Semaphore

Codesys V2.3 🇬🇧
Romuald
2010-07-05
2010-07-06

  • Romuald - 2010-07-05

    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.

     

  • Romuald - 2010-07-06

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

     

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