threading program runs with g++ but not with gcc

Question

I have compiled a c++ code using g++ -std=c++11 -o main main.cpp -pthread and it compiled fine however if I compile the same code using gcc -std=c++11 -o main main.cpp -pthread it does not compile and throws error. The program uses threading which properly taken care of using -pthread option while compiling. For the reference I am attaching the code below. Any help is highly appreciated.

#include <iostream>
#include <thread>


class foo
{
public:
    void bar(int loop_num)
    {
        for (int i = 0; i < loop_num; ++i) {
            std::cout << "Thread executing\n";
            ++n;
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
    int n = 0;
};

int main()
{
    int n = 0;
    foo f;
    std::thread t1(&foo::bar, &f, 5);

    t1.join();
}

Possible duplicate of [What is the difference between g++ and gcc?](https://stackoverflow.com/questions/172587/what-is-the-difference-between-g-and-gcc) — Mike Kinghan, Feb 18 '19 at 09:19

score 3 · Accepted Answer · answered Feb 22 '19 at 05:05

If you’ve written C++ code with GCC, you’ll know that you need to use the program g++, both for compilation and linking. For multi-module programs, this means every .cpp file gets compiled with g++, and then the entire program must be linked separately using g++. If you try to link the program using gcc, it will almost work, but you’ll get a lot of “undefined reference” errors, like this:

test.cpp:(.text+0x11): undefined reference to `std::cout' The need to use g++ to link the entire program causes trouble when you have a very complicated build process you don’t have full control of. For instance, I’m trying to link C++ code with Mercury, and I have to use the Mercury linker, which in turn calls gcc.

So just a quick tip: If you are forced to use gcc to link the program, just add the library “stdc++”, as you would any other library, and it will work. That is, add the option “-lstdc++” to your GCC linker command line. For example:

g++ -c hello.cpp

gcc -lstdc++ -o hello hello.o

score 1 · Answer 2 · answered Feb 18 '19 at 07:01

I assume the error you get looks something like this:

/tmp/ccUKAq0K.o: In function `main':
main.cpp:(.text+0x59): undefined reference to `std::thread::join()'
/tmp/ccUKAq0K.o: In function `__static_initialization_and_destruction_0(int, int)':
main.cpp:(.text+0xb6): undefined reference to `std::ios_base::Init::Init()'
main.cpp:(.text+0xcb): undefined reference to `std::ios_base::Init::~Init()'
/tmp/ccUKAq0K.o: In function `std::thread::~thread()':
main.cpp:(.text._ZNSt6threadD2Ev[_ZNSt6threadD5Ev]+0x1d): undefined reference to `std::terminate()'

(And so on.)

C++ programs which use the standard library (so most of them) need to be linked using g++, not gcc. Only the g++ compiler driver links in most of the standard library. The gcc compiler driver compiles C++ programs, just like g++, but when the linker is invoked, the program is treated as a C program, which usually leads to linker errors.

You need to use `g++` for linking. Why do you want to use `gcc` for linking if it does not work (and is not expected to work)? — Florian Weimer, Feb 18 '19 at 10:52

score 0 · Answer 3 · answered Feb 18 '19 at 04:40

0

Try adding the -lrt flag after the pthread one.

answered Feb 18 '19 at 04:40

Mr.Mips

379
2
18

threading program runs with g++ but not with gcc

3 Answers3