threading program runs with g++ but not with gcc - 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();
}

Answer 1

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

Answer 2

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.

Answer 3

Try adding the -lrt flag after the pthread one.