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libpq(3) |
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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 This man page is obsolete, though in sync with the Programmer’s Guide as of 1998/08/15.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.
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.
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)
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 routines described below can be used to retrieve the tuples returned by the query. Note that a SELECT that happens to retrieve zero tuples still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return tuples.
PQresStatus
Converts the
enumerated type returned by PQresultStatus into a string
constant describing the status code.
const char *PQresStatus(ExecStatusType status);
Older code may
perform this same operation by direct access to a constant
string array inside libpq,
extern const char * const pgresStatus[];
However, using the function is recommended instead, since it is more portable and will not fail on out-of-range values.
PQresultErrorMessage
returns the
error message associated with the query, or an empty string
if there was no error.
const char *PQresultErrorMessage(PGresult *res);
Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will return the same string as PQresultErrorMessage (on the result). However, a PGresult will retain its error message until destroyed, whereas the connection’s error message will change when subsequent operations are done. Use PQresultErrorMessage when you want to know the status associated with a particular PGresult; use PQerrorMessage when you want to know the status from the latest operation on the connection.
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);
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).
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.
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 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.
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.
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 char(16), d
float8)");
PQexec(conn, "copy foo from stdin");
PQputline(conn, "3\thello world\t4.5\n");
PQputline(conn,"4\tgoodbye world\t7.11\n");
...
PQputline(conn,"\\.\n");
PQendcopy(conn);
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.
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)
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.
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.
The query buffer is 8192 bytes long, and queries over that length will be rejected.
/*
* 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); */
}
/*
* testlibpq2.c
* Test of the asynchronous notification interface
*
* Start this program, then from psql in another window do
* NOTIFY TBL2;
*
* Or, if you want to get fancy, try this:
* 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);
*
* and 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);
}
/*
* 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);
}
|
libpq(3) | |