This tiny code will demonstrate how to log-off from windows.
Enjoy!

The problem could be that you release your mutex too soon. The program is calling pthread_mutex_unlock() right after open(). Try releasing the mutex after the close() function.

Can you show us...

What's in the "ifqcam.h" file? Did you write the qcam_ functions? If so, maybe the problem is in that code.

i agree. maybe pthread_mutex_unlock function will help?

try running two single threaded programs at the same time. see if two different cameras can be accessed at the same time.

What OS are you using?