Question

I have function in C that assumes stdin is open. I want to add an assertion in front of it to make sure stdin is not closed by anyone. How can I check that stdin isn’t closed by anyone?

assert(is_open(stdin));

Answer 1

You can’t find out whether a FILE* has been closed. fclose might free the FILE object it points to, so that object’s contents may be undefined after fclose. This is true even of stdin. The solution I proposed previously was wrong. Sorry about that.

The best you can do on a POSIX platform is something like

bool stdin_open()
{
    errno = 0;
    fcntl(STDIN_FILENO, F_GETFD);
    return errno == EBADF;
}

though that really tells you something about the standard input FD, rather than the stdin object.