continue with the normal result-reading sequence using PQgetResult. If the cancellation is effective, the current command will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all. Note that if the current command is part of a transaction block, cancellation will abort the whole transaction. PQrequestCancel can safely be invoked from a signal handler. So, it is also possible to use it in conjunction with plain PQexec, if the decision to cancel can be made in a signal handler. For example, psql invokes PQrequestCancel from a SIGINT signal handler, thus allowing interactive cancellation of commands that it issues through PQexec. By using the functions described above, it is possible to avoid blocking while waiting for input from the database server. However, it is still possible that the application will block waiting to send output to the server. This is relatively uncommon but can happen if very long SQL commands or data values are sent. (It is much more probable if the application sends data via COPY IN, however.) To prevent this possibility and achieve completely nonblocking database operation, the following additional functions may be used. PQsetnonblockingPQsetnonblocking Sets the nonblocking status of the connection. int PQsetnonblocking(PGconn *conn, int arg); Sets the state of the connection to nonblocking if arg is 1, or blocking if arg is 0. Returns 0 if OK, -1 if error. In the nonblocking state, calls to PQsendQuery, PQputline, PQputnbytes, and PQendcopy will not block but instead return an error if they need to be called again. Note that PQexec does not honor nonblocking mode; if it is called, it will act in blocking fashion anyway. PQisnonblockingPQisnonblocking Returns the blocking status of the database connection. int PQisnonblocking(const PGconn *conn); Returns 1 if the connection is set to nonblocking mode and 0 if blocking. PQflushPQflush Attempts to flush any queued output data to the server. Returns 0 if successful (or if the send queue is empty), -1 if it failed for some reason, or 1 if it was unable to send all the data in the send queue yet (this case can only occur if the connection is nonblocking). int PQflush(PGconn *conn); After sending any command or data on a nonblocking connection, call PQflush. If it returns 1, wait for the socket to be write-ready and call it again; repeat until it returns 0. Once PQflush returns 0, wait for the socket to be read-ready and then read the response as described above. fast path PostgreSQL provides a fast-path interface to send simple function calls to the server. This interface is somewhat obsolete, as one may achieve similar performance and greater functionality by setting up a prepared statement to define the function call. Then, executing the statement with binary transmission of parameters and results substitutes for a fast-path function call. The function PQfnPQfn requests execution of a server function via the fast-path interface: PGresult* PQfn(PGconn* conn, int fnid, int *result_buf, int *result_len, int result_is_int, const PQArgBlock *args, int nargs); typedef struct { int len; int isint; union { int *ptr; int integer; } u; } PQArgBlock; The fnid argument is the OID of the function to be executed. args and nargs define the parameters to be passed to the function; they must match the declared function argument list. When the isint field of a parameter struct is true, the u.integer value is sent to the server as an integer of the indicated length (this must be 1, 2, or 4 bytes); proper byte-swapping occurs. When isint is false, the indicated number of bytes at *u.ptr are sent with no processing; the data must be in the format expected by the server for binary transmission of the function's argument datatype. result_buf 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 result_len. If a 1, 2, or 4-byte integer result is expected, set result_is_int to 1, otherwise set it to 0. Setting result_is_int to 1 causes libpq to byte-swap the value if necessary, so that it is delivered as a proper int value for the client machine. When result_is_int is 0, the binary-format byte string sent by the server is returned unmodified. PQfn always returns a valid PGresult pointer. The result status should be checked before the result is used. The caller is responsible for freeing the PGresult with PQclear when it is no longer needed. Note that it is not possible to handle NULL arguments, NULL results, nor set-valued results when using this interface. NOTIFY in libpq PostgreSQL offers asynchronous notification via the LISTEN and NOTIFY commands. A client session registers its interest in a particular notification condition with the LISTEN command (and can stop listening with the UNLISTEN command). All sessions listening on a particular condition will be notified asynchronously when a NOTIFY command with that condition name is executed by any session. No additional information is passed from the notifier to the listener. Thus, typically, any actual data 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. libpq