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    <p>Use <code>fdopen()</code> and <code>dup()</code> as well as <code>freopen()</code>.</p> <pre><code>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 </code></pre> <p>I'm not sure whether you can do the assignment reliably elsewhere.</p> <h3>Dubious code that does actually work</h3> <p>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.</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;unistd.h&gt; int main(void) { printf("This goes to screen\n"); int old_stdout = dup(1); // Consider dup(STDOUT_FILENO) or dup(fileno(stdout)) FILE *fp1 = freopen("out.txt", "a", stdout); printf("This goes to out.txt\n"); fclose(stdout); FILE *fp2 = fdopen(old_stdout, "w"); *stdout = *fp2; // Unreliable! printf("This should go to screen too, but doesn't\n"); return 0; } </code></pre> <p>You can't say you weren't warned — <strong>this is playing with fire!</strong></p> <p>If you're on a system with the <code>/dev/fd</code> file system, you could create the name of the file implied by the file descriptor returned from <code>dup()</code> with <code>sprintf(buffer, "/dev/fd/%d", old_stdout)</code> and then use <code>freopen()</code> with that name. This would be a lot more reliable than the assignment used in this code.</p> <p>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:</p> <h3>mprintf.c</h3> <pre><code>#include "mprintf.h" #include &lt;stdarg.h&gt; 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); } </code></pre> <h3>mprintf.h</h3> <pre><code>#ifndef MPRINTF_H_INCLUDED #define MPRINTF_H_INCLUDED #include &lt;stdio.h&gt; extern void set_default_stream(FILE *fp); extern int mprintf(const char *fmt, ...); #endif </code></pre> <p>Clearly, you can create an mvprintf() and other functions as needed.</p> <h3>Example use of mprintf()</h3> <p>Then, in place of the original code, you can use:</p> <pre><code>#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; } </code></pre> <p>(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.)</p> <hr> <p>Caution:</p> <p>Note that if the original program is invoked as <code>./original_program &gt; file</code> or <code>./original_program | grep something</code> (with redirected output) or is run from a <code>cron</code> job, then opening <code>/dev/tty</code> is not usually appropriate as a way to reopen standard output because the original standard output was not the terminal.</p> <p>Also, note that if the redirection of standard output is used prior to forking and execing a child program and the original standard output is reinstated in the parent, then the sequence of operations is wrong. You should fork and then adjust the I/O of the child (only), without modifying the parent's I/O at all.</p>
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    1. POHow to redirect the output back to the screen after freopen("out.txt", "a", stdout)
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    1. USJonathan Leffler
    1. USJonathan Leffler
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    1. COYour first suggestion seems to be the right way to do it, but Patrick's suggestion is a lot simpler and does what I need. I did not test your answer but I think it should work on linux too. Thanks for the answer though. fprintf(fp,...) is not an option for me because I use exec() on the child process.
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      1. USHoffmann
    2. COHoffman, if you're using exec (presumably you first fork, if it's a child process) then why not do the redirection *after* you forked?
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      1. USMichael Krelin - hacker
    3. COThis is actually a really nice solution, Hoffmann. You could even simplify this down with a variable argument macro to wrap printf. This would only take one line.
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      1. USPatrick Niedzielski
 

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