SQLCA management for multithreaded or reentrant code

You can use embedded SQL statements in multithreaded or reentrant code. However, if you use a single connection, you are restricted to one active request per connection. In a multithreaded application, you should not use the same connection to the database on each thread unless you use a semaphore to control access.

There are no restrictions on using separate connections on each thread that wants to use the database. The SQLCA is used by the runtime library to distinguish between the different thread contexts. So, each thread wanting to use the database concurrently must have its own SQLCA. The exception is that a thread can use the db_cancel_request function to cancel a statement executing on a different thread using that thread's SQLCA.

The following is an example of multithreaded embedded SQL reentrant code.

#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <ctype.h>
#include <stdlib.h>
#include <process.h>
#include <windows.h>
EXEC SQL INCLUDE SQLCA;
EXEC SQL INCLUDE SQLDA;

#define TRUE 1
#define FALSE 0

// multithreading support

typedef struct a_thread_data {
    SQLCA sqlca;
    int num_iters;
    int thread;
    int done;
} a_thread_data;

// each thread's ESQL test

EXEC SQL SET SQLCA "&thread_data->sqlca";

static void PrintSQLError( a_thread_data * thread_data )
/******************************************************/
{
  char                buffer[200];

  printf( "%d: SQL error %d -- %s ... aborting\n",
      thread_data->thread,
      SQLCODE,
      sqlerror_message( &thread_data->sqlca, 
               buffer, sizeof( buffer ) ) );
    exit( 1 );
}

EXEC SQL WHENEVER SQLERROR { PrintSQLError( thread_data ); };

static void do_one_iter( void * data )
{
  a_thread_data *  thread_data = (a_thread_data *)data;
  int    i;
  EXEC SQL BEGIN DECLARE SECTION;
  char    user[ 20 ];
  EXEC SQL END DECLARE SECTION;

  if( db_init( &thread_data->sqlca ) != 0 ) {
    for( i = 0; i < thread_data->num_iters; i++ ) {
     EXEC SQL CONNECT "dba" IDENTIFIED BY "sql";
     EXEC SQL SELECT USER INTO :user;
     EXEC SQL DISCONNECT;
    }
    printf( "Thread %d did %d iters successfully\n",
        thread_data->thread, thread_data->num_iters );
    db_fini( &thread_data->sqlca );
  }
  thread_data->done = TRUE;
}

int main() 
{
  int num_threads = 4;
  int thread;
  int num_iters = 300;
  int num_done = 0;
  a_thread_data *thread_data;
  thread_data = (a_thread_data *)malloc( sizeof(a_thread_data) * num_threads );
  for( thread = 0; thread < num_threads; thread++ ) {
    thread_data[ thread ].num_iters = num_iters;
    thread_data[ thread ].thread = thread;
    thread_data[ thread ].done = FALSE;
    if( _beginthread( do_one_iter, 
      8096, 
      (void *)&thread_data[thread] ) <= 0 ) {
      printf( "FAILED creating thread.\n" );
      return( 1 );
    }
  }
    while( num_done != num_threads ) {
    Sleep( 1000 );
    num_done = 0;
    for( thread = 0; thread < num_threads; thread++ ) {
      if( thread_data[ thread ].done == TRUE ) {
        num_done++;
      }
    }
  }
  return( 0 );
}

SQLCA management for multithreaded or reentrant code

See also