applications submit LISTEN and UNLISTEN commands as ordinary SQL commands. The arrival of NOTIFY messages can subsequently be detected by calling PQnotifies.PQnotifies The function PQnotifies returns the next notification from a list of unhandled notification messages received from the server. It returns a null pointer if there are no pending notifications. Once a notification is returned from PQnotifies, it is considered handled and will be removed from the list of notifications. PGnotify* PQnotifies(PGconn *conn); typedef struct pgNotify { char *relname; /* notification condition name */ int be_pid; /* process ID of server process */ char *extra; /* notification parameter */ } PGnotify; After processing a PGnotify object returned by PQnotifies, be sure to free it with PQfreemem. It is sufficient to free the PGnotify pointer; the relname and extra fields do not represent separate allocations. At present the extra field is unused and will always point to an empty string. In PostgreSQL 6.4 and later, the be_pid is that of the notifying backend process, whereas in earlier versions it was always the PID of your own backend process. gives a sample program that illustrates the use of asynchronous notification. PQnotifies() does not actually read data from the server; it just returns messages previously absorbed by another libpq function. In prior releases of libpq, the only way to ensure timely receipt of NOTIFY messages was to constantly submit commands, even empty ones, and then check PQnotifies() after each PQexec(). While this still works, it is deprecated as a waste of processing power. A better way to check for NOTIFY messages when you have no useful commands to execute is to call PQconsumeInput(), then check PQnotifies(). You can use select() to wait for data to arrive from the server, thereby using no CPU power unless there is something to do. (See PQsocket() to obtain the file descriptor number to use with select().) Note that this will work OK whether you submit commands with PQsendQuery/PQgetResult or simply use PQexec. You should, however, remember to check PQnotifies() after each PQgetResult or PQexec, to see if any notifications came in during the processing of the command. COPY with libpq The COPY command in PostgreSQL has options to read from or write to the network connection used by libpq. The functions described in this section allow applications to take advantage of this capability by supplying or consuming copied data. The overall process is that the application first issues the SQL COPY command via PQexec or one of the equivalent functions. The response to this (if there is no error in the command) will be a PGresult object bearing a status code of PGRES_COPY_OUT or PGRES_COPY_IN (depending on the specified copy direction). The application should then use the functions of this section to receive or transmit data rows. When the data transfer is complete, another PGresult object is returned to indicate success or failure of the transfer. Its status will be PGRES_COMMAND_OK for success or PGRES_FATAL_ERROR if some problem was encountered. At this point further SQL commands may be issued via PQexec. (It is not possible to execute other SQL commands using the same connection while the COPY operation is in progress.) If a COPY command is issued via PQexec in a string that could contain additional commands, the application must continue fetching results via PQgetResult after completing the COPY sequence. Only when PQgetResult returns NULL is it certain that the PQexec command string is done and it is safe to issue more commands. The functions of this section should be executed only after obtaining a result status of PGRES_COPY_OUT or PGRES_COPY_IN from PQexec or PQgetResult. A PGresult object bearing one of these status values carries some additional data about the COPY operation that is starting. This additional data is available using functions that are also used in connection with query results: PQnfieldsPQnfieldswith COPY Returns the number of columns (fields) to be copied. PQbinaryTuplesPQbinaryTupleswith COPY 0 indicates the overall copy format is textual (rows separated by newlines, columns separated by separator characters, etc). 1 indicates the overall copy format is binary. See for more information. PQfformatPQfformatwith COPY Returns the format code (0 for text, 1 for binary) associated with each column of the copy operation. The per-column format codes will always be zero when the overall copy format is textual, but the binary format can support both text and binary columns. (However, as of the current implementation of COPY, only binary columns appear in a binary copy; so the per-column formats always match the overall format at present.) These additional data values are only available when using protocol 3.0. When using protocol 2.0, all these functions will return 0. These functions are used to send data during COPY FROM STDIN. They will fail if called when the connection is not in COPY_IN state. PQputCopyDataPQputCopyData Sends data to the server