semSignal (Function)

The semSignal() function will perform a traditional SIGNAL operation on the specified semaphore.

The semSignal() mechanism will:

•If there are threads blocked in the semWait() waiting queue for the semaphore, the first thread will be released to execute.

•Otherwise the semaphore value will be incremented by 1.

Also see the function semWait() and the section on thread synchronization for more information.

Input:

sem : SEMAPHORE

The semaphore to Signal.                

Returns: INT

Declaration:

FUNCTION semSignal : INT;
VAR_INPUT
   sem : SEMAPHORE;
END_VAR;

Example:

INCLUDE rtcu.inc
 
VAR
   sem : SEMAPHORE;
END_VAR;
 
PROGRAM test;
   ...
   // Initialize Semaphore
   sem := semInit(initval:=3);
   IF semValue(sem:=sem) = -1 THEN DebugMsg(message:="sem failed to init!"); END_IF;
   ...
BEGIN
   ...
   // Wait until resource is free or timeout
   IF semWait(sem:=sem,timeout:=1000) = 0 THEN
      // Only do these actions if we have access to the resource
      ...
      // Free the resource after use
      semSignal(sem:=sem);
   END_IF;
   ...
END;
 
END_PROGRAM;