redirect stdout/stderr to file under unix c++ - again

Question

What I want to do

redirect stdout and stderr to one or more files from inside c++

Why I need it

I am using an external, precompiled third-party library that produces a ridiculous amount of output, which I would like to redirect to a log file to keep the console clean.

Conditions

Compatibility is not a problem, the code will only run on Unix systems. The redirection should not only affect c++-style printing (std::cout << "hello world" << std::endl), but also c-style printing (printf("hello world\n")).

What I have tried so far

I have been browsing on stackoverflow for half a day, reading multiple answers to people having similar problems. With the help of these answers, I have been able to put together the following piece of code:

---

#include <stdio.h>
#include <iostream>
#include <fcntl.h>
#include "unistd.h"

const int stdoutfd(dup(fileno(stdout)));

int redirect_stdout(const char* fname){
  fflush(stdout);
  int newstdout = open(fname, O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP |     S_IROTH);
  dup2(newstdout, fileno(stdout));
  close(newstdout);
}

int restore_stdout(){
  fflush(stdout);
  dup2(stdoutfd, fileno(stdout));
  close(stdoutfd);
  return stdoutfd;
}

int main(){
  redirect_stdout("/dev/null");
  std::cout << "invisible 1" << std::endl;
  restore_stdout();
  std::cout << "visible 1" << std::endl;
  redirect_stdout("/dev/null");
  std::cout << "invisible 2" << std::endl;
  restore_stdout();
  std::cout << "visible 2" << std::endl;
  return 0;
}

---

What I would expect to see:

visible 1
visible 2

What I actually see

visible 1

That is, when using this mechanism for the first time, it works - but if used again, restoring the output will not work. Can somebody point out to me what I need to change in order to have the mechanism work infinitely often?

Answer 1

In addition to afr0ck answer of freopen() I want to say that while using freopen() we should be careful. Once a stream like stdout or stdin is reopened with assigning the new destination(here the 'output.txt' file) always it remains for a program unless it has been explicitly change.

freopen("output.txt", "a", stdout);

Here the standard output stream stdout is reopened and assigned with the 'output.txt' file. After that whenever we use printf() or any other stdout stream like - putchar() then every output will goes to the 'output.txt'. To get back the default behavior (that is printing the output in console/terminal) of printf() or putchar() we can use the following line of code -

• for gcc, linux distribution like ubuntu - freopen("/dev/tty", "w", stdout);
• for Mingw C/C++, windows - freopen("CON", "w", stdout);

See the code example below -

#include <stdio.h>

int main() {
    
    printf("No#1. This line goes to terminal/console\n");

    freopen("output.txt", "a", stdout);
    printf("No#2. This line goes to the \"output.txt\" file\n");
    printf("No#3. This line aslo goes to the \"output.txt\" file\n");

    freopen("/dev/tty", "w", stdout); /*for gcc, diffrent linux distro eg. - ubuntu*/
    //freopen("CON", "w", stdout); /*Mingw C++; Windows*/
    printf("No#4. This line again goes to terminal/console\n");        

}

This code generate a 'output.txt' file in your current directory and the No#2 and No#3 will be printed in the 'output.txt' file.

Thanks

Answer 2

If you want to be able to reuse it, don't close stdoutfd in restore_stdout.