postgres/src/man/libpq.3

.\" This is -*-nroff-*-
.\" XXX standard disclaimer belongs here....
.\" $Header: /cvsroot/pgsql/src/man/Attic/libpq.3,v 1.19 1998/07/09 03:30:49 scrappy Exp $
.TH LIBPQ INTRO 07/08/98 PostgreSQL PostgreSQL
.SH DESCRIPTION
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.
.PP
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.
.PP
There are several examples of Libpq applications in the following
directories:
.nf
\&../src/test/regress
\&../src/test/examples
\&../src/bin/psql
.fi
.PP
Frontend programs which use Libpq must include the header file
.B "libpq-fe.h"
and must link with the
.B libpq 
library.
.SH "Control and Initialization"
.PP
The following environment variables can be used to set up default
environment values to avoid hard-coding database names into
an application program:
.sp
\(bu
.B 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.
.sp
\(bu
.B PGOPTIONS
sets additional runtime options for the Postgres backend.
.sp
\(bu
.B PGPORT
sets the default port or local Unix domain socket file extension
for communicating with the Postgres backend.
.sp
\(bu
.B PGTTY
sets the file or tty on which debugging messages from the backend server
are displayed.
.sp
\(bu
.B PGDATABASE
sets the default Postgres database name.
.sp
\(bu
.B PGREALM
sets the
.I Kerberos
realm to use with Postgres, if it is different from the local realm.  If 
.B 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 
.I Kerberos
authentication is enabled.

.PP
The following environment variables can be used to specify user-level default behavior
for every Postgres session:
.sp
\(bu
.B PGDATESTYLE
sets the default style of date/time representation.
.sp
\(bu
.B PGTZ
sets the default time zone.

.PP
The following environment variables can be used to specify default internal
behavior for every Postgres session:
.sp
\(bu
.B PGGEQO
sets the default mode for the genetic optimizer.
.sp
\(bu
.B PGRPLANS
sets the default mode to allow or disable right-sided plans in the optimizer.
.sp
\(bu
.B PGCOSTHEAP
sets the default cost for heap searches for the optimizer.
.sp
\(bu
.B PGCOSTINDEX
sets the default cost for indexed searches for the optimizer.

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

.SH "Database Connection Functions"
.PP
The following routines deal with making a connection to a backend
from a C program.
.PP
.B PQsetdb
.br
.B PQsetdbLogin
.IP
Makes a new connection to a backend.
.B PQsetdb
is the method usually used to
connect to the database when username/password authentication is not
needed.
.nf
PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName); 
.fi

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

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. 
.IP
.I PQsetdb
and
.I PQsetdbLogin
always return a valid PGconn pointer.  The 
.I 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.
.IP

.B PQdb
returns the database name of the connection.
.nf
char *PQdb(PGconn *conn)
.fi

.B PQhost
returns the host name of the connection.
.nf
char *PQhost(PGconn *conn)
.fi

.B PQoptions
returns the pgoptions used in the connection.
.nf
char *PQoptions(PGconn *conn)
.fi

.B PQport
returns the pgport of the connection.
.nf
char *PQport(PGconn *conn)
.fi

.B PQtty
returns the pgtty of the connection.
.nf
char *PQtty(PGconn *conn)
.fi

.B PQstatus
Returns the status of the connection. The status can be CONNECTION_OK or
CONNECTION_BAD.  
.nf
ConnStatusType *PQstatus(PGconn *conn)
.fi

.B PQerrorMessage
returns the error message associated with the connection
.nf
char *PQerrorMessage(PGconn* conn);
.fi
.PP
.B PQfinish
.IP
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. 
.nf
void PQfinish(PGconn *conn)
.fi
.PP
.B PQreset
.IP
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.
.nf
void PQreset(PGconn *conn)
.fi
.PP
.SH "Query Execution Functions"
.PP
.B PQexec
.IP
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
.I PQerrorMessage
to get more information about the error.
.nf
PGresult *PQexec(PGconn *conn,
                 const char *query);
.fi
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. 
.PP
.B PQresultStatus
.IP
Returns the result status of the query.
.I PQresultStatus
can return one of the following values:
.nf
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
.fi
.IP
If the result status is PGRES_TUPLES_OK, then the following routines can
be used to retrieve the tuples returned by the query.
.IP

.B PQntuples
returns the number of tuples (instances) in the query result.

