Re-opening stdout and stdin file descriptors after

Question

I'm writing a function, which, given an argument, will either redirect the stdout to a file or read the stdin from a file. To do this I close the file descriptor associated with the stdout or stdin, so that when I open the file it opens under the descriptor that I just closed. This works, but the problem is that once this is done, I need to restore the stdout and stdin to what they should really be.

What I can do for stdout is open("/dev/tty",O_WRONLY); But I'm not sure why this works, and more importantly I don't know of an equivalent statement for stdin.

So I have, for stdout

close(1);
if (creat(filePath, O_RDWR) == -1)
{
    exit(1);
}

and for stdin

close(0);
if (open(filePath, O_RDONLY) == -1)
{
    exit(1);
}

Answer 1

You should use dup() and dup2() to clone a file descriptor.

int stdin_copy = dup(0);
int stdout_copy = dup(1);
close(0);
close(1);

int file1 = open(...);
int file2 = open(...);

< do your work. file1 and file2 must be 0 and 1, because open always returns lowest unused fd >

close(file1);
close(file2);
dup2(stdin_copy, 0);
dup2(stdout_copy, 1);
close(stdin_copy);
close(stdout_copy);

However, there's a minor detail you might want to be careful with (from man dup):

The two descriptors do not share file descriptor flags (the close-on-execflag). The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the duplicate descriptor is off.

If this is a problem, you might have to restore the close-on-exec flag, possibly using dup3() instead of dup2() to avoid race conditions.

Also, be aware that if your program is multi-threaded, other threads may accidentally write/read to your remapped stdin/stdout.

Answer 2

You could create a child process, and set up the redirection inside the child only. Then wait for the child to terminate, and continue working in the parent process. That way you don't have to worry about reversing your redirection at all.

Just look for examples of code using fork() and wait ().

Answer 3

I think you can "save" the descriptors before redirecting:

int save_in, save_out;

save_in = dup(STDIN_FILENO);
save_out = dup(STDOUT_FILENO);

Later on you can use dup2 to restore them:

/* Time passes, STDIN_FILENO isn't what it used to be. */
dup2(save_in, STDIN_FILENO);

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I am not doing any error checking in that example - you should.