Computer Science
LIBPQ(INTRO) PostgreSQL LIBPQ(INTRO)
DESCRIPTION
Current documentation for this topic is available in the
new Programmer's Guide chapter on libpq. This man page is
obsolete, though in sync with the Programmer's Guide as of
1998/08/15.
Libpq is the programmer's interface to Postgres. Libpq is
a set of library routines which allows client programs to
pass queries to the Postgres backend server and to receive
the results of these queries.
This version of the documentation describes the C inter-
face library. Three short programs are included at the
end of this section to show how to write programs that use
Libpq.
There are several examples of Libpq applications in the
following directories:
../src/test/regress
../src/test/examples
../src/bin/psql
Frontend programs which use Libpq must include the header
file libpq-fe.h and must link with the libpq library.
Control and Initialization
The following environment variables can be used to set up
default environment values to avoid hard-coding database
names into an application program:
o 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 sock-
ets.
o PGUSER sets the username used to connect to the database
and for authentication.
o PGOPTIONS sets additional runtime options for the Post-
gres backend.
o PGPORT sets the default port or local Unix domain socket
file extension for communicating with the Postgres back-
end.
o PGTTY sets the file or tty on which debugging messages
from the backend server are displayed.
o PGDATABASE sets the default Postgres database name.
o 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:
o PGDATESTYLE sets the default style of date/time repre-
sentation.
o PGTZ sets the default time zone.
The following environment variables can be used to specify
default internal behavior for every Postgres session:
o PGGEQO sets the default mode for the genetic optimizer.
o PGRPLANS sets the default mode to allow or disable
right-sided plans in the optimizer.
o PGCOSTHEAP sets the default cost for heap searches for
the optimizer.
o PGCOSTINDEX sets the default cost for indexed searches
for the optimizer. o PGQUERY_LIMIT sets the maximum num-
ber of rows returned by a query.
See the set(l) man page for information on the arguments
for these environment variables.
Database Connection Functions
The following routines deal with making a connection to a
backend from a C program.
PQsetdb
PQsetdbLogin
Makes a new connection to a backend. PQsetdb is
the method usually used to connect to the database
when username/password authentication is not
needed.
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
PQsetdbLogin is the method used to connect to the
database when username/password authentication is
needed.
PGconn *PQsetdbLogin(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName,
char *login,
char *pwd);
If any argument is NULL, then the corresponding
environment variable is checked. If the environ-
ment 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 con-
nection. 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 connec-
tion.
char *PQoptions(PGconn *conn)
PQport returns the pgport of the connection.
char *PQport(PGconn *conn)
PQtty returns the pgtty of the connection.
char *PQtty(PGconn *conn)
PQstatus Returns the status of the connection. The
status can be CONNECTION_OK or CONNECTION_BAD.
ConnStatusType *PQstatus(PGconn *conn)
PQerrorMessage returns the error message associated
with the connection
char *PQerrorMessage(PGconn* conn);
PQfinish
Close the connection to the backend. Also frees
memory used by the PGconn structure. The PGconn
pointer should not be used after PQfinish has been
called.
void PQfinish(PGconn *conn)
PQreset
Reset the communication port with the backend.
This function will close the IPC socket connection
to the backend and attempt to reestablish a new
connection to the same backend.
void PQreset(PGconn *conn)
Query Execution Functions
PQexec
Submit a query to Postgres. Returns a PGresult
pointer or possibly a NULL pointer. If a NULL is
returned, it should be treated like a
PGRES_FATAL_ERROR result: use PQerrorMessage to get
more information about the error.
PGresult *PQexec(PGconn *conn,
const char *query);
The PGresult structure encapsulates the query
result returned by the backend. Libpq programmers
should be careful to maintain the PGresult abstrac-
tion. 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. PQresult-
Status 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 PQresult-
Status 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 opera-
tions are done. Use PQresultErrorMessage when you
want to know the status associated with a particu-
lar 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 associ-
ated with the given field index. Field indices
start at 0.
char *PQfname(PGresult *res,
int field_index);
PQfnumber returns the field (attribute) index asso-
ciated 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 PQget-
value 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 man-
ner. The psql + program uses this function for its
output.
void PQprint(
FILE* fout, /* output stream */
PGresult* res, /* query results */
PQprintOpt *ps /* option structure */
);
PQprintOpt is a typedef'ed structure as defined
below. typedef struct _PQprintOpt {
bool header; /* print table headings
and row count */
bool align; /* fill align the fields
*/
bool standard; /* old brain dead format
(needs align) */
bool html3; /* output html3+ tables
*/
bool expanded; /* expand tables */
bool pager; /* use pager if needed
*/
char *fieldSep; /* field separator */
char *caption; /* html table caption
(or NULL) */
char **fieldName; /* null terminated array
of field names (or NULL) */ } PQprintOpt;
PQclear
Frees the storage associated with the PGresult.
