LIBPQ(INTRO)

DESCRIPTION
Current documentation for this topic is available in the new Programmer's Guide chapter on libpq. This man page is obsolete, though in sync with the Programmer's Guide as of 1998/08/15.

Libpq is the programmer's interface to Postgres. Libpq 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 C interface library. Three short programs are included at the end of this section to show how to write programs that use Libpq.

There are several examples of Libpq applications in the following directories:
../src/test/regress
../src/test/examples
../src/bin/psql

Frontend programs which use Libpq must include the header file libpq-fe.h and must link with the libpq library.

Control and Initialization

The following environment variables can be used to set up default environment values to avoid hard-coding database names into an application program:

• PGHOST sets the default server name. If it is set to a non-zero-length string, it causes TCP/IP communication to be used, rather than the default local Unix domain sockets.

• PGUSER sets the username used to connect to the database and for authentication.

• PGOPTIONS sets additional runtime options for the Postgres backend.

• PGPORT sets the default port or local Unix domain socket file extension for communicating with the Postgres backend.

• PGTTY sets the file or tty on which debugging messages from the backend server are displayed.

• PGDATABASE sets the default Postgres database name.

• PGREALM sets the Kerberos realm to use with Postgres, if it is different from the local realm. If PGREALM 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.

The following environment variables can be used to specify user-level default behavior for every Postgres session:

• PGDATESTYLE sets the default style of date/time representation.

• PGTZ sets the default time zone.

The following environment variables can be used to specify default internal behavior for every Postgres session:

• PGGEQO sets the default mode for the genetic optimizer.

• PGRPLANS sets the default mode to allow or disable right-sided plans in the optimizer.

• PGCOSTHEAP sets the default cost for heap searches for the optimizer.

• PGCOSTINDEX sets the default cost for indexed searches for the optimizer. • PGQUERY_LIMIT sets the maximum number of rows returned by a query.

See the set(l) man page for information on the arguments for these environment variables.

Database Connection Functions

The following routines deal with making a connection to a backend from a C program.

PQsetdb
PQsetdbLogin

    Makes a new connection to a backend. PQsetdb is the method usually used to connect to the database when username/password authentication is not needed.
    PGconn *PQsetdb(char *pghost,
                    char *pgport,
                    char *pgoptions,
                    char *pgtty,
                    char *dbName);

    PQsetdbLogin is the method used to connect to the database when username/password authentication is needed.
    PGconn *PQsetdbLogin(char *pghost,
                         char *pgport,
                         char *pgoptions,
                         char *pgtty,
                         char *dbName,
                         char *login,
                         char *pwd);
    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.

    PQsetdb and PQsetdbLogin always return a valid PGconn pointer. The PQstatus (see below) command should be called to ensure that a connection was properly made before queries are sent via the connection. Libpq programmers should be careful to maintain the 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.

    PQdb returns the database name of the connection.
    char *PQdb(PGconn *conn)
    PQhost returns the host name of the connection.
    char *PQhost(PGconn *conn)
    PQoptions returns the pgoptions used in the connection.
    char *PQoptions(PGconn *conn)
    PQport returns the pgport of the connection.
    char *PQport(PGconn *conn)
    PQtty returns the pgtty of the connection.
    char *PQtty(PGconn *conn)
    PQstatus Returns the status of the connection. The status can be CONNECTION_OK or CONNECTION_BAD.
    ConnStatusType *PQstatus(PGconn *conn)
    PQerrorMessage returns the error message associated with the connection
    char *PQerrorMessage(PGconn* conn);

PQfinish

    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.
    void PQfinish(PGconn *conn)

PQreset

    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.
    void PQreset(PGconn *conn)

Query Execution Functions

PQexec

    Submit a query to Postgres. Returns a PGresult pointer or possibly a NULL pointer. If a NULL is returned, it should be treated like a PGRES_FATAL_ERROR result: use PQerrorMessage to get more information about the error.
    PGresult *PQexec(PGconn *conn,
                     const char *query);
    The PGresult structure encapsulates the query result returned by the backend. Libpq 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.

