[ TOC ] [ Previous ] [ Next ]

# 12. LIBPQ

LIBPQ is the application programming 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.

## 12.1. Control and Initialization

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

- PGHOST sets the default server name.
- PGOPTIONS sets additional runtime options for the POSTGRES backend.
- PGPORT sets the default port 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.

## 12.2. Database Connection Functions

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)

PQtrace
  Enables tracing of messages passed between the frontend and the backend. The messages are echoed to the debug_port file stream.

                    void PQtrace(PGconn *conn,
                                 FILE* debug_port);

PQuntrace
  Disables tracing of messages passed between the frontend and the backend.

                    void PQuntrace(PGconn *conn);

## 12.3. Query Execution Functions

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);

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);

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);

PQprintTuples Prints out all the tuples and, optionally, the attribute names to the specified output stream. The programs psql and monitor both use PQprintTuples for output.

                  void PQprintTuples(
                         PGresult* res,
                         FILE* fout,      /* output stream */
                         int printAttName,/* print attribute names or not*/
                         int terseOutput, /* delimiter bars or not?*/
                         int width        /* width of column, variable width if 0*/
                         );

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.

                  void PQclear(PQresult *res);

## 12.4. Fast Path

POSTGRES provides a fast path interface to send function calls to the backend. This is a trapdoor into system internals and can be a potential security hole. Most users will not need this feature.

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

The fnid argument is the object identifier of the function to be executed. result_buf is the buffer in which to load the return value. The caller must have allocated sufficient space to store the return value. The result length will be returned in the storage pointed to by result_len. If the result is to be an integer value, than result_is_int should be set to 1; otherwise it should be set to 0. args and nargs specify the arguments to the function.

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

PQfn always returns a valid PGresult*. The resultStatus should be checked before the result is used. The caller is responsible for freeing the PGresult with PQclear when it is not longer needed.

## 12.5. Asynchronous Notification

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.

## 12.6. 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.

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 single character ".", 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 single character "." 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);

## 12.7. LIBPQ Tracing Functions

PQtrace
  Enable tracing of the frontend/backend communication to a debugging file stream.

                    void PQtrace(PGconn *conn
                                 FILE *debug_port)

PQuntrace
  Disable tracing started by PQtrace

                    void PQuntrace(PGconn *conn)

## 12.8. User Authentication Functions

If the user has generated the appropriate authentication credentials (e.g., obtaining Kerberos tickets), the frontend/backend authentication process is handled by PQexec without any further intervention. The following routines may be called by LIBPQ programs to tailor the behavior of the authentication process.

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.

## 12.9. BUGS

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

## 12.10. Sample Programs

### 12.10.1. 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.0, 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 failed0);
                 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 myportal CURSOR FOR select * from pg_database");
               if (PQresultStatus(res) != PGRES_COMMAND_OK) {
                 fprintf(stderr,"DECLARE CURSOR command failed0);
                 PQclear(res);
                 exit_nicely(conn);
               }
               PQclear(res);

               res = PQexec(conn,"FETCH ALL in myportal");
               if (PQresultStatus(res) != PGRES_TUPLES_OK) {
                 fprintf(stderr,"FETCH ALL command didn't return tuples properly0);
                 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("0);

               /* 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("0);
               }

               PQclear(res);

               /* close the portal */
               res = PQexec(conn, "CLOSE myportal");
               PQclear(res);

               /* end the transaction */
               res = PQexec(conn, "END");
               PQclear(res);

               /* close the connection to the database and cleanup */
               PQfinish(conn);

             /*   fclose(debug); */
             }

### 12.10.2. Sample Program 2

             /*
              * testlibpq2.c
              *   Test of the asynchronous notification interface
              *
                populate a database with the following:

             CREATE TABLE TBL1 (i int4);

             CREATE TABLE TBL2 (i int4);

             CREATE RULE r1 AS ON INSERT TO TBL1 DO [INSERT INTO TBL2 values (new.i); NOTIFY TBL2];

              * Then start up this program
              * After the program has begun, do

             INSERT INTO TBL1 values (10);

              *
              *
              */
             #include <stdio.h>
             #include "libpq-fe.h"

             void exit_nicely(PGconn* conn)
             {
               PQfinish(conn);
               exit(1);
             }

             main()
             {
               char *pghost, *pgport, *pgoptions, *pgtty;
               char* dbName;
               int nFields;
               int i,j;

               PGconn* conn;
               PGresult* res;
               PGnotify* notify;

               /* begin, by setting the parameters for a backend connection
                  if the parameters are null, then the system will try to use
                  reasonable defaults by looking up environment variables
                  or, failing that, using hardwired constants */
               pghost = NULL;  /* host name of the backend server */
               pgport = NULL;  /* port of the backend server */
               pgoptions = NULL; /* special options to start up the backend server */
               pgtty = NULL;     /* debugging tty for the backend server */
               dbName = getenv("USER"); /* change this to the name of your test database*/

               /* make a connection to the database */
               conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

               /* check to see that the backend connection was successfully made */
               if (PQstatus(conn) == CONNECTION_BAD) {
                 fprintf(stderr,"Connection to database '%s' failed.0, dbName);
                 fprintf(stderr,"%s",PQerrorMessage(conn));
                 exit_nicely(conn);
               }

               res = PQexec(conn, "LISTEN TBL2");
               if (PQresultStatus(res) != PGRES_COMMAND_OK) {
                 fprintf(stderr,"LISTEN command failed0);
                 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 = %s0, pgresStatus[PQresultStatus(res)]); */
                   /* check for asynchronous returns */
                   notify = PQnotifies(conn);
                   if (notify) {
                    fprintf(stderr,
                         "ASYNC NOTIFY of '%s' from backend pid '%d' received0,
                         notify->relname, notify->be_pid);
                    free(notify);
                    break;
                   }
                   PQclear(res);
               }

               /* close the connection to the database and cleanup */
               PQfinish(conn);

             }

### 12.10.3. 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.0, 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 failed0);
                 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 failed0);
                 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 properly0);
                 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] = %d0,
                       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: got0, i);
                 printf(" i = (%d bytes) %d,0,
                     PQgetlength(res,i,i_fnum), *ival);
                 printf(" d = (%d bytes) %f,0,
                     PQgetlength(res,i,d_fnum), *dval);
                 printf(" p = (%d bytes) %d points boundbox = (hi=%f/%f, lo = %f,%f)0,
                     PQgetlength(res,i,d_fnum),
                     pval->npts,
                     pval->boundbox.xh,
                     pval->boundbox.yh,
                     pval->boundbox.xl,
                     pval->boundbox.yl);
               }

               PQclear(res);

               /* close the portal */
               res = PQexec(conn, "CLOSE mycursor");
               PQclear(res);

               /* end the transaction */
               res = PQexec(conn, "END");
               PQclear(res);

               /* close the connection to the database and cleanup */
               PQfinish(conn);

             }

[ TOC ] [ Previous ] [ Next ]