I am working on an applicaiton where I have to deal with multiple threads and have to implement IPC mechanism to commnicate between threads, I am using message queue for this purpose.

However, at one instant mq_open() is returning 0 (zero) which is the integer file descriptor associated with the stream stdin, which is causing trouble for taking input from user as mq_close() is being called in the program.

Now my question is how should I ensure that mq_open() will never return any standard value which is implemeted in linux for varoius files and descriptors for itself.

mq_open doesn't return a file handle, so there is no point in comparing it with file handles such as 0,1,2, etc.

But in my case mq_open() is returning 0 (zero) that is why I am unable to use scanf, getchar, fgetc etc. because these calls returns immediately and gives a garbage value in variable used in function, such as if i have used scanf("%d",&choice) the value in choice is something random and it does not wait for user input. Same thing goes for all other calls such as getchar etc.

When I debugged my code then I came to know that mq_open is returning zero and as soon as my main thread call scanf the thread in which I called mq_close is closing the file decsriptor. But after some efforts I have successfully executed my program but only once, and now it is behaving in same manner as it was previously.

You must have misunderstood something, the return value of mq_open shouldn't effect the behaviour of standard input. If you think otherwise, provide a complete, minimal example program that demonstrates problem. (Note: it has to be compilable with flags -W -Wall -Wextra -Werror)

Note: multi-threaded programs are not for beginners. Or anyone else, really.

I also agree with you Nevem, but it is behaving in that way. So, now I switched from POSIX message queues to System V Message queues and now I am not facing any issues now.

Thanks a lot.

I also agree with you Nevem, but it is behaving in that way. So, now I switched from POSIX message queues to System V Message queues and now I am not facing any issues now.

Thanks a lot.