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<TITLE>The POSTGRES95 User Manual - LIBPQ</TITLE>
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<H1>12. <B>LIBPQ</B></H1>
<HR>
<B>LIBPQ</B> is the application programming interface to POSTGRES.
<B>LIBPQ</B> is a set of library routines which allows
client programs to pass queries to the POSTGRES backend
server and to receive the results of these queries.
This version of the documentation describes the <B>C</B>
interface library. Three short programs are included
at the end of this section to show how to write programs that use <B>LIBPQ</B>.
There are several examples of <B>LIBPQ</B> applications in the
following directories:
<pre> ../src/test/regress
../src/test/examples
../src/bin/psql
</pre>
Frontend programs which use <B>LIBPQ</B> must include the
header file <CODE>libpq-fe.h</CODE> and must link with the <B>libpq</B>
library.
<H2><A NAME="control-and-initialization">12.1. Control and Initialization</A></H2>
The following environment variables can be used to set
up default environment values to avoid hard-coding
database names into an application program:
<UL>
<LI><B>PGHOST</B> sets the default server name.
<LI><B>PGOPTIONS</B> sets additional runtime options for the POSTGRES backend.
<LI><B>PGPORT</B> sets the default port for communicating with the POSTGRES backend.
<LI><B>PGTTY</B> sets the file or tty on which debugging messages from the backend server are displayed.
<LI><B>PGDATABASE</B> sets the default POSTGRES database name.
<LI><B>PGREALM</B> sets the Kerberos realm to use with POSTGRES, if it is different from the local realm. If
<LI><B>PGREALM</B> is set, POSTGRES 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 enabled.
</UL>
<H2><A NAME="database-connection-functions">12.2. Database Connection Functions</A></H2>
The following routines deal with making a connection to
a backend from a <B>C</B> program.
<DL>
<DT><B>PQsetdb</B>
<DD>Makes a new connection to a backend.
<pre> PGconn &#42;PQsetdb(char &#42;pghost,
char &#42;pgport,
char &#42;pgoptions,
char &#42;pgtty,
char &#42;dbName);
</pre>
<DD>If any argument is NULL, then the corresponding
environment variable is checked. If the environment variable is also not set, then hardwired
defaults are used.
<DD>PQsetdb always returns a valid PGconn pointer.
<DD>The PQstatus (see below) command should be called
to ensure that a connection was properly made
before queries are sent via the connection. <B>LIBPQ</B>
programmers should be careful to maintain the
<DD>PGconn abstraction. Use the accessor functions
below to get at the contents of PGconn. Avoid
directly referencing the fields of the PGconn
structure as they are subject to change in the
future.<br>
<DT><B>PQdb</B>
<DD>Returns the database name of the connection.
<pre> char &#42;PQdb(PGconn &#42;conn)
</pre><br>
<DT><B>PQhost</B>
<DD>Returns the host name of the connection.
<pre> char &#42;PQhost(PGconn &#42;conn)
</pre><br>
<DT><B>PQoptions</B>
<DD>Returns the pgoptions used in the connection.
<pre> char &#42;PQoptions(PGconn &#42;conn)
</pre><br>
<DT><B>PQport</B>
<DD>Returns the pgport of the connection.
<pre> char &#42;PQport(PGconn &#42;conn)
</pre><br>
<DT><B>PQtty</B>
<DD>Returns the pgtty of the connection.
<pre> char &#42;PQtty(PGconn &#42;conn)
</pre><br>
<DT><B>PQstatus</B>
<DD>Returns the status of the connection.
<DD>The status can be CONNECTION_OK or CONNECTION_BAD.
<pre> ConnStatusType &#42;PQstatus(PGconn &#42;conn)
</pre><br>
<DT><B>PQerrorMessage</B>
<DD>Returns the error message associated with the connection
<pre> char &#42;PQerrorMessage(PGconn&#42; conn);
</pre><br>
<DT><B>PQfinish</B>
<DD>Close the connection to the backend. Also frees
memory used by the PGconn structure. The PGconn
pointer should not be used after PQfinish has been
called.
