Why is _GNU_SOURCE macro required for pthread_mutexattr_settype() while it is in POSIX/IEEE standard?

You should use

#define _POSIX_C_SOURCE 200112L

if you want to use POSIX features such as pthread_mutexattr_settype ... see http://pubs.opengroup.org/onlinepubs/007904975/functions/xsh_chap02_02.html

Another possibility is

#define _XOPEN_SOURCE 700

See http://man7.org/linux/man-pages/man7/feature_test_macros.7.html and http://pubs.opengroup.org/onlinepubs/9699919799/

Setting _GNU_SOURCE includes POSIX and lots of other definitions.

P.S. I would expect that including <pthread.h> includes <features.h>, which by default defines _POSIX_C_SOURCE as 200112L, but it's possible that you have defined something that overrides that ... see /usr/include/features.h on your system for details of the symbols and their usage.

It doesn't, your problem likely lies elsewhere.

I just compiled a trivial program with the following content:

#include <pthread.h>int main(int argc, char **argv){    pthread_mutexattr_t attr;    pthread_mutexattr_init(&attr);    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);    return 0;}

This compiles perfectly with gcc -pthread -Wall -Werror a.c.

It's possible that another part of your program causes this, by eg. doing something silly like defining _PTHREAD_H, or some other minor sabotage.

You might want to try to get a minimal test case by using a tool like delta or creduce, which will probably make the problem evident.

When you're using old libraries (e.g. 2.1.x) you should use

#define __USE_UNIX98

Using a macro beginning with "__" it's not usually a good idea, but sometimes it's the only way... see also this discussion