Return values of POSIX functions

Question

I try to write POSIX compatible code that is shared between two POSIX compliant operating systems, namely QNX and a Linux variant.

I just found that there are small differences regarding the specification of function return values e.g. of pthread_mutex_trylock(). One documentation says it returns EOK (which evaluates to 0) the other says it returns plain 0 in case of success.

I assume I can safely check return values == 0 or != 0 and avoid the QNX EOK macro.

My questions:

1. Does POSIX explicitely define which values have to be returned by functions in case of success and/or error?
2. Strictly spoken, is a function that is specified to return EOK (not 0) POSIX compliant?
3. Is EOK part of some standard?

Answer 1

1. There are POSIX manual pages, consult them to find out what the POSIX definition of a function is. For example: http://www.unix.com/man-page/all/3posix/pthread_mutex_trylock/. In general, POSIX is very consistent, and in all cases where the return value is only used to signal success or failure, 0 means success.
2. If EOK equals 0, then the QNX implementation is POSIX compliant. However, as you already said, use 0 in your own code to ensure it compiles on all platforms.
3. As far as I can tell, it's not part of a platform-independent standard. Either you avoid using it, or if you believe EOK is more clear, then you can write this in your program:

.

#ifndef EOK
#define EOK 0
#endif