关于libpq各个函数接口的说明，参考：http://www.postgres.cn/docs/9.3/libpq.html
例子1：
/* * testlibpq.c * * Test the C version of libpq,the Postgresql frontend library. */ #include <stdio.h> #include <stdlib.h> #include <libpq-fe.h> static void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } int main(int argc,char **argv) { const char *conninfo; PGconn *conn; PGresult *res; int nFields; int i,j; /* * If the user supplies a parameter on the command line,use it as the * conninfo string; otherwise default to setting dbname=postgres and using * environment variables or defaults for all other connection parameters. */ if (argc > 1) conninfo = argv[1]; else conninfo = "dbname = postgres"; /* Make a connection to the database */ conn = PQconnectdb(conninfo); /* Check to see that the backend connection was successfully made */ if (PQstatus(conn) != CONNECTION_OK) { fprintf(stderr,"Connection to database Failed: %s",PQerrorMessage(conn)); exit_nicely(conn); } /* * Our test case here involves using a cursor,for which we must be inside * a transaction block. We could do the whole thing with a single * PQexec() of "select * from pg_database",but that's too trivial to make * a good example. */ /* Start a transaction block */ res = PQexec(conn,"BEGIN"); if (PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr,"BEGIN command Failed: %s",PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } /* * Should PQclear PGresult whenever it is no longer needed to avoid memory * leaks */ PQclear(res); /* * Fetch rows from 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 Failed: %s",PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } PQclear(res); res = PQexec(conn,"FETCH ALL in myportal"); if (PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr,"FETCH ALL Failed: %s",PQerrorMessage(conn)); 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 rows */ 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 portal ... we don't bother to check for errors ... */ 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); return 0; }
例子2：
/* * testlibpq2.c * Test of the asynchronous notification interface * * Start this program,then from psql in another window do * NOTIFY TBL2; * Repeat four times to get this program to exit. * * Or,if you want to get fancy,try this: * populate a database with the following commands * (provided in src/test/examples/testlibpq2.sql): * * 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 this four times: * * INSERT INTO TBL1 VALUES (10); */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h> #include <sys/time.h> #include <libpq-fe.h> static void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } int main(int argc,char **argv) { const char *conninfo; PGconn *conn; PGresult *res; PGnotify *notify; int nnotifies; /* * If the user supplies a parameter on the command line,PQerrorMessage(conn)); exit_nicely(conn); } /* * Issue LISTEN command to enable notifications from the rule's NOTIFY. */ res = PQexec(conn,"LISTEN TBL2"); if (PQresultStatus(res) != PGRES_COMMAND_OK) { fprintf(stderr,"LISTEN command Failed: %s",PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } /* * should PQclear PGresult whenever it is no longer needed to avoid memory * leaks */ PQclear(res); /* Quit after four notifies are received. */ nnotifies = 0; while (nnotifies < 4) { /* * Sleep until something happens on the connection. We use select(2) * to wait for input,but you could also use poll() or similar * facilities. */ int sock; fd_set input_mask; sock = PQsocket(conn); if (sock < 0) break; /* shouldn't happen */ FD_ZERO(&input_mask); FD_SET(sock,&input_mask); if (select(sock + 1,&input_mask,NULL,NULL) < 0) { fprintf(stderr,"select() Failed: %s\n",strerror(errno)); exit_nicely(conn); } /* Now check for input */ PQconsumeInput(conn); while ((notify = PQnotifies(conn)) != NULL) { fprintf(stderr,"ASYNC NOTIFY of '%s' received from backend PID %d\n",notify->relname,notify->be_pid); PQfreemem(notify); nnotifies++; } } fprintf(stderr,"Done.\n"); /* close the connection to the database and cleanup */ PQfinish(conn); return 0; }
例子3：
