Experimental Physics and
Industrial Control System

Guy Jennings <[email protected]> · Wed, 06 Nov 1996 11:47:42 -0600

Epics folk:

I recently ran into a problem while attempting to use a sync group from
a C++ client application.  The code below is based on acctst.c in the
epics
distribution.

	/* int ca_sg_create(const unsigned int *g); */

	int	status;
	CA_SYNC_GID	gid=0;  /* const unsigned int gid = 0; */

	printf("Initially %ud\n", gid);

	status = ca_sg_create(&gid);
	SEVCHK(status, NULL);

	printf("Then %ud\n", gid);

If compiled with gcc the code works fine, with the ca_sg_create call
initialising
'gid' to a non-zero value (45 in my case).  If compiled with g++ the
code does not
work correctly and 'gid' retains the value 0 after the call to
ca_sg_create.  This
appears to be due to the definition of CA_SYNC_GID as 'const unsigned
int'. I think
it would be safer to remove the const from the typedef for CA_SYNC_GID
since at
present you are relying on undefined behavior - (typecasting away the
const in 
ca_sg_create).

As far as I can tell, CA_SYNC_GID is the only epics-defined type which
displays this
problem and the header already allows for CA_SYNC_GID not to be 'const'

Guy Jennings
BESSRC CAT
Argonne National Laboratory

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System