[c] Rerouting stdin and stdout from C

I want to reopen the stdin and stdout (and perhaps stderr while I'm at it) filehandles, so that future calls to printf() or putchar() or puts() will go to a file, and future calls to getc() and such will come from a file.

1) I don't want to permanently lose standard input/output/error. I may want to reuse them later in the program.

2) I don't want to open new filehandles because these filehandles would have to be either passed around a lot or global (shudder).

3) I don't want to use any open() or fork() or other system-dependent functions if I can't help it.

So basically, does it work to do this:

stdin = fopen("newin", "r");

And, if it does, how can I get the original value of stdin back? Do I have to store it in a FILE * and just get it back later?

freopen("/my/newstdin", "r", stdin);
freopen("/my/newstdout", "w", stdout);
freopen("/my/newstderr", "w", stderr);

... do your stuff

freopen("/dev/stdin", "r", stdin);
...
...

This peaks the needle on my round-peg-square-hole-o-meter, what are you trying to accomplish?

Edit:

Remember that stdin, stdout and stderr are file descriptors 0, 1 and 2 for every newly created process. freopen() should keep the same fd's, just assign new streams to them.

So, a good way to ensure that this is actually doing what you want it to do would be:

printf("Stdout is descriptor %d\n", fileno(stdout));
freopen("/tmp/newstdout", "w", stdout);
printf("Stdout is now /tmp/newstdout and hopefully still fd %d\n",
   fileno(stdout));
freopen("/dev/stdout", "w", stdout);
printf("Now we put it back, hopefully its still fd %d\n",
   fileno(stdout));

I believe this is the expected behavior of freopen(), as you can see, you're still only using three file descriptors (and associated streams).

This would override any shell redirection, as there would be nothing for the shell to redirect. However, its probably going to break pipes. You might want to be sure to set up a handler for SIGPIPE, in case your program finds itself on the blocking end of a pipe (not FIFO, pipe).

So, ./your_program --stdout /tmp/stdout.txt --stderr /tmp/stderr.txt should be easily accomplished with freopen() and keeping the same actual file descriptors. What I don't understand is why you'd need to put them back once changing them? Surely, if someone passed either option, they would want it to persist until the program terminated?

I think you're looking for something like freopen()

The os function dup2() should provide what you need (if not references to exactly what you need).

More specifically, you can dup2() the stdin file descriptor to another file descriptor, do other stuff with stdin, and then copy it back when you want.

The dup() function duplicates an open file descriptor. Specifically, it provides an alternate interface to the service provided by the fcntl() function using the F_DUPFD constant command value, with 0 for its third argument. The duplicated file descriptor shares any locks with the original.

On success, dup() returns a new file descriptor that has the following in common with the original:

• Same open file (or pipe)
• Same file pointer (both file descriptors share one file pointer)
• Same access mode (read, write, or read/write)

This is the most readily available, handy and useful way to do

freopen("dir","r",stdin);

freopen solves the easy part. Keeping old stdin around is not hard if you haven't read anything and if you're willing to use POSIX system calls like dup or dup2. If you're started to read from it, all bets are off.

Maybe you can tell us the context in which this problem occurs?

I'd encourage you to stick to situations where you're willing to abandon old stdin and stdout and can therefore use freopen.

And in the meantime, there's a C source code library that will do all this for you, redirecting stdout or stderr. But the cool part is that it lets you assign as many callback functions as you want to the intercepted streams, allowing you then to very easily send a single message to multiple destinations, a DB, a text file, etc.

On top of that, it makes it trivial to create new streams that look and behave the same as stdout and stderr, where you can redirect these new streams to multiple locations as well.

look for U-Streams C library on *oogle.

This is a modified version of Tim Post's method; I used /dev/tty instead of /dev/stdout. I don't know why it doesn't work with stdout (which is a link to /proc/self/fd/1):

freopen("log.txt","w",stdout);
...
...
freopen("/dev/tty","w",stdout);

By using /dev/tty the output is redirected to the terminal from where the app was launched.

Hope this info is useful.