[SOLVED] multithreading question

kvm1983 · 06-11-2010, 06:43 PM

Hello all,

I have a multithreaded C program where I do signaling using condition variables at various points.

For signaling using posix condition variables, this works:

Code:

pthread_cond_signal(&(struct1->struct2[1].cond_var), &(struct1->struct2[1].mutex));

but this doesnt work:

Code:

my_cond_var = struct1->struct2[1].cond_var;
my_mutex = struct1->struct2[1].mutex;
pthread_cond_signal(&my_cond_var, &my_mutex);

Why does this happen? Why doesnt copying the mutex and the condition variable to another set of variables work?

Thanks for your help.

- Kshitij

ichrispa · 06-11-2010, 07:55 PM

It wouldn't hurt to see the definitions of my_cond_var and the error.

If your my_cond_var and my_mutex are pointers, try removing the "address off"'s like so:

pthread_cond_signal(my_cond_var, my_mutex);

Otherwise you might be passing the address of the pointer.

bigearsbilly · 06-12-2010, 04:10 AM

from "programming with posix threads"

Quote:

you should never make a copy of a condition variable, because the result
of using a copied condition variable is undefined.

kvm1983 · 06-14-2010, 11:49 PM

ichrispa,
I have made sure that I am not passing incorrect values to the function (pointers instead of addresses and so on), so thats not the problem for sure.
As bigearsbilly suggested, looks like its not a good idea to make a copy of the condition variable.

Thank you,
Kshitij

bigearsbilly · 06-15-2010, 06:58 AM

yes.
you shouldn't make copies of anything in pthreads.
mutexes too.
it may work it may not, but behaviour is generally undefined.