How to redirect the output back to the screen after freopen("out.txt", "a", stdout)

Question

#include <stdio.h>

int main() {
    printf("This goes to screen\n");
    freopen("out.txt", "a", stdout);
    printf("This goes to out.txt");
    freopen("/dev/stdout", "a", stdout);
    printf("This should go to screen too, but doesn't\n");

    return 0;
}

I call freopen to redirect the stdout to out.txt then I print something on the file, now I want to redirect it back to the screen, but freopen("/dev/stdout", "a", stdout); doesn't work. Is there any way to do that using ANSI C or POSIX system calls?

Answer 1

Unfortunately, there doesn't seem to be a good way:

http://c-faq.com/stdio/undofreopen.html

The best recommendation is not to use freopen in this circumstance.

Answer 2

Generally speaking, you can't. You have closed the file, which could've been pipe or whatever. It's not reopenable. You might have saved stdout value, then assign some fopen to it and then close it and copy the old value back. Example:

FILE *o = stdout;
stdout=fopen("/tmp/crap.txt","a");
printf("Oh no!\n");
fclose(stdout);
stdout = o;

Mike Weller suggested below in comments that stdout might not always be writable. In this case something like that might help:

int o = dup(fileno(stdout));
freopen=fopen("/tmp/crap.txt","a",stdout);
printf("Oh no!\n");
dup2(o,fileno(stdout));
close(o);

Another edit: if you're using it to redirect output from the child process like your comment elsewhere suggest, you can redirect it after the fork.

Answer 3

I can't think of a way to do this in a cross-platform manner, but on GNU/Linux systems (and maybe other POSIX-compliant ones, too) you can freopen ("/dev/tty", "a", stdout). Is this what you were trying to do?

Answer 4

Use fdopen() and dup() as well as freopen().

int old_stdout = dup(1);  // Preserve original file descriptor for stdout.

FILE *fp1 = freopen("out.txt", "w", stdout);  // Open new stdout

...write to stdout...   // Use new stdout

FILE *fp2 = fdopen(old_stdout, "w");   // Open old stdout as a stream

...Now, how to get stdout to refer to fp2?
...Under glibc, I believe you can use:

fclose(stdout);    // Equivalent to fclose(fp1);
stdout = fp2;      // Assign fp2 to stdout
// *stdout = *fp2;   // Works on Solaris and MacOS X, might work elsewhere.

close(old_stdout);   // Close the file descriptor so pipes work sanely

I'm not sure whether you can do the assignment reliably elsewhere.

Dubious code that does actually work

The code below worked on Solaris 10 and MacOS X 10.6.2 - but I'm not confident that it is reliable. The structure assignment may or may not work with Linux glibc.

#include <stdio.h>

int main()
{
    printf("This goes to screen\n");
    int old_stdout = dup(1);
    FILE *fp1 = freopen("out.txt", "a", stdout);
    printf("This goes to out.txt\n");
    fclose(stdout);
    FILE *fp2 = fdopen(old_stdout, "w");
    *stdout = *fp2;
    printf("This should go to screen too, but doesn't\n");

    return 0;
}

You can't say you weren't warned -- this is playing with fire.

The better solutions either make the code use 'fprintf(fp, ...)' everywhere, or use a cover function that allows you set your own default file pointer:

mprintf.c

#include "mprintf.h"
#include <stdarg.h>

static FILE *default_fp = 0;

void set_default_stream(FILE *fp)
{
    default_fp = fp;
}

int mprintf(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);

    if (default_fp == 0)
        default_fp = stdout;

    int rv = vfprintf(default_fp, fmt, args);

    va_end(args);
    return(rv);
 }

mprintf.h

#ifndef MPRINTF_H_INCLUDED
#define MPRINTF_H_INCLUDED

#include <stdio.h>

extern void set_default_stream(FILE *fp);
extern int  mprintf(const char *fmt, ...);

#endif

Clearly, you can create an mvprintf() and other functions as needed.

Example use of mprintf()

Then, in place of the original code, you can use:

#include "mprintf.h"

int main()
{
    mprintf("This goes to screen\n");
    FILE *fp1 = fopen("out.txt", "w");
    set_default_stream(fp1);
    mprintf("This goes to out.txt\n");
    fclose(fp1);
    set_default_stream(stdout);
    mprintf("This should go to screen too, but doesn't\n");

    return 0;
}

(Warning: untested code - confidence level too high. Also, all code written assuming you use a C99 compiler, primarily because I declare variables when I first need them, not at the beginning of the function.)