during COPY_IN state. int PQputCopyData(PGconn *conn, const char *buffer, int nbytes); Transmits the COPY data in the specified buffer, of length nbytes, to the server. The result is 1 if the data was sent, zero if it was not sent because the attempt would block (this case is only possible if the connection is in nonblock mode), or -1 if an error occurred. (Use PQerrorMessage to retrieve details if the return value is -1. If the value is zero, wait for write-ready and try again.) The application may divide the COPY datastream into bufferloads of any convenient size. Bufferload boundaries have no semantic significance when sending. The contents of the datastream must match the data format expected by the COPY command; see for details. PQputCopyEndPQputCopyEnd Sends end-of-data indication to the server during COPY_IN state. int PQputCopyEnd(PGconn *conn, const char *errormsg); Ends the COPY_IN operation successfully if errormsg is NULL. If errormsg is not NULL then the COPY is forced to fail, with the string pointed to by errormsg used as the error message. (One should not assume that this exact error message will come back from the server, however, as the server might have already failed the COPY for its own reasons. Also note that the option to force failure does not work when using pre-3.0-protocol connections.) The result is 1 if the termination data was sent, zero if it was not sent because the attempt would block (this case is only possible if the connection is in nonblock mode), or -1 if an error occurred. (Use PQerrorMessage to retrieve details if the return value is -1. If the value is zero, wait for write-ready and try again.) After successfully calling PQputCopyEnd, call PQgetResult to obtain the final result status of the COPY command. One may wait for this result to be available in the usual way. Then return to normal operation. These functions are used to receive data during COPY TO STDOUT. They will fail if called when the connection is not in COPY_OUT state. PQgetCopyDataPQgetCopyData Receives data from the server during COPY_OUT state. int PQgetCopyData(PGconn *conn, char **buffer, int async); Attempts to obtain another row of data from the server during a COPY. Data is always returned one data row at a time; if only a partial row is available, it is not returned. Successful return of a data row involves allocating a chunk of memory to hold the data. The buffer parameter must be non-NULL. *buffer is set to point to the allocated memory, or to NULL in cases where no buffer is returned. A non-NULL result buffer must be freed using PQfreemem when no longer needed. When a row is successfully returned, the return value is the number of data bytes in the row (this will always be greater than zero). The returned string is always null-terminated, though this is probably only useful for textual COPY. A result of zero indicates that the COPY is still in progress, but no row is yet available (this is only possible when async is true). A result of -1 indicates that the COPY is done. A result of -2 indicates that an error occurred (consult PQerrorMessage for the reason). When async is true (not zero), PQgetCopyData will not block waiting for input; it will return zero if the COPY is still in progress but no complete row is available. (In this case wait for read-ready before trying again; it does not matter whether you call PQconsumeInput.) When async is false (zero), PQgetCopyData will block until data is available or the operation completes. After PQgetCopyData returns -1, call PQgetResult to obtain the final result status of the COPY command. One may wait for this result to be available in the usual way. Then return to normal operation. These functions represent older methods of handling COPY. Although they still work, they are deprecated due to poor error handling, inconvenient methods of detecting end-of-data, and lack of support for binary or nonblocking transfers. PQgetlinePQgetline Reads a newline-terminated line of characters (transmitted by the server) into a buffer string of size length. int PQgetline(PGconn *conn, char *buffer, int length); This function copies up to length-1 characters into the buffer and converts the terminating newline into a zero byte. PQgetline returns EOF at the end of input, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read. Note that the application must check to see if a new line consists of the two characters \., which indicates that the server has finished sending the results of the COPY command. If the application might receive lines that are more than length-1 characters long, care is needed to be sure it recognizes the \. line correctly (and does not, for example, mistake the end of a long data line for a terminator line). PQgetlineAsyncPQgetlineAsync Reads a row of COPY data (transmitted by the server) into a buffer without blocking. int PQgetlineAsync(PGconn *conn, char *buffer, int bufsize); This function is similar to PQgetline, but it can be used by applications that must read COPY data asynchronously, that is, without