The ViSession handle that you obtain from Ivi_SpecificDriverNew. The handle identifies a particular IVI session.

This parameter serves as a convenience. If you do not want to use this parameter, pass VI_NULL.

Use this parameter in complex functions to keep track of whether you obtain a lock and therefore need to unlock the session. Pass the address of a local ViBoolean variable. Initialize the local variable to VI_FALSE when you declare it. Pass the same address to any other calls you make to Ivi_LockSession or Ivi_UnlockSession in the same function.

The parameter is an input/output parameter. Ivi_LockSession and Ivi_UnlockSession each inspect the current value and take the following actions:

If the value is VI_TRUE, Ivi_LockSession does not lock the session again. If the value is VI_FALSE, Ivi_LockSession obtains the lock and sets the value of the parameter to VI_TRUE.

If the value is VI_FALSE, Ivi_UnlockSession does not attempt to unlock the session. If the value is VI_TRUE, Ivi_UnlockSession unlocks the lock and sets the value of the parameter to VI_FALSE.

Thus, you can, call Ivi_UnlockSession at the end of your function without worrying about whether you actually have the lock.

Example:

ViStatus PREFIX_Func (ViSession vi, ViInt32 flags){

ViStatus error = VI_SUCCESS;
ViBoolean haveLock = VI_FALSE;

if (flags & BIT_1)

{
viCheckErr( Ivi_LockSession(vi, &haveLock));
viCheckErr( TakeAction1(vi));

if (flags & BIT_2)

{
viCheckErr( Ivi_UnlockSession(vi, &haveLock));
viCheckErr( TakeAction2(vi));
viCheckErr( Ivi_LockSession(vi, &haveLock);
}

if (flags & BIT_3)

viCheckErr( TakeAction3(vi));

}

Error:

/*

At this point, you cannot really be sure that you have the lock. Fortunately, the haveLock variable takes care of that for you.

*/
Ivi_UnlockSession(vi, &haveLock);
return error;

}