I recently discovered a strange behaviour using std::thread and dlopen.
Basically, when I execute a std::thread in a library which is loaded using dlopen I receive a sigsev. The library itself is linked against pthread, the executable that calls dlopen is not.
Once I link the executable against pthread or the library itself everything works fine. However, we are using a plugin based infrastructure, where we do not know if the application itself is linked against pthread or not. Therefore, it is not an option to link the executable always against pthread.
Please find attached some code to reproduce the Issue. Currently I am not sure what causes the Issue. Is it a problem of gcc, glibc, libstdc++, or the ld.so? Is there a convenient way to work around this? I looks like this glibc bug is related, but I am using glibc2.27 (debian testing).
Calling pthread_create itself from the library seems to work.
hello.cpp
#include <thread>
#include <iostream>
void thread()
{
std::thread t ([](){std::cout << "hello world" << std::endl;});
t.join();
}
extern "C" {
void hello()
{
thread();
}
}
example.cpp
#include <iostream>
#include <dlfcn.h>
/** code from https://www.tldp.org/HOWTO/html_single/C++-dlopen/
*/
int main() {
std::cout << "C++ dlopen demo\n\n";
// open the library
std::cout << "Opening hello.so...\n";
void* handle = dlopen("./libhello.so", RTLD_LAZY);
if (!handle) {
std::cerr << "Cannot open library: " << dlerror() << '\n';
return 1;
}
// load the symbol
std::cout << "Loading symbol hello...\n";
typedef void (*hello_t)();
// reset errors
dlerror();
hello_t hello = (hello_t) dlsym(handle, "hello");
const char *dlsym_error = dlerror();
if (dlsym_error) {
std::cerr << "Cannot load symbol 'hello': " << dlsym_error <<
'\n';
dlclose(handle);
return 1;
}
// use it to do the calculation
std::cout << "Calling hello...\n";
hello();
// close the library
std::cout << "Closing library...\n";
dlclose(handle);
}
build.sh (build and execute the upper example. Example 1 crashes)
#!/bin/bash
echo "g++ -shared -fPIC -std=c++14 hello.cpp -o libhello.so -pthread"
g++ -shared -fPIC -std=c++14 hello.cpp -o libhello.so -pthread
echo "g++ example.cpp -o example1 -ldl"
g++ example.cpp -o example1 -ldl
echo "g++ example.cpp -o example2 -ldl -pthread"
g++ example.cpp -o example2 -ldl -pthread
echo "g++ example.cpp -o example3 -ldl -lhello -L ./"
g++ example.cpp -o example3 -ldl -lhello -L ./
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:$(pwd)
echo "===== example1 ====="
./example1
echo "===== end ====="
echo "===== example2 ====="
./example2
echo "===== end ====="
echo "===== example3 ====="
./example3
echo "===== end ====="
EDIT
I forgot to mention: If I am running the faulty example (i.e. example 1) using LD_DEBUG=all the program crashes during the lookup of pthread_create. Even more interesting is that a former lookup of pthread_create succeeds:
8111: symbol=_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: binding file ./libhello.so [0] to /usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]: normal symbol `_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_' [GLIBCXX_3.4]
8111: symbol=pthread_create; lookup in file=./example1 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libdl.so.2 [0]
8111: symbol=pthread_create; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libm.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libgcc_s.so.1 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libc.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib64/ld-linux-x86-64.so.2 [0]
8111: symbol=pthread_create; lookup in file=./libhello.so [0]
8111: symbol=pthread_create; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libm.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libgcc_s.so.1 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libpthread.so.0 [0]
8111: binding file ./libhello.so [0] to /lib/x86_64-linux-gnu/libpthread.so.0 [0]: normal symbol `pthread_create' [GLIBC_2.2.5]
8111: symbol=_ZTVNSt6thread6_StateE; lookup in file=./example1 [0]
8111: symbol=_ZTVNSt6thread6_StateE; lookup in file=/lib/x86_64-linux-gnu/libdl.so.2 [0]
8111: symbol=_ZTVNSt6thread6_StateE; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: binding file ./libhello.so [0] to /usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]: normal symbol `_ZTVNSt6thread6_StateE' [GLIBCXX_3.4.22]
...
8111: binding file ./libhello.so [0] to ./libhello.so [0]: normal symbol `_ZNSt10_Head_baseILm0EPNSt6thread6_StateELb0EE7_M_headERS3_'
8111: symbol=_ZNSt6thread15_M_start_threadESt10unique_ptrINS_6_StateESt14default_deleteIS1_EEPFvvE; lookup in file=./example1 [0]
8111: symbol=_ZNSt6thread15_M_start_threadESt10unique_ptrINS_6_StateESt14default_deleteIS1_EEPFvvE; lookup in file=/lib/x86_64-linux-gnu/libdl.so.2 [0]
8111: symbol=_ZNSt6thread15_M_start_threadESt10unique_ptrINS_6_StateESt14default_deleteIS1_EEPFvvE; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: binding file ./libhello.so [0] to /usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]: normal symbol `_ZNSt6thread15_M_start_threadESt10unique_ptrINS_6_StateESt14default_deleteIS1_EEPFvvE' [GLIBCXX_3.4.22]
8111: symbol=pthread_create; lookup in file=./example1 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libdl.so.2 [0]
8111: symbol=pthread_create; lookup in file=/usr/lib/x86_64-linux-gnu/libstdc++.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libm.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libgcc_s.so.1 [0]
8111: symbol=pthread_create; lookup in file=/lib/x86_64-linux-gnu/libc.so.6 [0]
8111: symbol=pthread_create; lookup in file=/lib64/ld-linux-x86-64.so.2 [0]
./build.sh: line 18: 8111 Segmentation fault (core dumped) LD_DEBUG=all ./example1
===== end =====
