c - How to check if stdout has been redirected to NUL on Windows (a.k.a. /dev/null on Linux)?

Question

How can I check if my program's stdout has been redirected to NUL?
That way I can avoid outputting data since it's pointless.

I mainly need this for Windows, but if you have a Linux solution, it might be helpful for others in the future, so feel free to post that as well.

Answer 1

There are probably other ways to do this (and it wouldn't be a surprise if there turns out to be a proper function for it I've overlooked), but here's one way:

enum
{
    Output_Console,
    Output_File,
    Output_NUL,
};

bool GetOutputHandleType(int* piType)
{
    HANDLE h = GetStdHandle(STD_OUTPUT_HANDLE);
    if (h)
    {
        BY_HANDLE_FILE_INFORMATION fi;
        if (GetFileInformationByHandle(h, &fi))
        {
            *piType = Output_File;
            return true;
        }
        if (GetLastError() == ERROR_INVALID_FUNCTION)
        {
            *piType = Output_NUL;
            return true;
        }
        if (GetLastError() == ERROR_INVALID_HANDLE)
        {
            *piType = Output_Console;
            return true;
        }
    }
    return false;
}