#define INCL_DOSSEMAPHORES USHORT rc = DosSemSetWait(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. It then invokes process2 to run asynchronously. Finally, process1 sets the semaphore and waits, with a timeout of 4.5 seconds, for process2 to open the semaphore and clear it. Process1 resumes after this clearance. The following example is different from the one given for DosSemSet and DosSemWait only in that the set and wait functions are performed in one atomic step, via DosSemSetWait. Hence, in the following example, there is no possibility for process1 to miss the notification because of a premature clear by process2.
/* ----- 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 indicator */
&SemHandle, /* Semaphore handle (returned) */
SEM_NAME); /* Semaphore name string */
printf("Process1.sem created; return code is %d \n", rc);
/* 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");
/************* SET semaphore and WAIT for a notification ************/
/************* from process2 on process1.sem ************/
if((rc = DosSemSetWait(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");
}
}