libpq Connection Status Functions in .NET

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Table 14-3 libpq Connection Status Functions
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The versionc program demonstrates connecting to a database, checking the status of the connection, and extracting the PostgreSQL server version:
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#include <stdioh> #include <stdlibh> #include "libpq-feh" int main(int argc, char **argv) { const char *conninfo; const char *serverversion; PGconn *conn; const char *paramtext = "server_version"; conninfo = "hostaddr = 127001 dbname = test user = earl password = auction"; conn = PQconnectdb(conninfo); if (PQstatus(conn) != CONNECTION_OK) { printf("Unable to establish connection: %s", PQerrorMessage(conn)); return 1; } else { printf("Connection established!\n"); serverversion = PQparameterStatus(conn, paramtext); printf("Server Version: %s\n", serverversion); } PQfinish(conn); return 0; }
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This small example demonstrates all the basics of connecting to and interacting with a PostgreSQL server in Visual C++ It connects to the test database on the localhost using the earl Login Role created in 12 If you have not created these database objects, you will have to use other values for these parameters on your system After attempting the connection, the PQstatus() function is used to test the status of the connection If the connection is OK, the PQparameterStatus() function is used to obtain the value of the server_version parameter from the PostgreSQL server If you are compiling the program on the command prompt, the command should look like:
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cl /I"c:\program files\postgresql\82\include" versionc libpqlib
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Remember the libpqlib file must be available in the same directory as the versionc file After compiling the application, the program versionexe is generated
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(assuming you do not have any typographical errors in your code) However, if you try to run this program, you may have a problem You may get a Windows error message that a component necessary to run the application was not found In order for libpq programs to run, they must have access to the PostgreSQL DLL files, normally located in the bin directory of the PostgreSQL installation To solve this problem, you must either include the PostgreSQL bin directory in your PATH environment variable or copy your applications to the PostgreSQL bin directory along with the other PostgreSQL applications After solving the DLL problem, the application should run with no trouble:
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C:\Program Files\PostgreSQL\82\bin>version Connection established! Server Version: 820 C:\Program Files\PostgreSQL\82\bin>
The program worked as expected The next section moves on to show more advanced functions that can be executed on the server
Executing SQL Commands
After establishing a connection with the PostgreSQL server, you will most likely want to execute SQL commands on the PostgreSQL server The libpq command execution functions are listed and described in Table 14-4 The PQexec() function is the basic function used for executing SQL commands on the PostgreSQL server from C++ programs Unlike the Npgsql library described in 13, the libpq library uses a single function to execute both query and nonquery SQL commands The result of all the commands is returned into the same PGresult data type object:
PGresult *PQexec(PGconn *conn, const char *command)
The PQexec() function requires two parameters The first parameter is the PGconn object created when connecting to the server The second parameter is a character string that contains the SQL command to execute on the server When the PQexec() function is executed in the program, the command is sent to the server and the program waits for a response The response is placed in an PGresult data object Since there are many types of output associated with different SQL commands, this data object must be capable of handling lots of possibilities This data object must first be checked using the PQresultStatus() function to determine the status of the command and the type of output available from the command The result status can be PGRES_COMMAND_OK Command processed okay but with no result set returned PGRES_TUPLES_OK Command processed okay and returned a result set (even an empty result set)
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