Compiling multithread code with g++

Question

I have the easiest code ever:

#include <iostream>
#include <thread>

void worker()
{
    std::cout << "another thread";
}

int main()
{
    std::thread t(worker);
    std::cout << "main thread" << std::endl;
    t.join();
    return 0;
}

though I still cannot compile it with g++ to run.

More details:

$ g++ --version
g++ (Ubuntu/Linaro 4.8.1-10ubuntu8) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Command to compile:

$ g++ main.cpp -o main.out -pthread -std=c++11

Running:

$ ./main.out 
terminate called after throwing an instance of 'std::system_error'
  what():  Enable multithreading to use std::thread: Operation not permitted
Aborted (core dumped)

And now I'm in stuck. In every related thread over the internet it's recommended to add -pthread while I have it already.

What am I doing wrong?

PS: It's a brand new ubuntu 13.10 installation. Only g++ package was installed and minor things like chromium etc

PPS:

$ ldd ./a.out 
linux-vdso.so.1 => (0x00007fff29fc1000) 
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fb85397d000) 
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fb853767000) 
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb85339e000) 
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb85309a000) 
/lib64/ld-linux-x86-64.so.2 (0x00007fb853c96000)

PPPS: with clang++ (v3.2) it compiles and runs fine

PPPPS: guys, it's not a duplicate of What is the correct link options to use std::thread in GCC under linux?

PPPPPS:

$ dpkg --get-selections | grep 'libc.*dev'
libc-dev-bin                    install
libc6-dev:amd64                 install
libclang-common-dev             install
linux-libc-dev:amd64                install

Grouping

It's a symlink to libpthread-2.17.so in the same directory. – Grouping

Yes, that is a correct link. Here is my dir lrwxrwxrwx. 1 root root 18 Sep 12 20:52 libpthread.so.0 -> libpthread-2.16.so – Shears

@Jason Enochs: it would work for me - I agree, if I didn't use buggy gcc version – Grouping

Yea, all of your libraries are a bit newer than mine: gcc version 4.7.2 20121109 (Red Hat 4.7.2-8) (GCC) – Shears

@Jason Enochs: hehe. For myself I'm already satisfied with "it's a bug" answer, and probably will try to run it on an older ubuntu tomorrow, just to be completely sure – Grouping

Answer 1

The answer was provided by a kind member of SO C++ chat.

It looks like this behaviour is caused by a bug in gcc.

The workaround provided in the last comment of that bug discussion does work and solves the issue:

-Wl,--no-as-needed

Grouping

What would I do without SO and guys like you? You saved me a headful of hair pulling. Thanks :) This is as obscure as it gets (for me at least) – Mcnutt

@scorpiodawg: your comment made my evening, thanks :-) – Grouping

I had to add all of the -pthread -lpthread -Wl,--no-as-needed flags in order to get this work. – Dinger

Answer 2

Adding -lpthread fixed the identical problem for me:

 g++ -std=c++11 foo.cpp -lpthread -o foo

Supplejack

Jep, this is the anwser and also completely reasonable. "-Wl,--no-as-needed" sounded weird and also doesn't work for me. – Analgesia

You definitely need -lpthread. You might need -pthread -Wl,--no-as-needed, depending on compiler version. gcc-4.8.2 needs it. – Sizeable

Answer 3

I have slightly more advanced version (4.8.4 instead of 4.8.1), and I tested all three answers above. In fact:

-pthread alone works:

g++ -std=c++11 -o main -pthread main.cpp

-Wl,--no-as-needed alone does not work.

-lpthread alone does not work.

-Wl,--no-as-needed and -lpthread together work:

g++ -std=c++11 -o main -Wl,--no-as-needed main.cpp -lpthread

My version is "g++ (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4".

Niklaus

Well, the solutions were provided for the different version and I can assure the checked answer did work at the moment I checked it. Anyway, thanks for the actual info. – Grouping

Answer 4

answer already was made for qtcreator:

LIBS += -pthread
QMAKE_CXXFLAGS += -pthread
QMAKE_CXXFLAGS += -std=c++11

for console g++: here

g++ -c main.cpp -pthread -std=c++11         // generate target object file
g++ main.o -o main.out -pthread -std=c++11  // link to target binary

Elmiraelmo

Thank you, your answer is the only one that makes it clear that the -pthread has to be passed to the compiler and the linker. – Halo