<pre> void PQfinish(PGconn &#42;conn)
</pre><br>
<DT><B>PQreset</B>
<DD>Reset the communication port with the backend.
This function will close the IPC socket connection
to the backend and attempt to reestablish a new
connection to the same backend.
<pre> void PQreset(PGconn &#42;conn)
</pre><br>
<DT><B>PQtrace</B>
<DD>Enables tracing of messages passed between the
frontend and the backend. The messages are echoed
to the debug_port file stream.
<pre> void PQtrace(PGconn &#42;conn,
FILE&#42; debug_port);
</pre><br>
<DT><B>PQuntrace</B>
<DD>Disables tracing of messages passed between the
frontend and the backend.
<pre> void PQuntrace(PGconn &#42;conn);
</pre><br>
</DL>
<H2><A NAME="query-execution-functions">12.3. Query Execution Functions</A></H2>
<DL>
<DT><B>PQexec</B>
<DD>Submit a query to POSTGRES. Returns a PGresult
pointer if the query was successful or a NULL otherwise. If a NULL is returned, PQerrorMessage can
be used to get more information about the error.
<pre> PGresult &#42;PQexec(PGconn &#42;conn,
char &#42;query);
</pre>
<DD>The <B>PGresult</B> structure encapsulates the query
result returned by the backend. <B>LIBPQ</B> programmers
should be careful to maintain the PGresult
abstraction. Use the accessor functions described
below to retrieve the results of the query. Avoid
directly referencing the fields of the PGresult
structure as they are subject to change in the
future.<br>
<DT><B>PQresultStatus</B>
<DD>Returns the result status of the query. PQresultStatus can return one of the following values:
<pre> PGRES_EMPTY_QUERY,
PGRES_COMMAND_OK, /&#42; the query was a command &#42;/
PGRES_TUPLES_OK, /&#42; the query successfully returned tuples &#42;/
PGRES_COPY_OUT,
PGRES_COPY_IN,
PGRES_BAD_RESPONSE, /&#42; an unexpected response was received &#42;/
PGRES_NONFATAL_ERROR,
PGRES_FATAL_ERROR
</pre>
<DD>If the result status is PGRES_TUPLES_OK, then the
following routines can be used to retrieve the
tuples returned by the query.<br>
<DT><B>PQntuples</B> returns the number of tuples (instances)
in the query result.
<pre> int PQntuples(PGresult &#42;res);
</pre><br>
<DT><B>PQnfields</B>
<DD>Returns the number of fields
(attributes) in the query result.
<pre> int PQnfields(PGresult &#42;res);
</pre><br>
<DT><B>PQfname</B>
<DD>Returns the field (attribute) name associated with the given field index. Field indices
start at 0.
<pre> char &#42;PQfname(PGresult &#42;res,
int field_index);
</pre><br>
<DT><B>PQfnumber</B>
<DD>Returns the field (attribute) index
associated with the given field name.
<pre> int PQfnumber(PGresult &#42;res,
char&#42; field_name);
</pre><br>
<DT><B>PQftype</B>
<DD>Returns the field type associated with the
given field index. The integer returned is an
internal coding of the type. Field indices start
at 0.
<pre> Oid PQftype(PGresult &#42;res,
int field_num);
</pre><br>
<DT><B>PQfsize</B>
<DD>Returns the size in bytes of the field
associated with the given field index. If the size
returned is -1, the field is a variable length
field. Field indices start at 0.
<pre> int2 PQfsize(PGresult &#42;res,
int field_index);
</pre><br>
<DT><B>PQgetvalue</B>
<DD>Returns the field (attribute) value.
For most queries, the value returned by PQgetvalue
is a null-terminated ASCII string representation
of the attribute value. If the query was a result
of a <B>BINARY</B> cursor, then the value returned by
PQgetvalue is the binary representation of the
type in the internal format of the backend server.