blocking. Having issued the COPY command and gotten a PGRES_COPY_OUT response, the application should call PQconsumeInput and PQgetlineAsync until the end-of-data signal is detected. Unlike PQgetline, this function takes responsibility for detecting end-of-data. On each call, PQgetlineAsync will return data if a complete data row is available in libpq's input buffer. Otherwise, no data is returned until the rest of the row arrives. The function returns -1 if the end-of-copy-data marker has been recognized, 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 PQendcopy, and then return to normal processing. The data returned will not extend beyond a data-row boundary. If possible a whole row will be returned at one time. But if the buffer offered by the caller is too small to hold a row sent by the server, then a partial data row will be returned. With textual data this can be detected by testing whether the last returned byte is \n or not. (In a binary COPY, actual parsing of the COPY data format will be needed to make the equivalent determination.) The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a bufsize one smaller than the room actually available.) PQputlinePQputline Sends a null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string. int PQputline(PGconn *conn, const char *string); The COPY datastream sent by a series of calls to PQputline has the same format as that returned by PQgetlineAsync, except that applications are not obliged to send exactly one data row per PQputline call; it is okay to send a partial line or multiple lines per call. Before PostgreSQL protocol 3.0, it was necessary for the application to explicitly send the two characters \. as a final line to indicate to the server that it had finished sending COPY data. While this still works, it is deprecated and the special meaning of \. can be expected to be removed in a future release. It is sufficient to call PQendcopy after having sent the actual data. PQputnbytesPQputnbytes Sends a non-null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string. int PQputnbytes(PGconn *conn, const char *buffer, int nbytes); This is exactly like PQputline, except that the data buffer need not be null-terminated since the number of bytes to send is specified directly. Use this procedure when sending binary data. PQendcopyPQendcopy Synchronizes with the server. int PQendcopy(PGconn *conn); This function waits until the server has finished the copying. It should either be issued when the last string has been sent to the server using PQputline or when the last string has been received from the server using PGgetline. It must be issued or the server will get out of sync with the client. Upon return from this function, the server is ready to receive the next SQL command. The return value is 0 on successful completion, nonzero otherwise. (Use PQerrorMessage to retrieve details if the return value is nonzero.) When using PQgetResult, the application should respond to a PGRES_COPY_OUT result by executing PQgetline repeatedly, followed by PQendcopy after the terminator line is seen. It should then return to the PQgetResult loop until PQgetResult returns a null pointer. Similarly a PGRES_COPY_IN result is processed by a series of PQputline calls followed by PQendcopy, then return to the PQgetResult loop. This arrangement will ensure that a COPY command embedded in a series of SQL commands will be executed correctly. Older applications are likely to submit a COPY via PQexec and assume that the transaction is done after PQendcopy. This will work correctly only if the COPY is the only SQL command in the command string. These functions control miscellaneous details of libpq's behavior. PQsetErrorVerbosityPQsetErrorVerbosity Determines the verbosity of messages returned by PQerrorMessage and PQresultErrorMessage. typedef enum { PQERRORS_TERSE, PQERRORS_DEFAULT, PQERRORS_VERBOSE } PGVerbosity; PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity); PQsetErrorVerbosity sets the verbosity mode, returning the connection's previous setting. In TERSE mode, returned messages include severity, primary text, and position only; this will normally fit on a single line. The DEFAULT mode produces messages that include the above plus any detail, hint, or context fields (these may span multiple lines). The VERBOSE mode includes all available fields. Changing the verbosity does not affect the messages available from already-existing PGresult objects, only subsequently-created ones. PQtracePQtrace Enables tracing of the client/server communication to a debugging file stream. void PQtrace(PGconn *conn, FILE *stream); PQuntracePQuntrace Disables tracing started by PQtrace. void PQuntrace(PGconn *conn); notice processing in libpq Notice and warning messages generated by the server are not returned by the query execution functions, since they do not imply failure of the query. Instead they are passed to a notice handling function, and execution continues normally after the handler returns. The default notice handling function prints