.nf
int PQntuples(PGresult *res);
.fi

.B PQnfields
returns the number of fields (attributes) in the query result.
.nf
int PQnfields(PGresult *res);
.fi

.B PQfname
returns the field (attribute) name associated with the given field index.
Field indices start at 0.
.nf
char *PQfname(PGresult *res,
             int field_index);
.fi

.B PQfnumber
returns the field (attribute) index associated with the given field name.
.nf
int PQfnumber(PGresult *res,
             char* field_name);
.fi

.B 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.
.nf
Oid PQftype(PGresult *res,
            int field_num);
.fi

.B 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. 
.nf
short PQfsize(PGresult *res,
              int field_index);
.fi

.B PQfmod
returns the type-specific modification data of the field
associated with the given field index.
Field indices start at 0.
.nf
short PQfmod(PGresult *res,
             int field_index);
.fi

.B PQgetvalue
returns the field (attribute) value.  For most queries, the value
returned by 
.I PQgetvalue
is a null-terminated ASCII string representation
of the attribute value.  If the query was a result of a 
.B BINARY
cursor, then the value returned by
.I 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 
.I 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.
.nf
char* PQgetvalue(PGresult *res,
                 int tup_num,
                 int field_num);
.fi

.B PQgetlength
returns the length of a field (attribute) in bytes.  If the field
is a
.I "struct varlena" ,
the length returned here does 
.B not
include the size field of the varlena, i.e., it is 4 bytes less.
.nf
int PQgetlength(PGresult *res,
                int tup_num,
                int field_num);
.fi

.B PQgetisnull
returns the NULL status of a field.
.nf
int PQgetisnull(PGresult *res,
                int tup_num,
                int field_num);
.fi

.PP
.B PQcmdStatus
.IP 
Returns the command status associated with the last query command.
.nf
char *PQcmdStatus(PGresult *res);
.fi

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

.PP
.B PQoidStatus
.IP
Returns a string with the object id of the tuple inserted if the last
query is an INSERT command.  Otherwise, returns an empty string.
.nf
char* PQoidStatus(PGresult *res);
.fi
.PP
.B PQprint
.IP
+ Prints out all the tuples in an intelligent manner. The
.B psql
+ program uses this function for its output.
.nf
void PQprint(
      FILE* fout,      /* output stream */
      PGresult* res,   /* query results */
      PQprintOpt *ps   /* option structure */
        );

.fi
.I PQprintOpt
is a typedef'ed structure as defined below.
.(C
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;
.fi
.LP
.B PQclear
.IP
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. 
.nf
void PQclear(PQresult *res);
.fi

.PP
.SH "Asynchronous Query Processing"
.PP
The PQexec function is adequate for submitting queries in simple synchronous
applications.  It has a couple of major deficiencies however:
.IP
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.
.IP
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.)
.IP
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.

.PP
Applications that do not like these limitations can instead use the
underlying functions that PQexec is built from: PQsendQuery and
PQgetResult.

.PP
.B PQsendQuery
.IP
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).
.nf
int PQsendQuery(PGconn *conn,
                const char *query);
.fi
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.

.PP
.B PQgetResult
.IP
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.
.nf
PGresult *PQgetResult(PGconn *conn);
.fi
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.

.PP
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:

.PP
.B PQconsumeInput
.IP
If input is available from the backend, consume it.
.nf
void PQconsumeInput(PGconn *conn);
.fi
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.

.PP
.B PQisBusy
.IP
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.
.nf
int PQisBusy(PGconn *conn);
.fi
PQisBusy will not itself attempt to read data from the backend;
therefore PQconsumeInput must be invoked first, or the busy
state will never end.

.PP
.B PQsocket
.IP
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.
.nf
int PQsocket(PGconn *conn);
.fi
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.

.PP
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.

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

.PP
.B PQrequestCancel
.IP
Request that <ProductName>Postgres</ProductName> abandon
processing of the current query.
.nf
int PQrequestCancel(PGconn *conn);
.fi
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.

.PP
Note that if the current query is part of a transaction, cancellation
will abort the whole transaction.

.PP
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.


.PP
.SH "Fast Path"
.PP
Postgres provides a 
.B "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. 
.nf
PGresult* PQfn(PGconn* conn,
	       int fnid, 
	       int *result_buf, 
	       int *result_len,
	       int result_is_int,
	       PQArgBlock *args, 
	       int nargs);
.fi
.PP
The
.I fnid
argument is the object identifier of the function to be executed.
.I 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 
.I result_len.
If the result is to be an integer value, than 
.I result_is_int
should be set to 1; otherwise it should be set to 0.
.I args
and 
.I nargs
specify the arguments to the function.
.nf
typedef struct {
    int len;
    int isint;
    union {
        int *ptr;	
	int integer;
    } u;
} PQArgBlock;
.fi
.PP
.I 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 
.I PQclear
when it is no longer needed.
.PP
.SH "Asynchronous Notification"
.PP
Postgres supports asynchronous notification via the 
.I LISTEN
and
.I 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.
.PP
libpq applications submit LISTEN commands as ordinary
SQL queries.  Subsequently, arrival of NOTIFY messages can be detected by
calling PQnotifies().
.PP
.B PQNotifies
.IP
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.
.nf
PGnotify* PQNotifies(PGconn *conn);
.fi
After processing a PGnotify object returned by PQnotifies,
be sure to free it with free() to avoid a memory leak.
.PP
The second sample program gives an example of the use of asynchronous
notification.
.PP
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.

