gistfile1.h

static void
io_fflush(f, fptr)
    FILE *f;
    OpenFile *fptr;
{
    int n;

    if (!rb_thread_fd_writable(fileno(f))) {
        rb_io_check_closed(fptr);
    }
    for (;;) {
        TRAP_BEG;
        n = fflush(f);
        TRAP_END;
        if (n != EOF) break;
        if (!rb_io_wait_writable(fileno(f)))
            rb_sys_fail(fptr->path); /* this line is executed */
    }
    fptr->mode &= ~FMODE_WBUF;
}