Member Avatar for rciprogrammer

How do I 'restore' my output (stdout) back to the terminal after redirecting it to a file. I want to be able to output to a file for a specific section of code, then close that file and put output back to the screen/terminal.

fprintf(stdout, "This will go to the terminal.\n");
stdout = fopen("/tmp/somefile.txt", "w");
fprintf(stdout, "This will go to the file /tmp/somefile.txt.\n");
...
fclose(stdout);
fprintf(stdout, "I WANT this to go to the terminal.\n");

Using this code, I get nothing after the close (obviously closing is wrong). How do I restore the output?

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Dani AI

Generated

Two clean approaches, depending on whether you are using C or C++:

  • C++ iostreams: instead of touching the process-wide stdout, temporarily swap std::cout's stream buffer. This redirects only std::cout (not other FILE* streams), and is easier to scope and test.
#include <iostream>
#include <fstream>

int main() {
    std::cout << "This goes to the terminal\n";

    std::ofstream file("/tmp/somefile.txt");
    auto* saved = std::cout.rdbuf(file.rdbuf());  // redirect
    std::cout << "This goes to the file\n";
    std::cout.flush();                             // ensure it is written

    std::cout.rdbuf(saved);                        // restore
    std::cout << "Back to the terminal\n";
}

For multithreaded programs, this still affects all threads that write to std::cout. Guard with a mutex during the redirection window. Avoid mixing printf/fprintf with iostreams during redirection unless you understand the buffering interaction; flush both sides if you must mix. See cppreference: basic_ios::rdbuf.

  • POSIX C (Linux): the C standard library does not define a portable way to "undo" a redirection of stdout; the usual POSIX technique is to duplicate and later restore file descriptor 1. If you need a known terminal even when stdout was not originally a TTY, you can reattach to /dev/tty (if available) or to the original descriptor path such as /proc/self/fd/1 on Linux. Check all return values and fflush(stdout) before switching. See POSIX dup/dup2, freopen, and isatty.

Practical tip: for robustness in larger codebases, prefer passing an explicit FILE*/std::ostream& (or a logger interface) into functions instead of mutating global stdout. This avoids cross-thread surprises.

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Member Avatar for rciprogrammer

I should have specified that I am running in Linux and not windows.

So according to this page you gave, something like:

fprintf(stdout, "This will go to the terminal.\n");

FILE *myOutput;
myOutput = fopen("/tmp/somefile.txt", "w");
fprintf(myOutput, "This will go to the file /tmp/somefile.txt.\n");
...
fclose(myOutput);
fprintf(stdout, "I WANT this to go to the terminal.\n");
Member Avatar for chaines51

Yes, that should work as you expect it to.

Member Avatar for rciprogrammer

This is easier if all your code is contained in one file, but what about multi-threaded programs - I will then have to pass this to every function, right?

Member Avatar for rciprogrammer

According to the post, this is how its done (for anyone interested):

fprintf(stdout, "This will go to the terminal.\n");

//Save position of current standard output
fpos_t pos;
fgetpos(stdout, &pos);
int fd = dup(fileno(stdout));
freopen("/tmp/somefile.txt", "w", stdout);

fprintf(stdout, "This will go to the file /tmp/somefile.txt.\n");

//Flush stdout so any buffered messages are delivered
fflush(stdout);
//Close file and restore standard output to stdout - which should be the terminal
dup2(fd, fileno(stdout));
close(fd);
clearerr(stdout);
fsetpos(stdout, &pos);

fprintf(stdout, "This will go to the terminal.\n");
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