http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53504
Bug #: 53504 Summary: configure script of platform TLS support. Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: lyricco...@gmail.com file: gcc-4.70/stdc++v3/configure line: 51506 #include <pthread.h> __thread int a; static int *volatile a_in_other_thread; static void * thread_func (void *arg) { a_in_other_thread = &a; return (void *)0; } int main () { pthread_t thread; void *thread_retval; int *volatile a_in_main_thread; a_in_main_thread = &a; if (pthread_create (&thread, (pthread_attr_t *)0, thread_func, (void *)0)) return 0; if (pthread_join (thread, &thread_retval)) return 0; return (a_in_other_thread == a_in_main_thread); ; return 0; } _ACEOF if ac_fn_c_try_run "$LINENO"; then : gcc_cv_have_tls=yes else gcc_cv_have_tls=no fi 1. if we have TLS support, &a will be difference value in main() and hread_func(), which means (a_in_other_thread == a_in_main_thread) eval to false. so, with TLS support => program exit with 0. 2. if "pthread_create" or "pthread_join" fails, program also exit with 0. "no threads, no TLS". so, without TLS support => program exit with 0. whatever ac_fn_c_try_run get, this can not be right.