It is the programmer's responsibility to cast and
convert the data to the correct C type. The value
returned by PQgetvalue points to storage that is
part of the PGresult structure. One must explicitly
copy the value into other storage if it is to
be used past the lifetime of the PGresult structure itself.
<pre> char&#42; PQgetvalue(PGresult &#42;res,
int tup_num,
int field_num);
</pre><br>
<DT><B>PQgetlength</B>
<DD>Returns the length of a field
(attribute) in bytes. If the field is a struct
varlena, the length returned here does not include
the size field of the varlena, i.e., it is 4 bytes
less.
<pre> int PQgetlength(PGresult &#42;res,
int tup_num,
int field_num);
</pre><br>
<DT><B>PQcmdStatus</B>
Returns the command status associated with the
last query command.
<pre>
char &#42;PQcmdStatus(PGresult &#42;res);
</pre><br>
<DT><B>PQoidStatus</B>
Returns a string with the object id of the tuple
inserted if the last query is an INSERT command.
Otherwise, returns an empty string.
<pre> char&#42; PQoidStatus(PGresult &#42;res);
</pre><br>
<DT><B>PQprintTuples</B>
Prints out all the tuples and, optionally, the
attribute names to the specified output stream.
The programs psql and monitor both use PQprintTuples for output.
<pre> void PQprintTuples(
PGresult&#42; res,
FILE&#42; fout, /&#42; output stream &#42;/
int printAttName,/&#42; print attribute names or not&#42;/
int terseOutput, /&#42; delimiter bars or not?&#42;/
int width /&#42; width of column, variable width if 0&#42;/
);
</pre><br>
<DT><B>PQclear</B>
Frees the storage associated with the PGresult.
Every query result should be properly freed when
it is no longer used. Failure to do this will
result in memory leaks in the frontend application.
<pre> void PQclear(PQresult &#42;res);
</pre><br>
</DL>
<H2><A NAME="fast-path">12.4. Fast Path</A></H2>
POSTGRES provides a fast path interface to send function calls to the backend. This is a trapdoor into
system internals and can be a potential security hole.
Most users will not need this feature.
<pre> PGresult&#42; PQfn(PGconn&#42; conn,
int fnid,
int &#42;result_buf,
int &#42;result_len,
int result_is_int,
PQArgBlock &#42;args,
int nargs);
</pre><br>
The fnid argument is the object identifier of the function to be executed. result_buf is the buffer in which
to load the return value. The caller must have allocated sufficient space to store the return value. The
result length will be returned in the storage pointed
to by result_len. If the result is to be an integer
value, than result_is_int should be set to 1; otherwise
it should be set to 0. args and nargs specify the
arguments to the function.
<pre> typedef struct {
int len;
int isint;
union {
int &#42;ptr;
int integer;
} u;
} PQArgBlock;
</pre>
PQfn always returns a valid PGresult&#42;. The resultStatus should be checked before the result is used. The
caller is responsible for freeing the PGresult with
PQclear when it is not longer needed.
<H2><A NAME="asynchronous-notification">12.5. Asynchronous Notification</A></H2>
POSTGRES supports asynchronous notification via the
LISTEN and NOTIFY commands. A backend registers its
interest in a particular relation with the LISTEN command. All backends listening on a particular relation
will be notified asynchronously when a NOTIFY of that
relation name is executed by another backend. No
additional information is passed from the notifier to
the listener. Thus, typically, any actual data that
needs to be communicated is transferred through the
relation.
<B>LIBPQ</B> applications are notified whenever a connected
backend has received an asynchronous notification.
However, the communication from the backend to the
frontend is not asynchronous. Notification comes
piggy-backed on other query results. Thus, an application must submit queries, even empty ones, in order to
receive notice of backend notification. In effect, the
<B>LIBPQ</B> application must poll the backend to see if there
is any pending notification information. After the
execution of a query, a frontend may call PQNotifies to
see if any notification data is available from the
backend.
<DL>
<DT><B>PQNotifies</B>
<DD>returns the notification from a list of unhandled
notifications from the backend. Returns NULL if
there are no pending notifications from the backend. PQNotifies behaves like the popping of a
stack. Once a notification is returned from PQnotifies, it is considered handled and will be
removed from the list of notifications.
