Experimental Physics and
Industrial Control System

"Zimoch Dirk \(PSI\) via Core-talk" <core-talk at aps.anl.gov> · Tue, 8 Jun 2021 15:26:10 +0000

The usage of epicsAtomicCmpAndSwapIntT for example in callback.c suggests that the function should return the previous
value. But the VxWorks implementation uses vxCas which is documented as returning "TRUE if the write actually occurs,
FALSE otherwise." (at least in VxWorks 6.7).

Comparing this with the gcc documentation:
bool __sync_bool_compare_and_swap (type *ptr, type oldval type newval, ...)
type __sync_val_compare_and_swap (type *ptr, type oldval type newval, ...)

These builtins perform an atomic compare and swap.  That is, if the current value of *ptr is oldval, then write newval
into *ptr.

The “bool” version returns true if the comparison is successful and newval was written.
The “val” version returns the contents of *ptr before the operation.

So it seems epicsAtomicCmpAndSwapIntT is expected to behave like __sync_val_compare_and_swap (and that is what is
actually implemented in libcom/src/osi/compiler/gcc/epicsAtomicCD.h) but vxCas as used in
libcom/src/osi/os/vxWorks/epicsAtomicOSD.h actually behaves like __sync_bool_compare_and_swap.

Dirk

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System