libpq简单示例

libpq是什么呢？英文原话是：libpq is the C application programmer’s interface to PostgreSQL，原来是postgresql的c语言应用程序接口，postgresql的官网是： http://www.postgresql.org/。

1. postgresql安装
在ubuntu下，使用下面的命令（12.04）来安装：
sudo apt-get install postgresql-9.1
sudo apt-get install libpq-dev

2. libpq测试程序

/*
 * 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");

	/* 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;
}

上面这个程序使用gcc -I/usr/include/postgresql -o test testlibpq.c -lpq命令来编译，然后运行，结果出现如下错误信息（root用户）：
Connection to database failed: FATAL:  role "root" does not exist

原来是libbq默认是没有这个用户的（root），默认用户是postgres，在/etc/passwd的确有找到postgres这条记录，可能是安装postgresql是添加进去的，使用su命令来切换到postgres用户，但是初始时并没有为postgres用户设置过密码，所以这里先使用sudo passwd postgres命令来为postgres用户设置一个初始密码，然后再使用su命令来切换，切换成功之后，执行编译之前的程序，结果如下：


当然，切换到postgres用户之后，也可以向postgresql添加新的用户，例如：