<pre> PGnotify&#42; PQNotifies(PGconn &#42;conn);
</pre><br>
</DL>
The second sample program gives an example of the use
of asynchronous notification.
<H2><A NAME="functions-associated-with-the-copy-command">12.6. Functions Associated with the COPY Command</A></H2>
The copy command in POSTGRES has options to read from
or write to the network connection used by <B>LIBPQ</B>.
Therefore, functions are necessary to access this network connection directly so applications may take full
advantage of this capability.
<DL>
<DT><B>PQgetline</B>
<DD>Reads a newline-terminated line of characters
(transmitted by the backend server) into a buffer
string of size length. Like fgets(3), this routine copies up to length-1 characters into string.
It is like gets(3), however, in that it converts
the terminating newline into a null character.
PQgetline returns EOF at EOF, 0 if the entire line
has been read, and 1 if the buffer is full but the
terminating newline has not yet been read.
Notice that the application must check to see if a
new line consists of the single character ".",
which indicates that the backend server has finished sending the results of the copy command.
Therefore, if the application ever expects to
receive lines that are more than length-1 characters long, the application must be sure to check
the return value of PQgetline very carefully.
The code in
<pre> ../src/bin/psql/psql.c
</pre>
contains routines that correctly handle the copy
protocol.
<pre> int PQgetline(PGconn &#42;conn,
char &#42;string,
int length)
</pre><br>
<DT><B>PQputline</B>
<DD>Sends a null-terminated string to the backend
server.
The application must explicitly send the single
character "." to indicate to the backend that it
has finished sending its data.
<pre> void PQputline(PGconn &#42;conn,
char &#42;string);
</pre><br>
<DT><B>PQendcopy</B>
<DD>Syncs with the backend. This function waits until
the backend has finished the copy. It should
either be issued when the last string has been
sent to the backend using PQputline or when the
last string has been received from the backend
using PGgetline. It must be issued or the backend
may get "out of sync" with the frontend. Upon
return from this function, the backend is ready to
receive the next query.
The return value is 0 on successful completion,
nonzero otherwise.
<pre> int PQendcopy(PGconn &#42;conn);
</pre><br>
As an example:
<pre> PQexec(conn, "create table foo (a int4, b char16, d float8)");
PQexec(conn, "copy foo from stdin");
PQputline(conn, "3&lt;TAB&gt;hello world&lt;TAB&gt;4.5\n");
PQputline(conn,"4&lt;TAB&gt;goodbye world&lt;TAB&gt;7.11\n");
...
PQputline(conn,".\n");
PQendcopy(conn);
</pre><br>
</DL>
<H2><A NAME="tracing-functions">12.7. <B>LIBPQ</B> Tracing Functions</A></H2>
<DL>
<DT><B>PQtrace</B>
<DD>Enable tracing of the frontend/backend communication to a debugging file stream.
<pre> void PQtrace(PGconn &#42;conn
FILE &#42;debug_port)
</pre><br>
<DT><B>PQuntrace</B>
<DD>Disable tracing started by PQtrace
<pre> void PQuntrace(PGconn &#42;conn)
</pre><br>
</DL>
<H2><A NAME="authentication-functions">12.8. User Authentication Functions</A></H2>
If the user has generated the appropriate authentication credentials (e.g., obtaining <B>Kerberos</B> tickets),
the frontend/backend authentication process is handled
by <B>PQexec</B> without any further intervention. The following routines may be called by <B>LIBPQ</B> programs to tailor the behavior of the authentication process.
<DL>
<DT><B>fe_getauthname</B>
<DD>Returns a pointer to static space containing whatever name the user has authenticated. Use of this
routine in place of calls to getenv(3) or getpwuid(3) by applications is highly recommended, as
it is entirely possible that the authenticated
user name is not the same as value of the <B>USER</B>
environment variable or the user's entry in
<CODE>/etc/passwd</CODE>.