Every query result should be properly freed when it
is no longer used. Failure to do this will result
in memory leaks in the frontend application. The
PQresult* passed in should be a value which is
returned from PQexec(). Calling PQclear() on an
uninitialized PQresult pointer will very likely
result in a core dump.
void PQclear(PQresult *res);
Asynchronous Query Processing
The PQexec function is adequate for submitting queries in
simple synchronous applications. It has a couple of major
deficiencies however:
PQexec waits for the query to be completed. The
application may have other work to do (such as
maintaining a user interface), in which case it
won't want to block waiting for the response.
Since control is buried inside PQexec, it is hard
for the frontend to decide it would like to try to
cancel the ongoing query. (It can be done from a
signal handler, but not otherwise.)
PQexec can return only one PGresult structure. If
the submitted query string contains multiple SQL
commands, all but the last PGresult are discarded
by PQexec.
Applications that do not like these limitations can
instead use the underlying functions that PQexec is built
from: PQsendQuery and PQgetResult.
PQsendQuery
Submit a query to Postgres without waiting for the
result(s). TRUE is returned if the query was suc-
cessfully 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 PQge-
tResult 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 PQgetRe-
sult 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 com-
mands, the results of those commands can be obtained indi-
vidually. (This allows a simple form of overlapped pro-
cessing, 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 PQcon-
sumeInput, 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 PQcon-
sumeInput to clear the select condition immedi-
ately, and then examine the results at leisure.
PQisBusy
Returns TRUE if a query is busy, that is, PQgetRe-
sult 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 connec-
tion 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 mes-
sages (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 PQgetRe-
sult. If the cancellation is effective, the cur-
rent 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 han-
dler. 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 PQrequest-
Cancel from a SIGINT signal handler, thus allowing inter-
active cancellation of queries that it issues through
PQexec. Note that PQrequestCancel will have no effect if
the connection is not currently open or the backend is not
currently processing a query.
Fast Path
Postgres provides a fast path interface to send function
calls to the backend. This is a trapdoor into system
internals and can be a potential security hole. Most
users will not need this feature.
PGresult* PQfn(PGconn* conn,
int fnid,
int *result_buf,
int *result_len,
int result_is_int,
PQArgBlock *args,
int nargs);
The fnid argument is the object identifier of the function
to be executed. result_buf is the buffer in which to load
the return value. The caller must have allocated suffi-
cient space to store the return value. The result length
will be returned in the storage pointed to by result_len.
If the result is to be an integer value, than
result_is_int should be set to 1; otherwise it should be
set to 0. args and nargs specify the arguments to the
function.
typedef struct {
int len;
int isint;
union {
int *ptr;
int integer;
} u;
} PQArgBlock;
PQfn always returns a valid PGresult*. The resultStatus
should be checked before the result is used. The caller
is responsible for freeing the PGresult with PQclear when
it is no longer needed.
Asynchronous Notification
Postgres supports asynchronous notification via the LISTEN
and NOTIFY commands. A backend registers its interest in
a particular notification condition with the LISTEN com-
mand. All backends listening on a particular condition
will be notified asynchronously when a NOTIFY of that con-
dition 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 communi-
cated is transferred through a database relation. Com-
monly 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 PQnoti-
fies() 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 PQnoti-
fies() after each PQgetResult or PQexec to see if any
notifications came in during the processing of the query.
Functions Associated with the COPY Command
The copy command in Postgres has options to read from or
write to the network connection used by Libpq. Therefore,
functions are necessary to access this network connection
directly so applications may take full advantage of this
capability.
These functions should be executed only after obtaining a
PGRES_COPY_OUT or PGRES_COPY_IN result object from PQexec
or PQgetResult.
PQgetline
Reads a newline-terminated line of characters
(transmitted by the backend server) into a buffer
string of size length . Like fgets(3), this rou-
tine 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. There-
fore, 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 char-
acters "\." 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 PQge-
tResult 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 cor-
rectly. Older applications are likely to submit a copy in
or copy out via PQexec and assume that the transaction is
done after PQendcopy. This will work correctly only if
the copy in/out is the only SQL command in the query
string.
LIBPQ Tracing Functions
PQtrace
Enable tracing of the frontend/backend communica-
tion to a debugging file stream.
void PQtrace(PGconn *conn
FILE *debug_port)
PQuntrace
Disable tracing started by PQtrace
void PQuntrace(PGconn *conn)
LIBPQ Control Functions
PQsetNoticeProcessor
Control reporting of notice and warning messages
generated by libpq.
void PQsetNoticeProcessor (PGconn * conn,
void (*noticeProcessor) (void * arg, const char * message),
void * arg)
By default, libpq prints "notice" messages from the
backend on stderr, as well as a few error messages
that it generates by itself. This behavior can be
overridden by supplying a callback function that
does something else with the messages. The call-
back function is passed the text of the error mes-
sage (which includes a trailing newline), plus a
void pointer that is the same one passed to PQset-
NoticeProcessor. (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 PQsetNoti-
ceProcessor just after any creation of a new PGconn
object.
User Authentication Functions
If the user has generated the appropriate authentication
credentials (e.g., obtaining Kerberos tickets), the fron-
tend/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 what-
ever name the user has authenticated. Use of this
routine in place of calls to getenv(3) or getp-
wuid(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 environ-
ment variable or the user's entry in /etc/passwd .
char *fe_getauthname(char* errorMessage)
fe_setauthsvc
Specifies that Libpq should use authentication ser-
vice name rather than its compiled-in default.
