Why the packaged task is terminating after throwing system error? [duplicate] - c++

Question

I got a strange core dump which I copied from a part of code in http://en.cppreference.com/w/cpp/thread/packaged_task,
#include <future>
#include <iostream>
#include <cmath>
void task_lambda() {
std::packaged_task<int(int,int)> task([](int a, int b) {
return std::pow(a, b);
});
std::future<int> result = task.get_future();
task(2, 9);
std::cout << "task_lambda:\t" << result.get() << '\n';
}
int main() {
task_lambda();
}
I got this
terminate called after throwing an instance of 'std::system_error'
what(): Unknown error -1
[1] 28373 abort (core dumped) ./a.out
The call stack is like below:
#0 0x00007ffff71a2428 in __GI_raise (sig=sig#entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
#1 0x00007ffff71a402a in __GI_abort () at abort.c:89
#2 0x00007ffff7ae484d in __gnu_cxx::__verbose_terminate_handler() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007ffff7ae26b6 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x00007ffff7ae2701 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5 0x00007ffff7ae2919 in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6 0x00007ffff7b0b7fe in std::__throw_system_error(int) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#7 0x0000000000404961 in std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) (__once=..., __f=<unknown type in /home/ace/test/a.out, CU 0x0, DIE 0x1246d>)
at /usr/include/c++/5/mutex:746
#8 0x0000000000403eb2 in std::__future_base::_State_baseV2::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>, bool) (
this=0x61ec30, __res=..., __ignore_failure=false) at /usr/include/c++/5/future:387
#9 0x0000000000402b76 in std::__future_base::_Task_state<task_lambda()::<lambda(int, int)>, std::allocator<int>, int(int, int)>::_M_run(<unknown type in /home/ace/test/a.out, CU 0x0, DIE 0x17680>, <unknown type in /home/ace/test/a.out, CU 0x0, DIE 0x17685>) (this=0x61ec30, __args#0=<unknown type in /home/ace/test/a.out, CU 0x0, DIE 0x17680>,
__args#1=<unknown type in /home/ace/test/a.out, CU 0x0, DIE 0x17685>) at /usr/include/c++/5/future:1403
#10 0x00000000004051c1 in std::packaged_task<int (int, int)>::operator()(int, int) (this=0x7fffffffdca0, __args#0=2, __args#1=9) at /usr/include/c++/5/future:1547
#11 0x0000000000401c7d in task_lambda () at aa.cc:12
#12 0x0000000000401d1b in main () at aa.cc:19
Then I added some sample code into my program, it became
#include <iostream>
#include <cmath>
#include <future>
#include <thread>
int f(int x, int y) { return std::pow(x,y); }
void task_thread() {
std::packaged_task<int(int,int)> task(f);
std::future<int> result = task.get_future();
std::thread task_td(std::move(task), 2, 10);
task_td.join();
std::cout << "task_thread:\t" << result.get() << '\n';
}
void task_lambda() {
std::packaged_task<int(int,int)> task([](int a, int b) {
return std::pow(a, b);
});
std::future<int> result = task.get_future();
task(2, 9);
std::cout << "task_lambda:\t" << result.get() << '\n';
}
int main() {
task_lambda();
}
the error was gone. How can I correct the program by adding a function even though I never call it?
gcc version
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)
Promgram compiled with command:
g++ -std=c++11 aa.cc -lpthread
With #fedepad's help, I got correct output by replace lpthread with pthread. But I still confused how second code work by add a dummy function!

Answer 1

I tried your first code snippet using the following version of g++:
g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
and compiled with the following
g++ -o test_threads test_threads.cpp -std=c++11 -pthread
and I can run the program with no problems, and getting the following output:
$ ./test_threads
task_lambda: 512
If I then use the -lpthread as you did with the following
g++ -o test_threads test_threads.cpp -std=c++11 -lpthread
I get
$ ./test_threads
terminate called after throwing an instance of 'std::system_error'
what(): Unknown error -1
[1] 7890 abort ./test_threads
So please use -pthread as a flag and not -lpthread.
This behavior is also mentioned in the following
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59830
There's a difference between -pthread and -lpthread.
Looking at the man page for g++
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.
To have a look to what flags are activated for both one can check with the following:
g++ -dumpspecs | grep pthread
g++ -dumpspecs | grep lpthread
As one can clearly see, there are some preprocessor macros that are not activated if one is using -lpthread.