errno doesn't change after putting a negative value in sqrt()

With errno, I am trying to check if cmath functions produce a valid result. But even after I put a negative value in sqrt() or log(), errno stays at value 0. Can anyone know why, and how I can make errno behave correctly?

The environment is macOS Monterey version 12.6.1, the compiler is gcc version 11.3.0 (Homebrew GCC 11.3.0_1) or Apple clang version 14.0.0 (clang-1400.0.29.202) (I tried the 2 compilers). The compile command is g++ test_errno.cpp -o test_errno -std=c++14.

The piece of code I tried is directly copied from this page. The following is the code.

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <clocale>
 
int main()
{
    double not_a_number = std::log(-1.0);
    std::cout << not_a_number << '\n';
    if (errno == EDOM) {
        std::cout << "log(-1) failed: " << std::strerror(errno) << '\n';
        std::setlocale(LC_MESSAGES, "de_DE.utf8");
        std::cout << "Or, in German, " << std::strerror(errno) << '\n';
    }
}

Its result didn't print the error messages, which should be printed if errno is set correctly.

Solution

It seems that on macOS, errno is not used, from this bug report from @GAVD's comment. I could check that via math_errhandling value from @Pete Becker's comment. It seems there are 2 ways of math error handling in C/C++, either with errno, or with floating-point exception, as the 2nd link above shows. We can check which way (or both of them) the system's math library employs, via checking if macro constant math_errhandling is equal to MATH_ERREXCEPT or MATH_ERRNO, like the following (copied from the 2nd link):

    std::cout << "MATH_ERRNO is "
              << (math_errhandling & MATH_ERRNO ? "set" : "not set") << '\n'
              << "MATH_ERREXCEPT is "
              << (math_errhandling & MATH_ERREXCEPT ? "set" : "not set") << '\n';

And on my system, the output is

MATH_ERRNO is not set
MATH_ERREXCEPT is set

, which means the system does not use errno for reporting math errors, but use floating-point exception. That's why errno stays at value 0 no matter what, and I should have used std::fetestexcept() to check error conditions. With floating-point exceptions, std::feclearexcept(FE_ALL_EXCEPT); corresponds to errno = 0;, and std::fetestexcept(FE_DIVBYZERO) corresponds to errno == ERANGE for example.