DosSemWait

#define INCL_DOSSEMAPHORES

USHORT  rc = DosSemWait(SemHandle, Timeout);

HSEM             SemHandle;     /* Semaphore handle */
LONG             Timeout;       /* Timeout (in milliseconds) */

USHORT           rc;            /* return code */

Example 

The following example illustrates the notification of an event between 
threads of different processes.  Process1 creates a nonexclusive system 
semaphore named process1.sem and sets it.  It then invokes process2 to run 
asynchronously. Process1 then waits, with a timeout of 4.5 seconds, for 
process2 to open the semaphore and clear it, thereby notifying process1 to 
resume. Notice that there is a small possibility of process1's missing the 
notification because process2 may clear the semaphore before process1 
issues DosSemWait. See the example for DosSemSetWait for an alternative 
that would correct this. 

/* ----- process1.c ---- */

#define INCL_DOSSEMAPHORES
#define INCL_DOSPROCESS

#include <os2.h>

#define SEM_NAME "\\SEM\\process1.sem" /* Semaphore name */
#define TIMEOUT 4500L                  /* Timeout period */
#define START_PROGRAM "process2.exe"   /* Name of program file */

main()
{
  HSEM          SemHandle;
  CHAR          ObjFail [50];
  PSZ           Args;
  PSZ           Envs;
  RESULTCODES   ReturnCodes;
  USHORT        rc;

  printf("Process1 now running. \n");
  rc = DosCreateSem(CSEM_PUBLIC,       /* Ownership - nonexclusive */
                    &SemHandle,        /* Semaphore handle (returned) */
                    SEM_NAME);         /* Semaphore name string */
  printf("Process1.sem created; return code is %d \n", rc);

  /*** SET the semaphore. ***/
  if((rc = DosSemSet(SemHandle)))      /* Semaphore handle */

  /****************************************/
  {
    /* Cannot set semaphore -- error processing */
  }
  /* Start a separate process */
  if(!(DosExecPgm(ObjFail,           /* Object name buffer */
                  sizeof(ObjFail),   /* Length of obj. name buffer */
                  EXEC_ASYNC,        /* Execution flag - asynchronous */
                  Args,              /* Ptr. to argument string */
                  Envs,              /* Ptr. to environment string */
                  &ReturnCodes,      /* Ptr. to resultcodes struct. */
                  START_PROGRAM)))   /* Name of program file */
    printf("Process2 started. Process1 will try to wait for notification. \n");

  /*** WAIT for a notification from process2 on process1.sem ***/
  if((rc = DosSemWait(SemHandle,     /* Semaphore handle */
                      TIMEOUT)))     /* Timeout period */

  /****************************************/
  {
    /* No notification (either interrupt or timeout); error processing */
  }
  else
  {
    /* Notification received.  Normal processing */
    printf("Process2 cleared semaphore; process1 running again. \n");
  }
}

/* ----- process2.c ----*/

#define INCL_DOSSEMAPHORES

#include <os2.h>

#define SEM_NAME "\\SEM\\process1.sem"    /* Semaphore name */

main()
{
  HSEM SemHandle;
  USHORT rc;

  /* Obtain access to semaphore created by process1 via OPEN */
  if((rc=DosOpenSem(&SemHandle,           /* Semaphore handle (returned) */
                    SEM_NAME)))           /* Semaphore Name */

  /****************************************/
  {
    /* Could not open -- error processing (switch on rc). */
  }
  else
  {
    /* Opened semaphore; normal processing.  Clear semaphore when done. */
    printf("Semaphore OPENED. \n");
    if(!(rc=DosSemClear(SemHandle)))      /* Semaphore handle */
      printf("Semaphore CLEARED. \n");
  }
}