Subj : Re: returning arguments
To   : comp.programming.threads
From : Huub
Date : Tue Jun 28 2005 09:34 pm

David Butenhof wrote:
> 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.
> 
I *thought* I understood it. Could you explain a bit more of the last 
part of your comment please, including the casting to a pointer to the 
structure?

.