Child Process runs even after parent process has exited? - c++

Question

I was writing a code for a research program. I have following requirement:
1. Main binary execution begins at main()
2. main() fork()
3. child process runs a linpack benchmark binary using execvp()
4. parent process runs some monitoring process and wait for child to exit.
The code is below:
main.cpp
extern ServerUncorePowerState * BeforeStates ;
extern ServerUncorePowerState * AfterStates;
int main(int argc, char *argv[]) {
power pwr;;
procstat st;
membandwidth_t data;
int sec_pause = 1; // sample every 1 second
pid_t child_pid = fork();
if (child_pid >= 0) { //fork successful
if (child_pid == 0) { // child process
int exec_status = execvp(argv[1], argv+1);
if (exec_status) {
std::cerr << "execv failed with error "
<< errno << " "
<< strerror(errno) << std::endl;
}
} else { // parent process
int status = 1;
waitpid(child_pid, &status, WNOHANG);
write_headers();
pwr.init();
st.init();
init_bandwidth();
while (status) {
cout << " Printing status Value: " << status << endl;
sleep (sec_pause);
time_t now;
time(&now);
struct tm *tinfo;
tinfo = localtime(&now);
pwr.loop();
st.loop();
data = getbandwidth();
write_samples(tinfo, pwr, st, data.read_bandwidth + data.write_bandwidth);
waitpid(child_pid, &status, WNOHANG);
}
wait(&status); // wait for child to exit, and store its status
//--------------------This code is not executed------------------------
std::cout << "PARENT: Child's exit code is: "
<< WEXITSTATUS(status)
<< std::endl;
delete[] BeforeStates;
delete[] AfterStates;
}
} else {
std::cerr << "fork failed" << std::endl;
return 1;
}
return 0;
}
What is expected that the child will exit and then parent exits but due to some unknown reason after 16 mins parent exits but child is still running.
Normally It is said that when parent exits the child dies automatically.
What could be the reason for this strange behavior???

Answer 1

Normally It is said that when parent exits the child dies automatically.
Well this is not always true, it depends on the system. When a parent process terminates, the child process is called an orphan process. In a Unix-like OS this is managed by relating the parent process of the orphan process to the init process, this is called re-parenting and it's automatically managed by the OS. In other types of OS, orphan processes are automatically killed by the system. You can find more details here.
From the code snippet I would think that maybe the issue is in the wait(&status) statement. The previous loop would end (or not be executed) when the return status is 0, which is the default return value from your final return 0 at the end, that could be yielded by the previous waitpid(child_pid, &status, WNOHANG) statements. This means that the wait(&status) statement would wait on a already terminated process, this may cause some issues.