#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");
}
}