Spent all day investigating this bug, and my colleagues are saying it looks like a linker or library bug. I've never had anything like that before, so I am here to document it and ask for help!
My executable segfaults before main is called
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x00007ffff7b47901 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007ffff7b47943 in std::locale::locale() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007ffff7b44724 in std::ios_base::Init::Init() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x0000000000400c1c in __static_initialization_and_destruction_0 (__initialize_p=1, __priority=65535)
at /usr/include/c++/4.8/iostream:74
#5 0x0000000000400c45 in _GLOBAL__sub_I__ZN9CrashTestC2Ev () at crash_test.cc:8
#6 0x0000000000400c9d in __libc_csu_init ()
#7 0x00007ffff7512e55 in __libc_start_main (main=0x400bea <main()>, argc=1, argv=0x7fffffffdca8,
init=0x400c50 <__libc_csu_init>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdc98)
at libc-start.c:246
#8 0x0000000000400ad9 in _start ()
(gdb)
The resulting crash seems similar to this question which leads to this bug report, but my code is different and very sensitive to changes. I narrowed down the issue to 5 requirements:
- Have an class implemented in a shared library.
- Declare an instance of that class in another, immediately following a std::string member declaration.
- Include iostream
- Link with pthreads
- Use g++-4.8 (or 4.9) and the gold linker
That's it. Change or omit any requirement, and the segfault does not occur.
I've created a minimal test case. Here is the executable header
// crash_test.h
#pragma once
#include <string>
#include "crash.h"
class CrashTest {
CrashTest(); // must have a constructor
std::string first_; // must be a string declared before Crash object
Crash crash_; // must be a value, not pointer
};
And the main function is empty. I don't even construct the class I defined!
#include "crash_test.h"
#include <iostream> // required
CrashTest::CrashTest() { } // must be here, not header
int main() {
return 0;
}
The Crash class can't get much simpler
// crash.h
#pragma once
struct Crash {
Crash();
};
But it does require an implementation to create a shared library.
#include "crash.h"
Crash::Crash() {} // must be here, not header
I've also tested this in a Docker container on a fresh install of Ubuntu 14.04, with g++-4.8 installed via apt-get.
Here is the build script.
#! /bin/sh
COMPILE="/usr/bin/x86_64-linux-gnu-g++-4.8 -Wall -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-missing-field-initializers -Werror -std=c++11 -O0 -g "
# Segfault only occurs using the gold linker.
LINKER="-fuse-ld=gold"
# Compile a shared library and then an executable.
# If the latter is linked with pthread, it segfaults when run.
# If the shared library is removed, there is no segfault.
$COMPILE -fPIC -o crash.o -c crash.cc \
&& $COMPILE -o crash_test.o -c crash_test.cc \
&& $COMPILE -fPIC $LINKER -shared -Wl,-soname,libcrash.so -o libcrash.so crash.o -Wl,-rpath=. \
&& $COMPILE $LINKER crash_test.o -o crash_test -rdynamic libcrash.so -pthread -Wl,-rpath=. \
&& echo "Compiled and linked..." \
&& ./crash_test \
&& echo "Did not crash!"
I've put all the code in a github repo: crash_test
Advice appreciated!
