g++ doesn't resolve pthread_once used in stdlibc++

Question

I was static linking a program using gtest, stdlibc++ and pthread, the link command is as follows

g++ -static -L/usr/local/lib -o test run_test.o -lgtest -lstdc++ -lgcc -lgcc_eh -lpthread -lc

it can produce the binary but it crashes every time, when I trace the core dump


    #0  0x0000000000000000 in ?? ()
    #1  0x000000000048bf11 in __gthread_once (__func=0x48beb0 , 
        __once=)
        at /home/abuild/rpmbuild/BUILD/gcc-4.8.5/obj-x86_64-suse-linux/x86_64-suse-linux/libstdc++-v3/include/x86_64-suse-linux/bits/gthr-default.h:699
    #2  std::locale::_S_initialize () at ../../../../../libstdc++-v3/src/c++98/locale_init.cc:276
    #3  0x000000000048bf53 in std::locale::locale (this=0x7e3a98 )
        at ../../../../../libstdc++-v3/src/c++98/locale_init.cc:210
    #4  0x000000000045798c in basic_streambuf (this=)
        at /home/abuild/rpmbuild/BUILD/gcc-4.8.5/obj-x86_64-suse-linux/x86_64-suse-linux/libstdc++-v3/include/streambuf:466
    #5  stdio_sync_filebuf (__f=0x7dd580 , this=)
        at /home/abuild/rpmbuild/BUILD/gcc-4.8.5/obj-x86_64-suse-linux/x86_64-suse-linux/libstdc++-v3/include/ext/stdio_sync_filebuf.h:77
    #6  std::ios_base::Init::Init (this=) at ../../../../../libstdc++-v3/src/c++98/ios_init.cc:85
    #7  0x00000000004028fe in __static_initialization_and_destruction_0 (__initialize_p=1, __priority=65535)
        at /usr/include/c++/4.8/iostream:74
    #8  0x0000000000402927 in _GLOBAL__sub_I_main () at run_test.cpp:7
    #9  0x00000000004ac017 in __libc_csu_init (argc=argc@entry=1, argv=argv@entry=0x7ffcdf8db6d8, envp=0x7ffcdf8db6e8)
        at elf-init.c:88
    #10 0x00000000004ababd in __libc_start_main (main=0x4028ae , argc=1, argv=0x7ffcdf8db6d8, 
        init=0x4abfa0 , fini=0x4ac030 , rtld_fini=0x0, stack_end=0x7ffcdf8db6c8)
        at libc-start.c:244
    #11 0x00000000004027c7 in _start () at ../sysdeps/x86_64/start.S:122

and disassembled the binary found in std::locale::_S_initialize () the generated binary callq 0 instead of invoke the address of pthread_once.


      48bf0c:   e8 ef 40 b7 ff          callq  0

used objdump to dump the symbol table and found


    0000000000000000  w      *UND*  0000000000000000 pthread_once
    0000000000000000  w      *UND*  0000000000000000 __pthread_once

It shows that linker know pthread_once is undefined weak symbol and refuse to resolve it. I have to add linker option -Wl,--undefined=pthread_once to force linker to resolve pthread_once to avoid crash.

Does anyone know why linker doesn't resolve pthread_once ?

Static linking of libpthread with libstdc++ is not supported. A workaround is to use -Wl,--whole-archive -lpthread -Wl,--no-whole-archive. See gcc.gnu.org/ml/gcc-help/2010-05/msg00029.html — n. 1.8e9-where's-my-share m.

Siyuan Xiang Siyuan Xiang · Accepted Answer · 2016-05-30T18:36:04

Checked -Wl,--whole-archive -lpthread -Wl,--no-whole-archive as n.m. mentioned, it works. A more related reference can be found in https://gcc.gnu.org/ml/gcc/2014-11/msg00246.html

The crash comes from how libstdc++ uses weakref attribute to implement a wrapper called gthread on top of pthread. In summary, crash happens because

libstdc++ assumes that if __pthread_key_create is defined, then pthread_once is defined, while this is not true for static linking.
pthread_once is not referenced or defined any where else in user's code.
gnu linker doesn't resolve undefined weak symbol pthread_once, there is no link error and the address of pthread_once is set to 0.
pthread_once is called.

I agree that --whole-archive link option might be the easiest work around for static linking of stdlibc++ and pthread.

g++ doesn't resolve pthread_once used in stdlibc++

1 Answers