GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
RedHat 5.2(Apollo) |
|
 |
libpq(3) |
 |
Libpq is the programmer’s interface to Postgres. Libpq is a set of library routines that allows queries to pass to the Postgres backend and instances to return through an IPC channel.
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.
• 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.
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
Makes a new
connection to a backend.
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
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 always returns 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 if the query was
successful or a NULL otherwise. If a NULL is returned,
PQerrorMessage can be used to get more information
about the error.
PGresult *PQexec(PGconn *conn,
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 */
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);
PQcmdTuples returns the number of tuples (instances) affected by INSERT, UPDATE, and DELETE queries.
char *PQcmdTuples(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.
int2 PQfsize(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);
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);
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 not longer needed.
Postgres supports asynchronous notification via the LISTEN and NOTIFY commands. A backend registers its interest in a particular relation with the LISTEN command. All backends listening on a particular relation will be notified asynchronously when a NOTIFY of that relation name is executed by another backend. No additional information is passed from the notifier to the listener. Thus, typically, any actual data that needs to be communicated is transferred through the relation.
Libpq applications are notified whenever a connected backend has received an asynchronous notification. However, the communication from the backend to the frontend is not asynchronous. Notification comes piggy-backed on other query results. Thus, an application must submit queries, even empty ones, in order to receive notice of backend notification. In effect, the Libpq application must poll the backend to see if there is any pending notification information. After the execution of a query, a frontend may call PQNotifies to see if any notification data is available from the backend.
PQNotifies
returns the
notification from a list of unhandled notifications from the
backend. Returns NULL if there are no pending notifications
from the backend. PQNotifies behaves like the popping
of a stack. Once a notification is returned from
PQnotifies, it is considered handled and will be
removed from the list of notifications.
PGnotify* PQNotifies(PGconn *conn);
The second sample program gives an example of the use of asynchronous notification.
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.
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 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 characters “\.” 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 char16, 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);
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)
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 silently truncated.
/*
* testlibpq.c
*
*
*/
#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); */
}
/*
* 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) {
/* async notification only come back as a result of a
query*/
/* we can send empty queries */
res = PQexec(conn, "");
/* printf("res->status = %s\n",
pgresStatus[PQresultStatus(res)]); */
/* check for asynchronous returns */
notify = PQnotifies(conn);
if (notify) {
|
fprintf(stderr, | |
|
"ASYNC NOTIFY of ’%s’ from backend pid ’%d’ received\n", | |
|
notify->relname, notify->be_pid); | |
|
free(notify); | |
|
break; |
}
PQclear(res);
}
/* 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 (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++) {
}
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);
|
PQgetlength(res,i,i_fnum), *ival); | ||
|
PQgetlength(res,i,d_fnum), *dval); | ||
|
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) | |