Semaphore

2020-02-27T23:16:42.981000Z

In fact there is a library more reliable for semaphores : SysLibSem.lib

Semaphore

2020-02-27T23:16:38.334000Z

Hi,

I need to use a semaphore to protect a global variable between several tasks. CoDeSys says that the FB SEMA is interruptable ! So if it is the case, is it reliable ?

Below a small program that would tend to prove that it is not reliable:

VAR_GLOBAL
  gSema: SEMA;
  gVar: DWORD;
END_VAR

PROGRAM TASK1
VAR
  var1: DWORD;
END_VAR
gSema(CLAIM := TRUE, RELEASE := FALSE);
IF NOT gSema.BUSY THEN
  var1 := var1 + 1;
  gVar := gVar1 + 1;
  gSema(CLAIM := FALSE, RELEASE := TRUE);
END_IF

PROGRAM TASK2
VAR
  var2: DWORD;
END_VAR
gSema(CLAIM := TRUE, RELEASE := FALSE);
IF NOT gSema.BUSY THEN
  var2 := var2 + 1;
  gVar := gVar1 + 1;
  gSema(CLAIM := FALSE, RELEASE := TRUE);
END_IF

After a time, gSema.BUSY is TRUE in the two tasks !

So what real alternatives I have to protect the variable ?

Thanks for your help,

Romuald.

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