PQresultStatus

    Returns the result status of the query. PQresultStatus can return one of the following values:
    PGRES_EMPTY_QUERY,
    PGRES_COMMAND_OK,  /* the query was a command returning no data */
    PGRES_TUPLES_OK,  /* the query successfully returned tuples */
    PGRES_COPY_OUT,
    PGRES_COPY_IN,
    PGRES_BAD_RESPONSE, /* an unexpected response was received */
    PGRES_NONFATAL_ERROR,
    PGRES_FATAL_ERROR

    If the result status is PGRES_TUPLES_OK, then the following routines can be used to retrieve the tuples returned by the query.

    PQntuples returns the number of tuples (instances) in the query result.
    int PQntuples(PGresult *res);
    PQnfields returns the number of fields (attributes) in the query result.
    int PQnfields(PGresult *res);
    PQfname returns the field (attribute) name associated with the given field index. Field indices start at 0.
    char *PQfname(PGresult *res,
                 int field_index);
    PQfnumber returns the field (attribute) index associated with the given field name.
    int PQfnumber(PGresult *res,
                 char* field_name);
    PQftype 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.
    Oid PQftype(PGresult *res,
                int field_num);
    PQfsize 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.
    short PQfsize(PGresult *res,
                  int field_index);
    PQfmod returns the type-specific modification data of the field associated with the given field index. Field indices start at 0.
    int PQfmod(PGresult *res,
               int field_index);
    PQgetvalue 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 BINARY 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.
    char* PQgetvalue(PGresult *res,
                     int tup_num,
                     int field_num);
    PQgetlength 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.
    int PQgetlength(PGresult *res,
                    int tup_num,
                    int field_num);
    PQgetisnull returns the NULL status of a field.
    int PQgetisnull(PGresult *res,
                    int tup_num,
                    int field_num);

PQcmdStatus

    Returns the command status associated with the last query command.
    char *PQcmdStatus(PGresult *res);

PQcmdTuples

    Returns the number of tuples (instances) affected by INSERT, UPDATE, and DELETE queries.
    char *PQcmdTuples(PGresult *res);

PQoidStatus

    Returns a string with the object id of the tuple inserted if the last query is an INSERT command. Otherwise, returns an empty string.
    char* PQoidStatus(PGresult *res);

PQprint

    + Prints out all the tuples in an intelligent manner. The psql + program uses this function for its output.
    void PQprint(
          FILE* fout,      /* output stream */
          PGresult* res,   /* query results */
          PQprintOpt *ps   /* option structure */
            );
    PQprintOpt is a typedef'ed structure as defined below. typedef struct _PQprintOpt {
        bool header; /* print table headings and row count */
        bool align; /* fill align the fields */
        bool standard; /* old brain dead format (needs align) */
        bool html3; /* output html3+ tables */
        bool expanded; /* expand tables */
        bool pager; /* use pager if needed */
        char *fieldSep; /* field separator */
        char *caption; /* html table caption (or NULL) */
        char **fieldName; /* null terminated array of field names (or NULL) */ } PQprintOpt;

PQclear

    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. The PQresult* passed in should be a value which is returned from PQexec(). Calling PQclear() on an uninitialized PQresult pointer will very likely result in a core dump.
    void PQclear(PQresult *res);

Asynchronous Query Processing

The PQexec function is adequate for submitting queries in simple synchronous applications. It has a couple of major deficiencies however:

    PQexec waits for the query 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 control is buried inside PQexec, it is hard for the frontend to decide it would like to try to cancel the ongoing query. (It can be done from a signal handler, but not otherwise.)

    PQexec can return only one PGresult structure. If the submitted query 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.

PQsendQuery

    Submit a query to Postgres without waiting for the result(s). TRUE is returned if the query was successfully dispatched, FALSE if not (in which case, use PQerrorMessage to get more information about the failure).
    int PQsendQuery(PGconn *conn,
                    const char *query);
    After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the query results. PQsendQuery may not be called again (on the same connection) until PQgetResult has returned NULL, indicating that the query is done.

PQgetResult

    Wait for the next result from a prior PQsendQuery, and return it. NULL is returned when the query is complete and there will be no more results.
    PGresult *PQgetResult(PGconn *conn);
    PQgetResult must be called repeatedly until it returns NULL, indicating that the query is done. (If called when no query is active, PQgetResult will just return NULL 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 query 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 query 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 frontend can be handling the results of one query while the backend is still working on later queries in the same query string.) However, calling PQgetResult will still cause the frontend to block until the backend completes the next SQL command. This can be avoided by proper use of three more functions:

PQconsumeInput

    If input is available from the backend, consume it.
    void PQconsumeInput(PGconn *conn);
    No direct return value is available from PQconsumeInput, but after calling it, 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. It will read available data and save it in a buffer, thereby causing a select(2) 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.

