How to use precompiled headers with gcc if linking against openmp

Question

Minimal example:

// file: main.cpp
#include "pch.h"

int main()
{
    std::cout << "test" << std::endl;
    return 0;
}

--

// file: pch.h
#include <iostream>

Works fine and as expected if I compile this with

g++ pch.h
g++ main.cpp -Winvalid-pch

However once I change the last line to:

g++ main.cpp -fopenmp -Winvalid-pch

usage of the precompiled header is disabled:

warning: pch.h.gch: not used because `_REENTRANT' is defined [-Winvalid-pch]

How can I still use precompiled headers while linking to OpenMP? Why does the _REENTRANT define conflict with using a precompiled header at all?

Answer 1

You must generate .pch and compile sources with identical flags. -fopenmp implies #pragma omp and -pthread.

g++ -fopenmp pch.h
g++ main.cpp -fopenmp -Winvalid-pch

Or at least

g++ -pthread pch.h
g++ main.cpp -fopenmp -Winvalid-pch