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clinuxbsd

Is it possible that `fileno(stdout) != 1` on a POSIX system?

Can stdout file descriptor differ from 1 (STDOUT_FILENO) assuming stdout need not be a modifiable lvalue?

For example, can freopen("/dev/null", "w", stdout) change fileno(stdout) result?

Solution

  • Yes.

    Test program:

    #include <stdio.h>

int main() {
    fclose(stdin);
    freopen("stdout.txt", "w+", stdout);
    fprintf(stderr, "%d\n", fileno(stdout));
    return 0;
}

    This prints 0 on my machine (OS X 10.9.4).

    File descriptors are typically reused starting from the lowest number first. By closing stdin, file descriptor 0 is freed up, and the subsequent freopen will use file descriptor 0 when opening the file.