Determine the number of cores at compile time in C/C++

Question

Is there a way to determine how many physical cores a target machine has at compile time in C/C++ in Linux under GCC?

I am aware of other methods like std::thread::hardware_concurrency() in C++11 or sysconf(_SC_NPROCESSORS_ONLN) but I am curious to know if there is actually a way to obtain this information at compile time.

Answer 1

You can query information during the build processes and pass it into the program as a pre-processor definition.

Example

g++ main.cpp -D PROC_COUNT=$(grep -c ^processor /proc/cpuinfo)

where main.cpp is

#include <iostream>
int main() {
    std::cout << PROC_COUNT << std::endl;
    return 0;
}

Edit

As pointed out in the comments. If the target machine differs from the build machine then you'll need to replace the method grep -c ^processor /proc/cpuinfo with something that queries the number of processors on the target machine. The details would depend on what form of access you have to the target machine during build.