Forking and Waiting in linux (C++). - c++

Question

I want to fork a process and then do the following in the parent:
Wait until it terminates naturally or timeout period set by the parent expires (something like waitforsingalobject in windows) after which I will kill the process using kill(pid);
Get the exit code of the child process (assuming it exited naturally)
I need to have access to the std::cout of the child process from the parent.
I attempted to use waitpid() however while this allows me access to the return code I cannot implement a timeout using this function.
I also looked at the following solution (https://www.linuxprogrammingblog.com/code-examples/signal-waiting-sigtimedwait) which allows me to implement a time-out however there doesnt seem a way to get the return code.
I geuss my question boils down to, Whats the correct way achieving this in linux?

Answer 1

You can do #1 and #2 with sigtimedwait function and #3 with pipe:
#include <unistd.h>
#include <signal.h>
#include <iostream>
int main() {
// Block SIGCHLD, so that it only gets delivered while in sigtimedwait.
sigset_t sigset;
sigemptyset(&sigset);
sigaddset(&sigset, SIGCHLD);
sigprocmask(SIG_BLOCK, &sigset, nullptr);
// Make a pipe to communicate with the child process.
int child_stdout[2];
if(pipe(child_stdout))
abort();
std::cout.flush();
std::cerr.flush();
auto child_pid = fork();
if(-1 == child_pid)
abort();
if(!child_pid) { // In the child process.
dup2(child_stdout[1], STDOUT_FILENO); // Redirect stdout into the pipe.
std::cout << "Hello from the child process.\n";
std::cout.flush();
sleep(3);
_exit(3);
}
// In the parent process.
dup2(child_stdout[0], STDIN_FILENO); // Redirect stdin to stdout of the child.
std::string line;
getline(std::cin, line);
std::cout << "Child says: " << line << '\n';
// Wait for the child to terminate or timeout.
timespec timeout = {1, 0};
siginfo_t info;
auto signo = sigtimedwait(&sigset, &info, &timeout);
if(-1 == signo) {
if(EAGAIN == errno) { // Timed out.
std::cout << "Killing child.\n";
kill(child_pid, SIGTERM);
}
else
abort();
}
else { // The child has terminated.
std::cout << "Child process terminated with code " << info.si_status << ".\n";
}
}
Outputs:
Child says: Hello from the child process.
Killing child.
If sleep is commented out:
Child says: Hello from the child process.
Child process terminated with code 3.