PQisBusy

    Returns TRUE if a query is busy, that is, PQgetResult would block waiting for input. A FALSE 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 backend; therefore PQconsumeInput must be invoked first, or the busy state will never end.

PQsocket

    Obtain the file descriptor number for the backend connection socket. A valid descriptor will be >= 0; a result of -1 indicates that no backend connection is currently open.
    int PQsocket(PGconn *conn);
    PQsocket should be used to obtain the backend socket descriptor in preparation for executing select(2). This allows an application to wait for either backend responses or other conditions. If the result of select(2) indicates that data can be read from the backend socket, then PQconsumeInput should be called to read the data; after which, PQisBusy, PQgetResult, and/or PQnotifies can be used to process the response.

A typical frontend using these functions will have a main loop that uses select(2) to wait for all the conditions that it must respond to. One of the conditions will be input available from the backend, which in select's terms is 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. It can also call PQnotifies to detect NOTIFY messages (see "Asynchronous Notification", below). An example is given in the sample programs section.

A frontend that uses PQsendQuery/PQgetResult can also attempt to cancel a query that is still being processed by the backend.

PQrequestCancel

    Request that <ProductName>Postgres</ProductName> abandon processing of the current query.
    int PQrequestCancel(PGconn *conn);
    The return value is TRUE if the cancel request was successfully dispatched, FALSE 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 query will terminate early and return an error result. If the cancellation fails (say because the backend was already done processing the query), then there will be no visible result at all.

Note that if the current query is part of a transaction, 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 queries that it issues through PQexec. Note that PQrequestCancel will have no effect if the connection is not currently open or the backend is not currently processing a query.

Fast Path

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.
PGresult* PQfn(PGconn* conn,
	       int fnid,
	       int *result_buf,
	       int *result_len,
	       int result_is_int,
	       PQArgBlock *args,
	       int nargs);

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.
typedef struct {
    int len;
    int isint;
    union {
        int *ptr;
	int integer;
    } u;
} PQArgBlock;

PQfn always returns a valid PGresult*. The resultStatus should be checked before the result is used. The caller is responsible for freeing the PGresult with PQclear when it is no longer needed.

Asynchronous Notification

Postgres supports asynchronous notification via the LISTEN and NOTIFY commands. A backend registers its interest in a particular notification condition with the LISTEN command. All backends listening on a particular condition will be notified asynchronously when a NOTIFY of that condition name is executed by any 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 a database relation. Commonly the condition name is the same as the associated relation, but it is not necessary for there to be any associated relation.

libpq applications submit LISTEN commands as ordinary SQL queries. Subsequently, arrival of NOTIFY messages can be detected by calling PQnotifies().

PQNotifies

    Returns the next notification from a list of unhandled notification messages received from the backend. Returns NULL if there are no pending notifications. 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.
    PGnotify* PQNotifies(PGconn *conn);
    After processing a PGnotify object returned by PQnotifies, be sure to free it with free() to avoid a memory leak.

The second sample program gives an example of the use of asynchronous notification.

PQnotifies() does not actually read backend data; 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 queries, 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 queries to make is to call PQconsumeInput(), then check PQnotifies(). You can use select(2) to wait for backend data to arrive, thereby using no CPU power unless there is something to do. Note that this will work OK whether you use PQsendQuery/PQgetResult or plain old PQexec for queries. You should, however, remember to check PQnotifies() after each PQgetResult or PQexec to see if any notifications came in during the processing of the query.

Functions Associated with the COPY Command

The copy command in Postgres has options to read from or write to the network connection used by Libpq. Therefore, functions are necessary to access this network connection directly so applications may take full advantage of this capability.

These functions should be executed only after obtaining a PGRES_COPY_OUT or PGRES_COPY_IN result object from PQexec or PQgetResult.

PQgetline

    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 two characters “\.”, 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
    ../src/bin/psql/psql.c
    contains routines that correctly handle the copy protocol.
    int PQgetline(PGconn *conn,
                  char *string,
                  int length)

PQputline

    Sends a null-terminated string to the backend server.