<pre> char &#42;fe_getauthname(char&#42; errorMessage)
</pre><br>
<DT><B>fe_setauthsvc</B>
<DD>Specifies that <B>LIBPQ</B> should use authentication
service name rather than its compiled-in default.
<DD>This value is typically taken from a command-line
switch.
<pre> void fe_setauthsvc(char &#42;name,
char&#42; errorMessage)
</pre>
<DD>Any error messages from the authentication
attempts are returned in the errorMessage argument.
</DL>
<H2><A NAME="bugs">12.9. BUGS</A></H2>
The query buffer is 8192 bytes long, and queries over
that length will be silently truncated.
<H2><A NAME="sample-programs">12.10. Sample Programs</H2>
<p>
<H3><A NAME="sample-program-1">12.10.1. Sample Program 1</A></H3>
<pre>
/&#42;
&#42; testlibpq.c
&#42; Test the C version of LIBPQ, the POSTGRES frontend library.
&#42;
&#42;
&#42;/
#include &lt;stdio.h&gt;
#include "libpq-fe.h"
<p>
void
exit_nicely(PGconn&#42; conn)
{
PQfinish(conn);
exit(1);
}
<p>
main()
{
char &#42;pghost, &#42;pgport, &#42;pgoptions, &#42;pgtty;
char&#42; dbName;
int nFields;
int i,j;
<p>
/&#42; FILE &#42;debug; &#42;/
<p>
PGconn&#42; conn;
PGresult&#42; res;
<p>
/&#42; 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 &#42;/
pghost = NULL; /&#42; host name of the backend server &#42;/
pgport = NULL; /&#42; port of the backend server &#42;/
pgoptions = NULL; /&#42; special options to start up the backend server &#42;/
pgtty = NULL; /&#42; debugging tty for the backend server &#42;/
dbName = "template1";
<p>
/&#42; make a connection to the database &#42;/
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
<p>
/&#42; check to see that the backend connection was successfully made &#42;/
if (PQstatus(conn) == CONNECTION_BAD) {
fprintf(stderr,"Connection to database '&#37;s' failed.0, dbName);
fprintf(stderr,"&#37;s",PQerrorMessage(conn));
exit_nicely(conn);
}
<p>
/&#42; debug = fopen("/tmp/trace.out","w"); &#42;/
/&#42; PQtrace(conn, debug); &#42;/
<p>
/&#42; start a transaction block &#42;/
res = PQexec(conn,"BEGIN");
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"BEGIN command failed0);
PQclear(res);
exit_nicely(conn);
}
/&#42; should PQclear PGresult whenever it is no longer needed to avoid
memory leaks &#42;/
PQclear(res);
<p>
/&#42; fetch instances from the pg_database, the system catalog of databases&#42;/
res = PQexec(conn,"DECLARE myportal CURSOR FOR select &#42; from pg_database");
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"DECLARE CURSOR command failed0);
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
<p>
res = PQexec(conn,"FETCH ALL in myportal");
if (PQresultStatus(res) != PGRES_TUPLES_OK) {
fprintf(stderr,"FETCH ALL command didn't return tuples properly0);
PQclear(res);
exit_nicely(conn);
}
<p>
/&#42; first, print out the attribute names &#42;/
nFields = PQnfields(res);
for (i=0; i &lt; nFields; i++) {
printf("&#37;-15s",PQfname(res,i));
}
printf("0);
<p>
/&#42; next, print out the instances &#42;/
for (i=0; i &lt; PQntuples(res); i++) {
