Subj : Re: returning arguments
To   : comp.programming.threads
From : David Butenhof
Date : Tue Jun 28 2005 06:36 pm

Huub wrote:

> Is the right way to do it?
> 
> struct displaygegevens
> {
>     int gasstatus;
>     int rookstatus;
>     int temperatuur;
> };
> 
> void *LeesSensoren(void *threadid)
> {
>     ...
>     try
>     {
>         ...
>         displaygegevens = {gasstatus, rookstatus, temperatuur};
>         return (void*)displaygegevens;

Here you're casting your STRUCTURE (not a pointer) to "void*". That 
won't work. But even if you changed it to "(void*)&displaygegevens", 
you're still returning a pointer to a single statically allocated 
structure. That MAY be OK if your application logic ensures that there 
can never be more than one thread running "LeesSensoren" at a time. 
Note, that doesn't just mean you can't have two threads running at the 
same time... you can't start a new LeesSensoren thread until 
pthread_join() has completed for the previous thread AND you've 
extracted all the information out of the static structure. If you can't 
be sure that guarantee will always hold, then you'd need synchronization 
to protect your access. You'd be better off just using malloc() to have 
each thread make its own copy of the structure, and return a pointer to 
that copy. (Again, after extracting the data the joiner would simply 
free() the structure.)

>     }
>     catch(..)
>     {
>     }
> }
> 
> void x()
> {
>     try
>     {
>         int retv;
>         void *retp;
>         rc = pthread_create(...);
>         pthread_exit(NULL);
>         rc = pthread_join(.....);
>         assert (rc == 0);
>         return(retv = (int)retp);

You've copied too much and not enough of Loic's logic. You don't show 
that you're actually extracting the returned value from pthread_join(), 
which makes me a little nervous given the rest of your code. But 
presuming you really understood that part, you're also casting retp (now 
the ADDRESS of your structure) to an int for return from a void 
function. (This wouldn't even compile.) You need to cast the returned 
void* to a pointer to your structure type, and then read the members of 
the structure.

-- 
Dave Butenhof, David.Butenhof@hp.com
HP Utility Pricing software, POSIX thread consultant
Manageability Solutions Lab (MSL), Hewlett-Packard Company
110 Spit Brook Road, ZK2/3-Q18, Nashua, NH 03062

.