Hi, Why can't I: #include <stdlib.h> #include <stdio.h> #include <pthread.h> int main () { printf("Value of PTHREAD_MUTEX_RECURSIVE is: %d\n", PTHREAD_MUTEX_RECURSIVE); return 0; } I have to define _XOPEN_SOURCE 500 or _GNU_SOURCE to get it to work OS is SuSE 10.2 with provided version of GCC. Man page for pthread.h does not describe this requirement in any way and it seems just plain weird that the cross-platform / standard behaviour is NOT available without some voodoo... Eddy