The usage of epicsAtomicCmpAndSwapIntT() 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."
vxAtomicLib.html:
vxCas( )
NAME
vxCas( ) - atomically compare-and-swap the contents of a memory location
SYNOPSIS
BOOL vxCas
(
atomic_t * target,
atomicVal_t oldValue,
atomicVal_t newValue
)
DESCRIPTION
This routine performs an atomic compare-and-swap; testing that *target contains oldValue, and if it does, setting the value of *target to newValue.
RETURNS
TRUE if the swap is actually executed, FALSE otherwise.
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.
That means epicsAtomicCmpAndSwapIntT() behaves behaves like __sync_bool_compare_and_swap(). Simply casting the return value from BOOL to the type of oldVal does not change that.
But the usage implies an expected bahavior like __sync_val_compare_and_swap (), wich is used in the gcc implementation of epicsAtomicCmpAndSwapIntT().
This bugs prevents the callback threads from being started in vxWorks 6.7, maybe more or all vxWorks 6 versions.
This patch makes vxWorks use the generic version of epicsAtomicCmpA