This example shows how a thread enters and exits a critical section of code.

 #define INCL_DOSPROCESS      /* Process values */
 #define INCL_DOSERRORS       /* Error values */
 #include <os2.h>
 #include <stdio.h>

 int main(VOID)
   {
   APIRET  rc = NO_ERROR;    /* Return code */

    rc = DosEnterCritSec();

    if (rc != NO_ERROR) {
        printf("DosEnterCritSec error: return code = %u\n",rc);
        return 1;
    }
      /***********************************************/
      /* Add critical section code here.  While this */
      /* code is running, all other threads are      */
      /* stopped.  CALL NO LIBRARY OR OS/2 FUNCTIONS */
      /* HERE UNLESS YOU KNOW THEY DO NOT REQUIRE    */
      /* ACTION BY ANOTHER THREAD IN THE PROCESS.    */
      /***********************************************/

    rc =  DosExitCritSec();

    if (rc != NO_ERROR) {
        printf("DosExitCritSec error: return code = %u\n",rc);
        return 1;
    }

    return NO_ERROR;
    }