    The application must explicitly send the two characters “\.” on a final line to indicate to the backend that it has finished sending its data.
    void PQputline(PGconn *conn,
                   char *string);

PQendcopy

    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.
    int PQendcopy(PGconn *conn);
    As an example:
    PQexec(conn, "create table foo (a int4, b name, d float8)");
    PQexec(conn, "copy foo from stdin");
    PQputline(conn, "3<TAB>hello world<TAB>4.5\n");
    PQputline(conn,"4<TAB>goodbye world<TAB>7.11\n");
    ...
    PQputline(conn,"\.\n");
    PQendcopy(conn);

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 NULL. 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 in or copy out command embedded in a series of SQL commands will be executed correctly. Older applications are likely to submit a copy in or copy out via PQexec and assume that the transaction is done after PQendcopy. This will work correctly only if the copy in/out is the only SQL command in the query string.

LIBPQ Tracing Functions

PQtrace

    Enable tracing of the frontend/backend communication to a debugging file stream.
    void PQtrace(PGconn *conn
                 FILE *debug_port)

PQuntrace

    Disable tracing started by PQtrace
    void PQuntrace(PGconn *conn)

LIBPQ Control Functions

PQsetNoticeProcessor

    Control reporting of notice and warning messages generated by libpq.
    void PQsetNoticeProcessor (PGconn * conn,
    		void (*noticeProcessor) (void * arg, const char * message),
    		void * arg)
    By default, libpq prints "notice" messages from the backend on stderr, as well as a few error messages that it generates by itself. This behavior can be overridden by supplying a callback function that does something else with the messages. The callback function is passed the text of the error message (which includes 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);
    }
    To use a special notice processor, call PQsetNoticeProcessor just after any creation of a new PGconn object.

User Authentication Functions

If the user has generated the appropriate authentication credentials (e.g., obtaining Kerberos tickets), the frontend/backend authentication process is handled by PQexec without any further intervention. The authentication method is now determined entirely by the DBA (see pga_hba.conf(5)). The following routines no longer have any effect and should not be used.

fe_getauthname

    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 USER environment variable or the user's entry in /etc/passwd .
    char *fe_getauthname(char* errorMessage)

fe_setauthsvc

    Specifies that Libpq should use authentication service name rather than its compiled-in default. This value is typically taken from a command-line switch.
    void fe_setauthsvc(char *name,
                       char* errorMessage)
    Any error messages from the authentication attempts are returned in the errorMessage argument.

BUGS

The query buffer is 8192 bytes long, and queries over that length will be rejected.

Sample Programs

Sample Program 1

/*
 * testlibpq.c
 *  Test the C version of Libpq, the Postgres frontend library.
 *
 *
 */
#include <stdio.h>
#include "libpq-fe.h"
void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}
main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;
/*  FILE *debug; */
    PGconn     *conn;
    PGresult   *res;
    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */
    dbName = "template1";
    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }
/*  debug = fopen("/tmp/trace.out","w");  */
/*   PQtrace(conn, debug);  */
    /* start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);
    /*
     * fetch instances from the pg_database, the system catalog of
     * databases
     */
    res = PQexec(conn, "DECLARE mycursor CURSOR FOR select * from pg_database");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);
    res = PQexec(conn, "FETCH ALL in mycursor");
    if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
        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 instances */
    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 cursor */
    res = PQexec(conn, "CLOSE mycursor");
    PQclear(res);
    /* commit the transaction */
    res = PQexec(conn, "COMMIT");
    PQclear(res);
    /* close the connection to the database and cleanup */
    PQfinish(conn);
/*   fclose(debug); */
}

Sample Program 2

/*
 * testlibpq2.c
 *  Test of the asynchronous notification interface
 *
   populate a database with the following:
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];
 * Then start up this program
 * After the program has begun, do
INSERT INTO TBL1 values (10);
 *
 *
 */
#include <stdio.h>
#include "libpq-fe.h"
void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}
main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;
    PGconn     *conn;
    PGresult   *res;
    PGnotify   *notify;
    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */
    dbName = getenv("USER");    /* change this to the name of your test
                                 * database */
    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }
    res = PQexec(conn, "LISTEN TBL2");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "LISTEN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);
    while (1)
    {
	/* wait a little bit between checks;
	 * waiting with select() would be more efficient.
	 */
	sleep(1);
	/* collect any asynchronous backend messages */
	PQconsumeInput(conn);
	/* check for asynchronous notify messages */
	while ((notify = PQnotifies(conn)) != NULL) {
            fprintf(stderr,
                "ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
                    notify->relname, notify->be_pid);
            free(notify);
        }
    }
    /* close the connection to the database and cleanup */
    PQfinish(conn);
}

