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

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

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.
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?