Why is _GNU_SOURCE macro required for pthread_mutexattr_settype() while it is in POSIX/IEEE standard?
c, client-server, gcc, language-features, multithreading
Solution
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.
Problem
I have written a multithread server program in C, which echoes back all the data that a client sends. Initially, I used `poll()` function in my program to detect `POLLRDHUP` event, for that I defined `_GNU_SOURCE` macro (This event is defined here). Later I updated my code & removed `poll()` function, however I forgot to remove `_GNU_SOURCE` macro. Now my code is finally complete (and a little long to post, more than 250 lines). Before removing macro I was compiling my program using: ``` gcc multi_thread_socket_v4.c -Wall -Werror -g -lpthread -o multi_thread_socket ``` and it worked fine: No errors, no warnings After I removed the macro definition, and compiled using same command-line, the output of gcc was: ``` multi_thread_socket_v4.c: In function ‘main’: multi_thread_socket_v4.c:194: warning: implicit declaration of function ‘pthread_mutexattr_settype’ multi_thread_socket_v4.c:194: error: ‘PTHREAD_MUTEX_ERRORCHECK’ undeclared (first use in this function) multi_thread_socket_v4.c:194: error: (Each undeclared identifier is reported only once multi_thread_socket_v4.c:194: error: for each function it appears in.) ``` I have included all the required libraries as it worked fine initially. I peeked into `pthread.h` at `/usr/include/pthread.h` and found out this: ``` /* Mutex types. */ enum { PTHREAD_MUTEX_TIMED_NP, PTHREAD_MUTEX_RECURSIVE_NP, PTHREAD_MUTEX_ERRORCHECK_NP, PTHREAD_MUTEX_ADAPTIVE_NP #ifdef __USE_UNIX98 , PTHREAD_MUTEX_NORMAL = PTHREAD_MUTEX_TIMED_NP, PTHREAD_MUTEX_RECURSIVE = PTHREAD_MUTEX_RECURSIVE_NP, PTHREAD_MUTEX_ERRORCHECK = PTHREAD_MUTEX_ERRORCHECK_NP, PTHREAD_MUTEX_DEFAULT = PTHREAD_MUTEX_NORMAL #endif #ifdef __USE_GNU /* For compatibility. */ , PTHREAD_MUTEX_FAST_NP = PTHREAD_MUTEX_TIMED_NP #endif }; ``` and this: ``` #ifdef __USE_UNIX98 /* Return in *KIND the mutex kind attribute in *ATTR. */ extern int pthread_mutexattr_gettype (__const pthread_mutexattr_t *__restrict __attr, int *__restrict __kind) __THROW __nonnull ((1, 2)); /* Set the mutex kind attribute in *ATTR to KIND (either PTHREAD_MUTEX_NORMAL, PTHREAD_MUTEX_RECURSIVE, PTHREAD_MUTEX_ERRORCHECK, or PTHREAD_MUTEX_DEFAULT). */ extern int pthread_mutexattr_settype (pthread_mutexattr_t *__attr, int __kind) __THROW __nonnull ((1)); ``` I checked out here to check if `__USE_UNIX98` is a feature test macro, but it was not there. So please help me understanding the reasons for the error, because the function & the macro where `gcc` shows error are defined in POSIX standard. I do not know what more info regarding my problem will be required so please tell me, I will update my question.