Sample Program 3

/*
 * testlibpq3.c
 *  Test the C version of Libpq, the Postgres frontend library.
 *   tests the binary cursor interface
 *
 *
 *
 populate a database by doing the following:
CREATE TABLE test1 (i int4, d float4, p polygon);
INSERT INTO test1 values (1, 3.567, '(3.0, 4.0, 1.0, 2.0)'::polygon);
INSERT INTO test1 values (2, 89.05, '(4.0, 3.0, 2.0, 1.0)'::polygon);
 the expected output is:
tuple 0: got
 i = (4 bytes) 1,
 d = (4 bytes) 3.567000,
 p = (4 bytes) 2 points         boundbox = (hi=3.000000/4.000000, lo = 1.000000,2.000000)
tuple 1: got
 i = (4 bytes) 2,
 d = (4 bytes) 89.050003,
 p = (4 bytes) 2 points         boundbox = (hi=4.000000/3.000000, lo = 2.000000,1.000000)
 *
 */
#include <stdio.h>
#include "libpq-fe.h"
#include "utils/geo-decls.h"    /* for the POLYGON type */
void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}
main()
{
    char       *pghost,
               *pgport,
               *pgoptions,
               *pgtty;
    char       *dbName;
    int         nFields;
    int         i,
                j;
    int         i_fnum,
                d_fnum,
                p_fnum;
    PGconn     *conn;
    PGresult   *res;
    /*
     * begin, by setting the parameters for a backend connection if the
     * parameters are null, then the system will try to use reasonable
     * defaults by looking up environment variables or, failing that,
     * using hardwired constants
     */
    pghost = NULL;              /* host name of the backend server */
    pgport = NULL;              /* port of the backend server */
    pgoptions = NULL;           /* special options to start up the backend
                                 * server */
    pgtty = NULL;               /* debugging tty for the backend server */
    dbName = getenv("USER");    /* change this to the name of your test
                                 * database */
    /* make a connection to the database */
    conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }
    /* start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    /*
     * should PQclear PGresult whenever it is no longer needed to avoid
     * memory leaks
     */
    PQclear(res);
    /*
     * fetch instances from the pg_database, the system catalog of
     * databases
     */
    res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR select * from test1");
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR command failed\n");
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);
    res = PQexec(conn, "FETCH ALL in mycursor");
    if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
        PQclear(res);
        exit_nicely(conn);
    }
    i_fnum = PQfnumber(res, "i");
    d_fnum = PQfnumber(res, "d");
    p_fnum = PQfnumber(res, "p");
    for (i = 0; i < 3; i++)
    {
        printf("type[%d] = %d, size[%d] = %d\n",
               i, PQftype(res, i),
               i, PQfsize(res, i));
    }
    for (i = 0; i < PQntuples(res); i++)
    {
        int        *ival;
        float      *dval;
        int         plen;
        POLYGON    *pval;
        /* we hard-wire this to the 3 fields we know about */
        ival = (int *) PQgetvalue(res, i, i_fnum);
        dval = (float *) PQgetvalue(res, i, d_fnum);
        plen = PQgetlength(res, i, p_fnum);
        /*
         * plen doesn't include the length field so need to increment by
         * VARHDSZ
         */
        pval = (POLYGON *) malloc(plen + VARHDRSZ);
        pval->size = plen;
        memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen);
        printf("tuple %d: got\n", i);
        printf(" i = (%d bytes) %d,\n",
               PQgetlength(res, i, i_fnum), *ival);
        printf(" d = (%d bytes) %f,\n",
               PQgetlength(res, i, d_fnum), *dval);
        printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
               PQgetlength(res, i, d_fnum),
               pval->npts,
               pval->boundbox.xh,
               pval->boundbox.yh,
               pval->boundbox.xl,
               pval->boundbox.yl);
    }
    PQclear(res);
    /* close the cursor */
    res = PQexec(conn, "CLOSE mycursor");
    PQclear(res);
    /* commit the transaction */
    res = PQexec(conn, "COMMIT");
    PQclear(res);
    /* close the connection to the database and cleanup */
    PQfinish(conn);
}