.PP
.SH "Functions Associated with the COPY Command"
.PP
The
.I 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.
.PP
These functions should be executed only after obtaining a PGRES_COPY_OUT
or PGRES_COPY_IN result object from PQexec or PQgetResult.
.PP
.B PQgetline
.IP
Reads a newline-terminated line of characters (transmitted by the
backend server) into a buffer 
.I string 
of size
.I length .
Like
.I fgets(3),
this routine copies up to 
.I length "-1"
characters into 
.I string .
It is like 
.I gets(3),
however, in that it converts the terminating newline into a null
character.
.IP
.I 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.
.IP
Notice that the application must check to see if a new line consists
of the two characters \*(lq\\.\*(rq, which indicates that the backend
server has finished sending the results of the 
.I copy
command.  Therefore, if the application ever expects to receive lines
that are more than
.I length "-1"
characters long, the application must be sure to check the return
value of 
.I PQgetline
very carefully.
.IP
The code in
.nf
\&../src/bin/psql/psql.c
.fi
contains routines that correctly handle the copy protocol.
.nf
int PQgetline(PGconn *conn,
              char *string,
              int length)
.fi
.PP
.B PQputline
.IP
Sends a null-terminated 
.I string
to the backend server.
.IP
The application must explicitly send the two characters \*(lq\\.\*(rq
on a final line
to indicate to the backend that it has finished sending its data.
.nf
void PQputline(PGconn *conn,
               char *string);
.fi
.PP
.B PQendcopy
.IP
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
.I PQputline
or when the last string has been received from the backend using
.I PGgetline .
It must be issued or the backend may get \*(lqout of sync\*(rq with
the frontend.  Upon return from this function, the backend is ready to
receive the next query.
.IP
The return value is 0 on successful completion, nonzero otherwise.
.nf
int PQendcopy(PGconn *conn);
.fi
As an example:
.nf
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\en");
PQputline(conn,"4<TAB>goodbye world<TAB>7.11\en");
\&...
PQputline(conn,"\\.\en");
PQendcopy(conn);
.fi
.PP
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.

.PP
.SH "LIBPQ Tracing Functions"
.PP
.B PQtrace
.IP
Enable tracing of the frontend/backend communication to a debugging file
stream. 
.nf
void PQtrace(PGconn *conn
             FILE *debug_port)
.fi
.PP
.B PQuntrace 
.IP
Disable tracing started by 
.I PQtrace
.nf
void PQuntrace(PGconn *conn)
.fi
.PP
.SH "User Authentication Functions"
.PP
If the user has generated the appropriate authentication credentials
(e.g., obtaining
.I Kerberos
tickets), the frontend/backend authentication process is handled by
.I 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.
.PP
.B fe_getauthname
.IP
Returns a pointer to static space containing whatever name the user
has authenticated.  Use of this routine in place of calls to
.I getenv(3)
or 
.I getpwuid(3)
by applications is highly recommended, as it is entirely possible that
the authenticated user name is 
.B not
the same as value of the
.B USER
environment variable or the user's entry in
.I /etc/passwd .
.nf
char *fe_getauthname(char* errorMessage)
.fi
.PP
.B fe_setauthsvc
.IP
Specifies that Libpq should use authentication service
.I name
rather than its compiled-in default.  This value is typically taken
from a command-line switch.
.nf
void fe_setauthsvc(char *name,
                   char* errorMessage)
.fi
Any error messages from the authentication attempts are returned in the
errorMessage argument.
.PP
.SH "BUGS"
.PP
The query buffer is 8192 bytes long, and queries over that length will
be rejected.
.PP
.SH "Sample Programs"
.bp
.SH "Sample Program 1"
.PP
.nf M
/*
 * 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 (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 (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 (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); */
}

.fi
.bp
.SH "Sample Program 2"
.PP
.nf M
/*
 * 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 (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);

}

.fi
.bp
.SH "Sample Program 3"
.PP
.nf M
/*
 * 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 (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 (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 (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);

}
.fi