Subj : Re: returning arguments
To   : comp.programming.threads
From : loic-dev
Date : Mon Jun 27 2005 09:51 am

Salut Huub,


> I know this isn't compilable code, but it's just to give an idea. My
> question(s):
> - Do threads *have* to be void?

>From a pure semantic point of view, thread should be function returning
a void*, that is a pointer to something.


> - Can I pass on return values directly from the thread?

Yes, this is possible if the returned type can pad on a void*, that is
its size is <= sizeof(void*).

For instance to return an integer, assuming that
sizeof(int)<=sizeof(void*), then you can proceed as follows:

void*
thread (void* args)
{
int result;
....
return (void*) result;
}

and where you're joining the thread:
void *retp;
int   retv;
....
rc   = pthread_join (thread_id, &retp);
assert (rc==0);
retv = (int) retp;


> - If not, how do I pass them on?

If you need to return a type that is bigger than a void*, you can
proceed as follows:

In the thread:
- allocate the type on the heap using e.g. malloc
- return a pointer to the type located on the heap

void*
thread (void* args)
{
void* retp;
....
retp = malloc (sizeof(myType);
// fills-up (*retp)
....
return retp;
}

where you're joining the thread:
- reads the type
- free the heap

void *retp;
....
rc   = pthread_join (thread_id, &retp);
// read *(retp)
....
free (retp); 


HTH,
Loic.

.