Redirect FROM stderr to another file descriptor

Question

My program calls library functions which print to stderr. I want to intervene so that all write calls to file descriptor #2 will instead get sent to somewhere else.

Here is my first attempt:

bool redirect_stderr (int fd)
{
    return dup2 (2, fd) > 0;
}

Here, fd was successfully obtained from open("/foo/bar",O_APPEND|O_CREAT)

After this function returns true, std::cerr<<"blah" goes to the terminal and not to the file.

What am I doing wrong?

Thanks.

UPDATE

Thanks, larsmans, but I'm not there yet...

void redirect_stderr_to (const char * output_file)
{
    int fd = open (output_file, O_APPEND | O_CREAT, S_IRUSR | S_IWUSR);

    if (fd < 0) {
        throw RUNTIME_ERROR;
    }
    else {
        if (-1 == dup2 (fd, STDERR_FILENO))
            throw RUNTIME_ERROR;

        std :: cout << (std::cerr ? "Fine\n" : "Bad\n");
        char x [100];
        std :: cerr
            << "Output to " << getcwd (x, 100) << " / " << output_file
            <<  " yields " << fd << " errno: " << errno << "\n";
        std :: cout << (std::cerr ? "Fine\n" : "Bad\n");
    }
}

This outputs

Fine
Bad

to stdout and the given file is empty. (It is correctly created if it doesn't exist.)

Heritor

Please accept some answers before you post your next question. Click the checkmark next to the best answer for your previous questions. – Favoritism

Answer 1

You reversed the arguments: it's

dup2(from_fd, to_fd)

i.e.

dup2(fd, 2)

(see POSIX.2008 or your manpages.)

Favoritism

man page says int dup2(int oldfd, int newfd); and 2 is the old fd, no? – Heritor

No: oldfd is the fd copied from, and newfd is the fd copied to. (oldfd and newfd aren't very good names for the arguments. Is that a Linux manpage?) – Favoritism

Answer 2

Just for completeness: You could even achieve your goal with freopen(name, mode, stderr), which is a Standard/ANSI/ISO C89 and C99 function.

Guardi