for (j=0 ; j &lt; nFields; j++) {
printf("&#37;-15s", PQgetvalue(res,i,j));
}
printf("0);
}
<p>
PQclear(res);
<p>
/&#42; close the portal &#42;/
res = PQexec(conn, "CLOSE myportal");
PQclear(res);
<p>
/&#42; end the transaction &#42;/
res = PQexec(conn, "END");
PQclear(res);
<p>
/&#42; close the connection to the database and cleanup &#42;/
PQfinish(conn);
/&#42; fclose(debug); &#42;/
}
</pre>
<p>
<H3><A NAME="sample-program-2">12.10.2. Sample Program 2</A></H3>
<pre>
/&#42;
&#42; testlibpq2.c
&#42; Test of the asynchronous notification interface
&#42;
populate a database with the following:
<p>
CREATE TABLE TBL1 (i int4);
<p>
CREATE TABLE TBL2 (i int4);
<p>
CREATE RULE r1 AS ON INSERT TO TBL1 DO [INSERT INTO TBL2 values (new.i); NOTIFY TBL2];
<p>
&#42; Then start up this program
&#42; After the program has begun, do
<p>
INSERT INTO TBL1 values (10);
<p>
&#42;
&#42;
&#42;/
#include &lt;stdio.h&gt;
#include "libpq-fe.h"
<p>
void exit_nicely(PGconn&#42; conn)
{
PQfinish(conn);
exit(1);
}
<p>
main()
{
char &#42;pghost, &#42;pgport, &#42;pgoptions, &#42;pgtty;
char&#42; dbName;
int nFields;
int i,j;
<p>
PGconn&#42; conn;
PGresult&#42; res;
PGnotify&#42; notify;
<p>
/&#42; 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 &#42;/
pghost = NULL; /&#42; host name of the backend server &#42;/
pgport = NULL; /&#42; port of the backend server &#42;/
pgoptions = NULL; /&#42; special options to start up the backend server &#42;/
pgtty = NULL; /&#42; debugging tty for the backend server &#42;/
dbName = getenv("USER"); /&#42; change this to the name of your test database&#42;/
<p>
/&#42; make a connection to the database &#42;/
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/&#42; check to see that the backend connection was successfully made &#42;/
if (PQstatus(conn) == CONNECTION_BAD) {
fprintf(stderr,"Connection to database '&#37;s' failed.0, dbName);
fprintf(stderr,"&#37;s",PQerrorMessage(conn));
exit_nicely(conn);
}
<p>
res = PQexec(conn, "LISTEN TBL2");
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"LISTEN command failed0);
PQclear(res);
exit_nicely(conn);
}
/&#42; should PQclear PGresult whenever it is no longer needed to avoid
memory leaks &#42;/
PQclear(res);
<p>
while (1) {
/&#42; async notification only come back as a result of a query&#42;/
/&#42; we can send empty queries &#42;/
res = PQexec(conn, " ");
/&#42; printf("res-&gt;status = &#37;s0, pgresStatus[PQresultStatus(res)]); &#42;/
/&#42; check for asynchronous returns &#42;/
notify = PQnotifies(conn);
if (notify) {
fprintf(stderr,
"ASYNC NOTIFY of '&#37;s' from backend pid '&#37;d' received0,
notify-&gt;relname, notify-&gt;be_pid);
free(notify);
break;
}
PQclear(res);
}
<p>
/&#42; close the connection to the database and cleanup &#42;/
PQfinish(conn);
<p>
}
</pre>
<p>
<H3><A NAME="sample-program-3">12.10.3. Sample Program 3</A></H3>
<pre>
/&#42;
&#42; testlibpq3.c
&#42; Test the C version of LIBPQ, the POSTGRES frontend library.
