In libc/sysdeps/linux/common/bits/uClibc_pthread.h: extern void weak_function _pthread_cleanup_push_defer(...) This *weak_function* declaration will cause nptl/cleanup_defer_compat.c: strong_alias (...) !!!FAIL!!!, because it include pthreadP.h->pthread.h ->uClibc_pthread.h That means: Readelf -s libpthread/nptl/cleanup_defer_compat.o you will get: 18: 00000000 198 FUNC WEAK DEFAULT 1 _pthread_cleanup_push_def Readelf -s libc/misc/internals/__uClibc_main.o you will also get: 32: 00000038 58 FUNC WEAK DEFAULT 1 _pthread_cleanup_push_def Final: gcc malloc_pthread_test.c -lpthread The libc/stdlib/malloc-standard/malloc.c:839 (__MALLOC_LOCK-> _pthread_cleanup_push_def) will use the one in __uClibc_main.o !!!not in cleanup_defer_compat.o!!!, becasue two cleanup_defer_compat in libc.a with the same weak declarations and the __uClibc_main.o is close to front. ==== All of malloc/free will failed in muti-threads' race conditions probabilistic. As it happens, uClibc-0.9.33.2 is OK, Becasue: It's seperated in libpthread.a and libc.a, and the libc.a is the last lib for ld internal-cmd, and malloc will get right cleanup_defer_compat from libpthread.a. This BUG is from 2016-09-24 to now: commit 29ff9055c80efe77a7130767a9fcb3ab8c67e8ce Author: Waldemar Brodkorb &lt;wbx(a)uclibc-ng.org&gt; Date: Sat Sep 24 02:55:31 2016 +0200 use a single libc and deduplicate threading code Similar to musl libc a single libc has many benefits and solves some open issues with uClibc-ng. - no pthread_mutex_* weak symbols exported anymore - applications no longer failing to link when either -lrt or -lpthread are missing for dynamic and static linking mode - smaller C library - slightly better runtime performance <<< Perharps we need carefully check all of the impact about that commit. Signed-off-by: Guo Ren &lt;ren_guo(a)c-sky.com&gt; --- libpthread/nptl/sysdeps/pthread/pthread.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libpthread/nptl/sysdeps/pthread/pthread.h b/libpthread/nptl/sysdeps/pthread/pthread.h index b2897b3..1fba7fc 100644 --- a/libpthread/nptl/sysdeps/pthread/pthread.h +++ b/libpthread/nptl/sysdeps/pthread/pthread.h @@ -29,7 +29,7 @@ #include <bits/pthreadtypes.h> #include <bits/setjmp.h> #include <bits/wordsize.h> -#if defined _LIBC && ( defined IS_IN_libc || defined NOT_IN_libc ) +#if defined _LIBC && ( defined IS_IN_libc || !defined NOT_IN_libc ) #include <bits/uClibc_pthread.h> #endif -- 2.7.4