the message on stderr, but the application can override this behavior by supplying its own handling function. For historical reasons, there are two levels of notice handling, called the notice receiver and notice processor. The default behavior is for the notice receiver to format the notice and pass a string to the notice processor for printing. However, an application that chooses to provide its own notice receiver will typically ignore the notice processor layer and just do all the work in the notice receiver. The function PQsetNoticeReceiver notice receiverPQsetNoticeReceiver sets or examines the current notice receiver for a connection object. Similarly, PQsetNoticeProcessor notice processorPQsetNoticeProcessor sets or examines the current notice processor. typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res); PQnoticeReceiver PQsetNoticeReceiver(PGconn *conn, PQnoticeReceiver proc, void *arg); typedef void (*PQnoticeProcessor) (void *arg, const char *message); PQnoticeProcessor PQsetNoticeProcessor(PGconn *conn, PQnoticeProcessor proc, void *arg); Each of these functions returns the previous notice receiver or processor function pointer, and sets the new value. If you supply a null function pointer, no action is taken, but the current pointer is returned. When a notice or warning message is received from the server, or generated internally by libpq, the notice receiver function is called. It is passed the message in the form of a PGRES_NONFATAL_ERROR PGresult. (This allows the receiver to extract individual fields using PQresultErrorField, or the complete preformatted message using PQresultErrorMessage.) The same void pointer passed to PQsetNoticeReceiver is also passed. (This pointer can be used to access application-specific state if needed.) The default notice receiver simply extracts the message (using PQresultErrorMessage) and passes it to the notice processor. The notice processor is responsible for handling a notice or warning message given in text form. It is passed the string text of the message (including a trailing newline), plus a void pointer that is the same one passed to PQsetNoticeProcessor. (This pointer can be used to access application-specific state if needed.) The default notice processor is simply static void defaultNoticeProcessor(void * arg, const char * message) { fprintf(stderr, "%s", message); } Once you have set a notice receiver or processor, you should expect that that function could be called as long as either the PGconn object or PGresult objects made from it exist. At creation of a PGresult, the PGconn's current notice handling pointers are copied into the PGresult for possible use by functions like PQgetvalue. environment variable The following environment variables can be used to select default connection parameter values, which will be used by PQconnectdb, PQsetdbLogin and PQsetdb if no value is directly specified by the calling code. These are useful to avoid hard-coding database connection information into simple client applications, for example. PGHOST PGHOST sets the database server name. 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 (default /tmp). PGHOSTADDR PGHOSTADDR specifies the numeric IP address of the database server. This can be set instead of PGHOST to avoid DNS lookup overhead. See the documentation of these parameters, under PQconnectdb above, for details on their interaction. PGPORT PGPORT sets the TCP port number or Unix-domain socket file extension for communicating with the PostgreSQL server. PGDATABASE PGDATABASE sets the PostgreSQL database name. PGUSER PGUSER sets the user name used to connect to the database. PGPASSWORD PGPASSWORD sets the password used if the server demands password authentication. This environment variable is deprecated for security reasons; consider migrating to use the $HOME/.pgpass file (see ). PGSERVICE PGSERVICE sets the service name to be looked up in pg_service.conf. This offers a shorthand way of setting all the parameters. PGREALM PGREALM sets the Kerberos realm to use with PostgreSQL, if it is different from the local realm. If PGREALM is set, libpq 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 used if Kerberos authentication is selected by the server. PGOPTIONS PGOPTIONS sets additional run-time options for the PostgreSQL server. PGSSLMODE PGSSLMODE determines whether and with what priority an SSL connection will be negotiated with the server. There are four modes: disable will attempt only an unencrypted SSL connection; allow will negotiate, trying first a non-SSL connection, then if that fails, trying an SSL connection; prefer (the default) will negotiate, trying first an SSL connection, then if that fails, trying a regular non-SSL connection; require will try only an SSL connection. If PostgreSQL is compiled without SSL support, using option require will cause an error, and options allow and prefer will be tolerated but libpq will be unable to negotiate an SSL connection. PGREQUIRESSL PGREQUIRESSL sets whether or not the connection must be made