DosRequestRamSem obtains a RAM semaphore. This is used at the entry to a range of code that must not be executed simultaneously in multiple threads.

#define INCL_DOSSEMAPHORES
#include <os2.h>

PRAMSEM    pramsem;  /*  Pointer to the RAM semaphore structure. */
LONG       timeout;
APIRET     rc;       /*  Return code. */

rc = DosRequestRamSem(pramsem, timeout);