std::cin is not blocking when process is forked - c++

Question

I have a C++ console application which forks itself and closes the main process.
std::cin in the childprocess is not blocking anymore if a press any key. This results in an endless loop. If I don't fork before then the application behaves as expected.
I have tried different combinations of cin::ignore, cin::fail, cin::clear and close to get this fixed but without success.
I'm using Ubuntu 18.04.
Why this is happening and how can I fix that?
/* includes */
#include <iostream>
#include <unistd.h>
#include <limits>
void fork_to_background()
{
pid_t f_return = fork();
if (f_return == -1)
{
exit(1);
}
if (f_return != 0)
{
exit(0);
}
}
int main(int argc, char** argv)
{
fork_to_background();
std::string commands;
while(true)
{
std::cin >> commands;
std::cout << "Loop" << std::endl;
//std::cin.clear();
//std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}

Answer 1

std::cin >> commands; does not block when you reach EOF, you can see it checking if >> success, for instance
if (! std::cin >> commands) {
std::cout << "EOF" << std::endl;
break;
}
the two lines you put in comment are useless here because you read a string, they are useful in case you read for instance a number and the input is not a valid number
note the parent process exit immediately after the fork, closing stdin for the child because they share stdin
if I modify your program to have :
/* includes */
#include <stdlib.h>
#include <iostream>
#include <unistd.h>
#include <limits>
#include <sys/wait.h>
void fork_to_background()
{
pid_t f_return = fork();
if (f_return == -1)
{
exit(1);
}
if (f_return != 0)
{
waitpid(f_return, NULL, 0); // wait end of child
exit(0);
}
}
int main(int argc, char** argv)
{
fork_to_background();
std::string commands;
while(std::cin >> commands)
{
std::cout << commands << std::endl;
std::cout << "Loop" << std::endl;
}
std::cout << "done" << std::endl;
}
Compilation and execution :
/tmp % g++ -Wall f.cc
/tmp % echo "aze qsd" | ./a.out
aze
Loop
qsd
Loop
done
/tmp %