over SSL. If set to 1, libpq will refuse to connect if the server does not accept an SSL connection (equivalent to sslmode prefer). This option is deprecated in favor of the sslmode setting, and is only available if PostgreSQL is compiled with SSL support. PGCONNECT_TIMEOUT PGCONNECT_TIMEOUT sets the maximum number of seconds that libpq will wait when attempting to connect to the PostgreSQL server. If unset or set to zero, libpq will wait indefinitely. It is not recommended to set the timeout to less than 2 seconds. The following environment variables can be used to specify default behavior for each PostgreSQL session. (See also the ALTER USER and ALTER DATABASE commands for ways to set default behavior on a per-user or per-database basis.) PGDATESTYLE PGDATESTYLE sets the default style of date/time representation. (Equivalent to SET datestyle TO ....) PGTZ PGTZ sets the default time zone. (Equivalent to SET timezone TO ....) PGCLIENTENCODING PGCLIENTENCODING sets the default client character set encoding. (Equivalent to SET client_encoding TO ....) PGGEQO PGGEQO sets the default mode for the genetic query optimizer. (Equivalent to SET geqo TO ....) Refer to the SQL command SET for information on correct values for these environment variables. password file .pgpass The file .pgpass 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: hostname:port:database:username:password Each of the first four fields may be a literal value, or *, which matches anything. The password field from the first line that matches the current connection parameters will be used. (Therefore, put more-specific entries first when you are using wildcards.) If an entry needs to contain : or \, escape this character with \. The permissions on .pgpass must disallow any access to world or group; achieve this by the command chmod 0600 ~/.pgpass. If the permissions are less strict than this, the file will be ignored. threads with libpq libpq is thread-safe if the library is compiled using configure's --enable-thread-safety command-line option. (In addition, you might need to use other threading command-line options to compile your client code.) One restriction is that no two threads attempt to manipulate the same PGconn object at the same time. In particular, you cannot issue concurrent commands from different threads through the same connection object. (If you need to run concurrent commands, start up multiple connections.) PGresult objects are read-only after creation, and so can be passed around freely between threads. The deprecated functions PQoidStatus and fe_setauthsvc are not thread-safe and should not be used in multithread programs. PQoidStatus can be replaced by PQoidValue. There is no good reason to call fe_setauthsvc at all. libpq applications that use the crypt authentication method rely on the crypt() operating system function, which is often not thread-safe.cryptthread safety It is better to use the md5 method, which is thread-safe on all platforms. compiling libpq applications To build (i.e., compile and link) your libpq programs you need to do all of the following things: Include the libpq-fe.h header file: #include <libpq-fe.h> If you failed to do that then you will normally get error messages from your compiler similar to 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) Point your compiler to the directory where the PostgreSQL header files were installed, by supplying the -Idirectory 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: cc -c -I/usr/local/pgsql/include testprog.c If you are using makefiles then add the option to the CPPFLAGS variable: CPPFLAGS += -I/usr/local/pgsql/include 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 pg_configpg_configwith libpq to find out where the header files are on the local system: $ pg_config --includedir /usr/local/include Failure to specify the correct option to the compiler will result in an error message such as testlibpq.c:8:22: libpq-fe.h: No such file or directory When linking the final program, specify the option -lpq so that the libpq library gets pulled in, as well as the option -Ldirectory to point the compiler to the directory where the libpq library resides. (Again, the compiler will search some directories by default.) For maximum portability, put the -L option before the -lpq option. For example: cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq You can find out the library directory using pg_config as well: $ pg_config --libdir /usr/local/pgsql/lib Error messages that point to problems in this area could look like the following. 