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DosWaitEventSem blocks until the specified event semaphore is posted. Event semaphores are edge-triggered. This means that if the semaphore is posted and reset before a waiting thread has a chance to run, the waiting thread will still act as though the semaphore is posted.


DosWaitEventSem( hevSemaphore, ulTimeOut )


HEV hevSemaphore (input)
The handle of the semaphore wait on.
ULONG ulTimeout (input)
The number of milliseconds the function will wait before returning.
  • Set to SEM_IMMEDIATE_RETURN (0), the function will return immediately regardless of whether or not the semaphore is posted.
  • Set to SEM_INDEFINITE_WAIT (-1), the function will block indefinitely (forever) until the semaphore is posted.


The following values can be returned:
0 NO_ERROR Successfully waited for the event semaphore to be posted to
6 ERROR_INVALID_HANDLE Error, The value in phevSemaphore does not point to a valid semaphore, The calling process must first have access to the semaphore in question
8 ERROR_NOT_ENOUGH_MEMORY Error, The system memory limit has been exhausted
95 ERROR_INTERRUPT Error, The thread has become unblocked by an external event such as an exception, ownership has not been obtained
640 ERROR_TIMEOUT Error, The caller was blocked for ulTimeout milliseconds but no post was issued within this time limit, Time has expired


The process calling DosWaitEventSem must first obtain access to the semaphore in question or ERROR_INVALID_HANDLE will be returned. If the thread to be blocked, waiting on an event semaphore, is a PM thread, WinWaitEventSem should be used instead of DosWaitEventSem. This will allow the message queue thread to continue processing.

Sample Code

#include <os2.h>

HEV  hevMySemaphore;    /* MySemaphore handle */
ULONG TimeOut= -1;      /* the number of milliseconds the */
                        /* the caller will block for the sem */
                        /* this example will block forever */

	/* access is gained to the semaphore in question */
	/* either by DosCreateEventSem ... */
	/* ... or by DosOpenEventSem */
	/* its handle is placed in hevMySemaphore */

	rc = DosWaitEventSem(hevMySemaphore, TimeOut);

	if (rc != 0)
	  /* We got an error to take care of. */

See Also

DosCloseEventSem, DosCreateEventSem, DosOpenEventSem, DosPostEventSem, DosQueryEventSem, DosResetEventSem, WinWaitEventSem