&#42; tests the binary cursor interface
&#42;
&#42;
&#42;
populate a database by doing the following:
<p>
CREATE TABLE test1 (i int4, d float4, p polygon);
<p>
INSERT INTO test1 values (1, 3.567, '(3.0, 4.0, 1.0, 2.0)'::polygon);
<p>
INSERT INTO test1 values (2, 89.05, '(4.0, 3.0, 2.0, 1.0)'::polygon);
<p>
the expected output is:
<p>
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)
<p>
&#42;
&#42;/
#include &lt;stdio.h&gt;
#include "libpq-fe.h"
#include "utils/geo-decls.h" /&#42; for the POLYGON type &#42;/
<p>
void exit_nicely(PGconn&#42; conn)
{
PQfinish(conn);
exit(1);
}
<p>
main()
{
char &#42;pghost, &#42;pgport, &#42;pgoptions, &#42;pgtty;
char&#42; dbName;
int nFields;
int i,j;
int i_fnum, d_fnum, p_fnum;
<p>
PGconn&#42; conn;
PGresult&#42; res;
<p>
/&#42; 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 &#42;/
pghost = NULL; /&#42; host name of the backend server &#42;/
pgport = NULL; /&#42; port of the backend server &#42;/
pgoptions = NULL; /&#42; special options to start up the backend server &#42;/
pgtty = NULL; /&#42; debugging tty for the backend server &#42;/
<p>
dbName = getenv("USER"); /&#42; change this to the name of your test database&#42;/
<p>
/&#42; make a connection to the database &#42;/
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
<p>
/&#42; check to see that the backend connection was successfully made &#42;/
if (PQstatus(conn) == CONNECTION_BAD) {
fprintf(stderr,"Connection to database '&#37;s' failed.0, dbName);
fprintf(stderr,"&#37;s",PQerrorMessage(conn));
exit_nicely(conn);
}
<p>
/&#42; start a transaction block &#42;/
res = PQexec(conn,"BEGIN");
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"BEGIN command failed0);
PQclear(res);
exit_nicely(conn);
}
/&#42; should PQclear PGresult whenever it is no longer needed to avoid
memory leaks &#42;/
PQclear(res);
<p>
/&#42; fetch instances from the pg_database, the system catalog of databases&#42;/
res = PQexec(conn,"DECLARE mycursor BINARY CURSOR FOR select &#42; from test1");
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"DECLARE CURSOR command failed0);
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
<p>
res = PQexec(conn,"FETCH ALL in mycursor");
if (PQresultStatus(res) != PGRES_TUPLES_OK) {
fprintf(stderr,"FETCH ALL command didn't return tuples properly0);
PQclear(res);
exit_nicely(conn);
}
<p>
i_fnum = PQfnumber(res,"i");
d_fnum = PQfnumber(res,"d");
p_fnum = PQfnumber(res,"p");
<p>
for (i=0;i&lt;3;i++) {
printf("type[&#37;d] = &#37;d, size[&#37;d] = &#37;d0,
i, PQftype(res,i),
i, PQfsize(res,i));
}
for (i=0; i &lt; PQntuples(res); i++) {
int &#42;ival;
float &#42;dval;
int plen;
POLYGON&#42; pval;
/&#42; we hard-wire this to the 3 fields we know about &#42;/
ival = (int&#42;)PQgetvalue(res,i,i_fnum);
dval = (float&#42;)PQgetvalue(res,i,d_fnum);
plen = PQgetlength(res,i,p_fnum);
<p>
/&#42; plen doesn't include the length field so need to increment by VARHDSZ&#42;/
pval = (POLYGON&#42;) malloc(plen + VARHDRSZ);
pval-&gt;size = plen;
memmove((char&#42;)&amp;pval-&gt;npts, PQgetvalue(res,i,p_fnum), plen);
printf("tuple &#37;d: got0, i);
printf(" i = (&#37;d bytes) &#37;d,0,
PQgetlength(res,i,i_fnum), &#42;ival);
printf(" d = (&#37;d bytes) &#37;f,0,
PQgetlength(res,i,d_fnum), &#42;dval);
printf(" p = (&#37;d bytes) &#37;d points boundbox = (hi=&#37;f/&#37;f, lo = &#37;f,&#37;f)0,
PQgetlength(res,i,d_fnum),
pval-&gt;npts,
pval-&gt;boundbox.xh,
pval-&gt;boundbox.yh,
pval-&gt;boundbox.xl,
pval-&gt;boundbox.yl);
}
<p>
PQclear(res);
<p>
/&#42; close the portal &#42;/
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
<p>
/&#42; end the transaction &#42;/
res = PQexec(conn, "END");
PQclear(res);
<p>
/&#42; close the connection to the database and cleanup &#42;/
PQfinish(conn);
<p>
}
</pre>
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