I have a stripped down simple program with a static variable (‘abc.cpp’):
#include <iostream>
int main(int, char**)
{
static const std::string a("123");
std::cout << "Hello world" << std::endl;
return 0;
}
I compile it and it works:
> g++ -ggdb abc.cpp -o abc > ./abc Hello world
However, if I link in the pthread library….
> g++ -ggdb -lpthread abc.cpp -o abc
> ./abc
Segmentation fault (core dumped)
> gdb abc
(gdb) run
Starting program: abc
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) where
#0 0x0000000000000000 in ?? ()
#1 0x00007ffff7b01681 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007ffff7b016c3 in std::locale::locale() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007ffff7afe244 in std::ios_base::Init::Init() ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x0000000000400d03 in __static_initialization_and_destruction_0 (__initialize_p=1,
__priority=65535) at /usr/include/c++/4.9/iostream:74
#5 0x0000000000400d2c in _GLOBAL__sub_I_main () at abc.cpp:8
#6 0x0000000000400d7d in __libc_csu_init ()
#7 0x00007ffff74a6e55 in __libc_start_main (main=0x400c06 <main(int, char**)>, argc=1,
argv=0x7fffffffdb58, init=0x400d30 <__libc_csu_init>, fini=<optimised out>,
rtld_fini=<optimised out>, stack_end=0x7fffffffdb48) at libc-start.c:246
#8 0x0000000000400b39 in _start ()
I know it doesn’t use threading here, but in the actual non-stripped-down program, it links to a library that does use threading. It feels like it should be okay to link to pthread even though threading is not actually used.
Interestingly, adding the sanitizer makes it not crash (not sure if that’s an “undefined”/unstable fix for it or not…).
> g++ -ggdb -fsanitize=undefined -lpthread abc.cpp -o abc > ./abc Hello world
Why does this cause a segfault?
Side note: Clang works.
> clang++ -ggdb -lpthread abc.cpp -o abc > ./abc Hello world
Version info:
> g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.9.2-0ubuntu1~14.04' --with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.9 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.9.2 (Ubuntu 4.9.2-0ubuntu1~14.04) > dpkg -l 'libstdc++6*' Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad) ||/ Name Version Architecture Description +++-===================-==============-==============-=========================================== ii libstdc++6:amd64 5-20150329-1ub amd64 GNU Standard C++ Library v3 un libstdc++6-4.0-dbg <none> <none> (no description available) un libstdc++6-4.1-dbg <none> <none> (no description available) un libstdc++6-4.2-dbg <none> <none> (no description available) un libstdc++6-4.3-dbg <none> <none> (no description available) un libstdc++6-4.4-dbg <none> <none> (no description available) un libstdc++6-4.5-dbg <none> <none> (no description available) un libstdc++6-4.6-dbg <none> <none> (no description available) un libstdc++6-4.7-dbg <none> <none> (no description available) un libstdc++6-4.8-dbg <none> <none> (no description available) ii libstdc++6-4.9-dbg: 4.9.2-0ubuntu1 amd64 GNU Standard C++ Library v3 (debugging file un libstdc++6-5-dbg <none> <none> (no description available) un libstdc++6-dbg <none> <none> (no description available)
Here’s the ldd abc for gcc build:
linux-vdso.so.1 => (0x00007ffef8f2f000) libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f87b167c000) libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f87b1465000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f87b109f000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f87b0d99000) /lib64/ld-linux-x86-64.so.2 (0x00007f87b1a11000)
And the ldd abc for clang build (note the pthread here and not in gcc):
linux-vdso.so.1 => (0x00007fffa4cc7000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fab1f10d000) libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fab1ed94000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fab1ea8d000) libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fab1e876000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fab1e4b1000) /lib64/ld-linux-x86-64.so.2 (0x00007fab1f347000)
For what it’s worth, my locale:
> locale LANG=en_AU.UTF-8 LANGUAGE=en_AU:en LC_CTYPE="en_AU.UTF-8" LC_NUMERIC="en_AU.UTF-8" LC_TIME="en_AU.UTF-8" LC_COLLATE="en_AU.UTF-8" LC_MONETARY="en_AU.UTF-8" LC_MESSAGES="en_AU.UTF-8" LC_PAPER="en_AU.UTF-8" LC_NAME="en_AU.UTF-8" LC_ADDRESS="en_AU.UTF-8" LC_TELEPHONE="en_AU.UTF-8" LC_MEASUREMENT="en_AU.UTF-8" LC_IDENTIFICATION="en_AU.UTF-8" LC_ALL=
Setting the default locale results in the same (and the same stack trace too):
> LC_ALL=C ./abc Segmentation fault (core dumped)
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Answer
It turns out it was the GCC compiler or libstdc++ that is buggy/broken. Linking with pthread shouldn’t cause a segfault.
Clang didn’t segfault, which is using the same libstdc++, so that suggests it may be the compiler. I tested with gcc version 5.1.0 (Ubuntu 5.1.0-0ubuntu11~14.04.1) and found it worked, so it confirms the GCC/libstdc++ bug.
The original GCC 4.9 which was used was from an unofficial test compiler repository (https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test) – not the official Ubuntu repository. However the GCC 5.1 that was tested, was also from the same repository, so at least the bug was fixed there. It is not known as to whether the official Ubuntu GCC 4.8 causes this segfault or not.