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' This means you forgot -lpq. /usr/bin/ld: cannot find -lpq This means you forgot the -L option or did not specify the right directory. libpq-int.h If your codes references the header file libpq-int.h and you refuse to fix your code to not use it, starting in PostgreSQL 7.2, this file will be found in includedir/postgresql/internal/libpq-int.h, so you need to add the appropriate -I option to your compiler command line. These examples and others can be found in the directory src/test/examples in the source code distribution. /* * testlibpq.c * * Test the C version of LIBPQ, the POSTGRES frontend library. */ #include <stdio.h> #include <stdlib.h> #include "libpq-fe.h" static void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } int main(int argc, char **argv) { const char *conninfo; PGconn *conn; PGresult *res; int nFields; int i, j; /* * If the user supplies a parameter on the command line, use it as * the conninfo string; otherwise default to setting dbname=template1 * and using environment variables or defaults for all other connection * parameters. */ if (argc > 1) conninfo = argv[1]; else conninfo = "dbname = template1"; /* Make a connection to the database */ conn = PQconnectdb(conninfo); /* Check to see that the backend connection was successfully made */ if (PQstatus(conn) != CONNECTION_OK) { fprintf(stderr, "Connection to database '%s' failed.\n", PQdb(conn)); fprintf(stderr, "%s", PQerrorMessage(conn)); exit_nicely(conn); } /* * Our test case here involves using a cursor, for which we must be * inside a transaction block. We could do the whole thing with a * single PQexec() of "select * from pg_database", but that's too * trivial to make a good example. */ /* Start a transaction block */ res = PQexec(conn, "BEGIN"); if (PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } /* * Should PQclear PGresult whenever it is no longer needed to avoid * memory leaks */ PQclear(res); /* * Fetch rows from pg_database, the system catalog of databases */ res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database"); if (PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } PQclear(res); res = PQexec(conn, "FETCH ALL in myportal"); if (PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } /* first, print out the attribute names */ nFields = PQnfields(res); for (i = 0; i < nFields; i++) printf("%-15s", PQfname(res, i)); printf("\n\n"); /* next, print out the rows */ for (i = 0; i < PQntuples(res); i++) { for (j = 0; j < nFields; j++) printf("%-15s", PQgetvalue(res, i, j)); printf("\n"); } PQclear(res); /* close the portal ... we don't bother to check for errors ... */ res = PQexec(conn, "CLOSE myportal"); PQclear(res); /* end the transaction */ res = PQexec(conn, "END"); PQclear(res); /* close the connection to the database and cleanup */ PQfinish(conn); return 0; } /* * testlibpq2.c * Test of the asynchronous notification interface * * Start this program, then from psql in another window do * NOTIFY TBL2; * Repeat four times to get this program to exit. * * Or, if you want to get fancy, try this: * populate a database with the following commands * (provided in src/test/examples/testlibpq2.sql): * * CREATE TABLE TBL1 (i int4); * * CREATE TABLE TBL2 (i int4); * * CREATE RULE r1 AS ON INSERT TO TBL1 DO * (INSERT INTO TBL2 VALUES (new.i); NOTIFY TBL2); * * and do this four times: * * INSERT INTO TBL1 VALUES (10); */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h> #include <sys/time.h> #include "libpq-fe.h" static void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } int main(int argc, char **argv) { const char *conninfo; PGconn *conn; PGresult *res; PGnotify *notify; int nnotifies; /* * If the user supplies a parameter on the command line, use it as * the conninfo string; otherwise default to setting dbname=template1 * and using environment variables or defaults for all other connection * parameters. */ if (argc > 1) conninfo = argv[1]; else conninfo = "dbname = template1"; /* Make a connection to the database */ conn = PQconnectdb(conninfo); /* Check to see that the backend connection was successfully made */ if (PQstatus(conn) != CONNECTION_OK) { fprintf(stderr, "Connection to database '%s' failed.\n", PQdb(conn)); fprintf(stderr, "%s", PQerrorMessage(conn)); exit_nicely(conn); } /* * Issue LISTEN command to enable notifications from the rule's NOTIFY. */ res = PQexec(conn, "LISTEN TBL2"); if (PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr, "LISTEN command failed: %s", PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } /* * should PQclear PGresult whenever it is no longer needed to avoid * memory leaks */ PQclear(res); /* Quit after four notifies are received. */ nnotifies = 0; while (nnotifies < 4) { /* * Sleep until something happens on the connection. We use select(2) * to wait for input, but you could also use poll() or similar * facilities. */ int sock; fd_set input_mask; sock = PQsocket(conn); if (sock < 0) break; /* shouldn't happen */ FD_ZERO(&input_mask); FD_SET(sock, &input_mask); if (select(sock + 1, &input_mask, NULL, NULL, NULL) < 0) { fprintf(stderr, "select() failed: %s\n", strerror(errno)); exit_nicely(conn); } /* Now check for input */ PQconsumeInput(conn); while ((notify = PQnotifies(conn)) != NULL) { fprintf(stderr, "ASYNC NOTIFY of '%s' received from backend pid %d\n", notify->relname, notify->be_pid); PQfreemem(notify); nnotifies++; } } fprintf(stderr, "Done.\n"); /* close the connection to the database and cleanup */ PQfinish(conn); return 0; }