When I define DISPATCH_DEBUG=1 and build the library under Linux, almost all of the test programs fail. I checked a couple of them, and they both fail a dispatch_assert() test at the same place: in _dispatch_ccache_init(). See below for a backtrace: $ gdb ./dispatch_api GNU gdb (GDB) 7.0-ubuntu Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... Reading symbols from /home/mheily/proj/libdispatch/trunk/testing/.libs/dispatch_api...done. (gdb) r Starting program: /home/mheily/proj/libdispatch/trunk/testing/.libs/dispatch_api [Thread debugging using libthread_db enabled] ================================================== [TEST] Dispatch (Public) API [PID] 17941 ================================================== Actual: 0x6020a0 Expected: 0x6020a0 [PASS] dispatch_get_main_queue Program received signal SIGABRT, Aborted. 0x00007ffff72604b5 in *__GI_raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64 64 ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory. in ../nptl/sysdeps/unix/sysv/linux/raise.c (gdb) bt #0 0x00007ffff72604b5 in *__GI_raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64 #1 0x00007ffff7263f50 in *__GI_abort () at abort.c:92 #2 0x00007ffff7bd023e in _dispatch_abort (line=<value optimized out>, val=<value optimized out>) at queue.c:1774 #3 0x00007ffff7bd02ab in _dispatch_ccache_init (context=<value optimized out>) at queue.c:1697 #4 0x00007ffff7bcfb15 in dispatch_once_f (val=0x7ffff7dd8dc8, ctxt=0x4615, func=0x6) at once.c:43 #5 0x00007ffff7bd02ec in _dispatch_continuation_alloc_from_heap () at queue.c:1707 #6 0x00007ffff7bd05f1 in _dispatch_async_f_slow (dq=0x4615, context=0x4615, func=0x6) at queue.c:681 #7 0x0000000000400bd8 in main () (gdb)