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); }
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Answer
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.