How to connect to ODBC DSN through c++

I made a PostgreSQL30 data source name. I tested it and I managed to connect to that dsn through ODBC data source window manager. There was a button Test and It showed message dialog, telling me that connection was success.

Now I wonder how would I connect to that DSN through c++ code and get some data.

This is my code, i saw few examplex online and come up with this code

enter code here

HENV hEnv = NULL; // for allocating memory usingSQLAllocEnv
HDBC hDBC = NULL; // connection handler
HSTMT hStmt = NULL; // statement handler
const char* szDSN = "PostgreSQL30"; // DataSourceName (config in windows 
control panel)
const char* szUID = "postgres"; //username of the database
const char* szPasswd = "postgres"; //password of the database

RETCODE retcode;
int rcod = 1;
int i, j, no = 2;



int main()
{ 
  retcode = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &hEnv);

  std::cout << retcode << std::endl;

  SQLSetEnvAttr(hEnv, SQL_ATTR_ODBC_VERSION, (void*)SQL_OV_ODBC3, 0);

  retcode = SQLAllocHandle(SQL_HANDLE_DBC, hEnv, &hDBC);

  std::cout << retcode << std::endl;

   retcode = SQLConnectA(hDBC,(SQLCHAR*)szDSN, SQL_NTS, (SQLCHAR*)szUID, SQL_NTS, 
  (SQLCHAR*)szPasswd, 
  SQL_NTS);


std::cout << (SQLCHAR*)szUID << std::endl;


if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO)
{
    // connected!!
    std::cout << "Hello World!\n SUCCESSS COME ON PLEASE";
}
else {
    std::cout << retcode << std::endl;
}


SQLFreeHandle(SQL_HANDLE_ENV, &hEnv);
SQLFreeHandle(SQL_HANDLE_DBC, &hDBC);

}

But I cant comprehend why would this work, I mean how is this code knows to call SQLAllocHandle implemented by Postgres driver ?

My question is how would I connect to DSN thorugh C++ code and get some data

Solution

In fact ODBC Driver Manager makes these calls to ODBC driver when performing your SQLConnect call: on this stage it "knows" which driver you want to use. Look at the SQLConnect function description, "Comments" section:

"The Driver Manager does not connect to a driver until the application calls a function (SQLConnect, SQLDriverConnect, or SQLBrowseConnect) to connect to the driver. Until that point, the Driver Manager works with its own handles and manages connection information. When the application calls a connection function, the Driver Manager checks whether a driver is currently connected to for the specified ConnectionHandle:

If a driver is not connected to, the Driver Manager connects to the driver and calls SQLAllocHandle with a HandleType of SQL_HANDLE_ENV, SQLAllocHandle with a HandleType of SQL_HANDLE_DBC, SQLSetConnectAttr (if the application specified any connection attributes), and the connection function in the driver. The Driver Manager returns SQLSTATE IM006 (Driver's SQLSetConnectOption failed) and SQL_SUCCESS_WITH_INFO for the connection function if the driver returned an error for SQLSetConnectAttr. For more information, see Connecting to a Data Source or Driver.
If the specified driver is already connected to on the ConnectionHandle, the Driver Manager calls only the connection function in the driver. In this case, the driver must make sure that all connection attributes for the ConnectionHandle maintain their current settings.
If a different driver is connected to, the Driver Manager calls SQLFreeHandle with a HandleType of SQL_HANDLE_DBC, and then, if no other driver is connected to in that environment, it calls SQLFreeHandle with a HandleType of SQL_HANDLE_ENV in the connected driver and then disconnects that driver. It then performs the same operations as when a driver is not connected to.

The driver then allocates handles and initializes itself."

Full text here: SQLConnect function comments