/* * testlibpq3.c * Test out-of-line parameters and binary I/O. * * Before running this,populate a database with the following commands * (provided in src/test/examples/testlibpq3.sql): * * CREATE TABLE test1 (i int4,t text,b bytea); * * INSERT INTO test1 values (1,'joe''s place','\\000\\001\\002\\003\\004'); * INSERT INTO test1 values (2,'ho there','\\004\\003\\002\\001\\000'); * * The expected output is: * * tuple 0: got * i = (4 bytes) 1 * t = (11 bytes) 'joe's place' * b = (5 bytes) \000\001\002\003\004 * * tuple 0: got * i = (4 bytes) 2 * t = (8 bytes) 'ho there' * b = (5 bytes) \004\003\002\001\000 */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <libpq-fe.h> /* for ntohl/htonl */ #include <netinet/in.h> #include <arpa/inet.h> static void exit_nicely(PGconn *conn) { PQfinish(conn); exit(1); } /* * This function prints a query result that is a binary-format fetch from * a table defined as in the comment above. We split it out because the * main() function uses it twice. */ static void show_binary_results(PGresult *res) { int i,j; int i_fnum,t_fnum,b_fnum; /* Use PQfnumber to avoid assumptions about field order in result */ i_fnum = PQfnumber(res,"i"); t_fnum = PQfnumber(res,"t"); b_fnum = PQfnumber(res,"b"); for (i = 0; i < PQntuples(res); i++) { char *iptr; char *tptr; char *bptr; int blen; int ival; /* Get the field values (we ignore possibility they are null!) */ iptr = PQgetvalue(res,i_fnum); tptr = PQgetvalue(res,t_fnum); bptr = PQgetvalue(res,b_fnum); /* * The binary representation of INT4 is in network byte order,which * we'd better coerce to the local byte order. */ ival = ntohl(*((uint32_t *) iptr)); /* * The binary representation of TEXT is,well,text,and since libpq * was nice enough to append a zero byte to it,it'll work just fine * as a C string. * * The binary representation of BYTEA is a bunch of bytes,which could * include embedded nulls so we have to pay attention to field length. */ blen = PQgetlength(res,b_fnum); printf("tuple %d: got\n",i); printf(" i = (%d bytes) %d\n",PQgetlength(res,i_fnum),ival); printf(" t = (%d bytes) '%s'\n",t_fnum),tptr); printf(" b = (%d bytes) ",blen); for (j = 0; j < blen; j++) printf("\\%03o",bptr[j]); printf("\n\n"); } } int main(int argc,char **argv) { const char *conninfo; PGconn *conn; PGresult *res; const char *paramValues[1]; int paramLengths[1]; int paramFormats[1]; uint32_t binaryIntVal; /* * If the user supplies a parameter on the command line,PQerrorMessage(conn)); exit_nicely(conn); } /* * The point of this program is to illustrate use of PQexecParams() with * out-of-line parameters,as well as binary transmission of data. * * This first example transmits the parameters as text,but receives the * results in binary format. By using out-of-line parameters we can * avoid a lot of tedious mucking about with quoting and escaping,even * though the data is text. Notice how we don't have to do anything * special with the quote mark in the parameter value. */ /* Here is our out-of-line parameter value */ paramValues[0] = "joe's place"; res = PQexecParams(conn,"SELECT * FROM test1 WHERE t = $1",1,/* one param */ NULL,/* let the backend deduce param type */ paramValues,/* don't need param lengths since text */ NULL,/* default to all text params */ 1); /* ask for binary results */ if (PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr,"SELECT Failed: %s",PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } show_binary_results(res); PQclear(res); /* * In this second example we transmit an integer parameter in binary * form,and again retrieve the results in binary form. * * Although we tell PQexecParams we are letting the backend deduce * parameter type,we really force the decision by casting the parameter * symbol in the query text. This is a good safety measure when sending * binary parameters. */ /* Convert integer value "2" to network byte order */ binaryIntVal = htonl((uint32_t) 2); /* Set up parameter arrays for PQexecParams */ paramValues[0] = (char *) &binaryIntVal; paramLengths[0] = sizeof(binaryIntVal); paramFormats[0] = 1; /* binary */ res = PQexecParams(conn,"SELECT * FROM test1 WHERE i = $1::int4",paramLengths,paramFormats,1); /* ask for binary results */ if (PQresultStatus(res) != PGRES_TUPLES_OK) { fprintf(stderr,PQerrorMessage(conn)); PQclear(res); exit_nicely(conn); } show_binary_results(res); PQclear(res); /* close the connection to the database and cleanup */ PQfinish(conn); return 0; }