This value is typically taken from a command-line
switch.
void fe_setauthsvc(char *name,
char* errorMessage)
Any error messages from the authentication attempts
are returned in the errorMessage argument.
BUGS
The query buffer is 8192 bytes long, and queries over that
length will be rejected.
Sample Programs
Sample Program 1
/*
* testlibpq.c
* Test the C version of Libpq, the Postgres frontend library.
*
*
*/
#include <stdio.h>
#include "libpq-fe.h"
void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
main()
{
char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
/* FILE *debug; */
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* debugging tty for the backend server */
dbName = "template1";
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/* check to see that the backend connection was successfully made */
if (PQstatus(conn) == CONNECTION_BAD)
{
fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* debug = fopen("/tmp/trace.out","w"); */
/* PQtrace(conn, debug); */
/* start a transaction block */
res = PQexec(conn, "BEGIN");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
/*
* fetch instances from the pg_database, the system catalog of
* databases
*/
res = PQexec(conn, "DECLARE mycursor CURSOR FOR select * from pg_database");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in mycursor");
if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
PQclear(res);
exit_nicely(conn);
}
/* first, print out the attribute names */
nFields = PQnfields(res);
for (i = 0; i < nFields; i++)
printf("%-15s", PQfname(res, i));
printf("\n\n");
/* next, print out the instances */
for (i = 0; i < PQntuples(res); i++)
{
for (j = 0; j < nFields; j++)
printf("%-15s", PQgetvalue(res, i, j));
printf("\n");
}
PQclear(res);
/* close the cursor */
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
/* commit the transaction */
res = PQexec(conn, "COMMIT");
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
/* fclose(debug); */
}
PostgreSQL 08/08/98 1
LIBPQ(INTRO) PostgreSQL LIBPQ(INTRO)
Sample Program 2
/*
* 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);
}
PostgreSQL 08/08/98 2
LIBPQ(INTRO) PostgreSQL LIBPQ(INTRO)
Sample Program 3
/*
* testlibpq3.c
* Test the C version of Libpq, the Postgres frontend library.
* tests the binary cursor interface
*
*
*
populate a database by doing the following:
CREATE TABLE test1 (i int4, d float4, p polygon);
INSERT INTO test1 values (1, 3.567, '(3.0, 4.0, 1.0, 2.0)'::polygon);
INSERT INTO test1 values (2, 89.05, '(4.0, 3.0, 2.0, 1.0)'::polygon);
the expected output is:
tuple 0: got
i = (4 bytes) 1,
d = (4 bytes) 3.567000,
p = (4 bytes) 2 points boundbox = (hi=3.000000/4.000000, lo = 1.000000,2.000000)
tuple 1: got
i = (4 bytes) 2,
d = (4 bytes) 89.050003,
p = (4 bytes) 2 points boundbox = (hi=4.000000/3.000000, lo = 2.000000,1.000000)
*
*/
#include <stdio.h>
#include "libpq-fe.h"
#include "utils/geo-decls.h" /* for the POLYGON type */
void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
main()
{
char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
int i_fnum,
d_fnum,
p_fnum;
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* debugging tty for the backend server */
dbName = getenv("USER"); /* change this to the name of your test
* database */
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/* check to see that the backend connection was successfully made */
if (PQstatus(conn) == CONNECTION_BAD)
{
fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* start a transaction block */
res = PQexec(conn, "BEGIN");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
/*
* fetch instances from the pg_database, the system catalog of
* databases
*/
res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR select * from test1");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in mycursor");
if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
PQclear(res);
exit_nicely(conn);
}
i_fnum = PQfnumber(res, "i");
d_fnum = PQfnumber(res, "d");
p_fnum = PQfnumber(res, "p");
for (i = 0; i < 3; i++)
{
printf("type[%d] = %d, size[%d] = %d\n",
i, PQftype(res, i),
i, PQfsize(res, i));
}
for (i = 0; i < PQntuples(res); i++)
{
int *ival;
float *dval;
int plen;
POLYGON *pval;
/* we hard-wire this to the 3 fields we know about */
ival = (int *) PQgetvalue(res, i, i_fnum);
dval = (float *) PQgetvalue(res, i, d_fnum);
plen = PQgetlength(res, i, p_fnum);
/*
* plen doesn't include the length field so need to increment by
* VARHDSZ
*/
pval = (POLYGON *) malloc(plen + VARHDRSZ);
pval->size = plen;
memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen);
printf("tuple %d: got\n", i);
printf(" i = (%d bytes) %d,\n",
PQgetlength(res, i, i_fnum), *ival);
printf(" d = (%d bytes) %f,\n",
PQgetlength(res, i, d_fnum), *dval);
printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
PQgetlength(res, i, d_fnum),
pval->npts,
pval->boundbox.xh,
pval->boundbox.yh,
pval->boundbox.xl,
pval->boundbox.yl);
}
PQclear(res);
/* close the cursor */
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
/* commit the transaction */
res = PQexec(conn, "COMMIT");
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
}
PostgreSQL 08/08/98 3
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