Linux shuts down after compiling basic children process - c

Question

In this simple C code, I am trying to create 10 children. Then do some work (such as printing the time) in them individually. Then I try to kill all of them in parent process. You will easily understand the code below. The problem is that, whenever I run this, first it gives the various outputs from children then Linux shuts down. Then I start from passing user password and everything is closed. Why this happens?
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdlib.h>
#include <time.h>
int main() {
pid_t pids[10];
int i;
int n= 10;
for (i= 0; i<n; ++i) {
if ((pids[i] = fork()) <0) {
perror("fork");
abort();
}
else if (pids[i] == 0) {
struct timeval tv;
time_t t;
struct tm*info;
char buffer[64];
gettimeofday(&tv,NULL);
t = tv.tv_sec;
info = localtime(&t);
sleep(1);
printf("CHILD PID: %d and the time is %s\n",getpid(),asctime(info));
}
}
int status;
pid_t pid;
sleep(5);
while (n >0) {
pid = wait(&status);
kill((long)pid, SIGTERM);
printf("CHILD %d killed.",pid);
--n;
}
}

Answer 1

In process created by fork() all variables have the same value as before fork. After fork in this your code
for (i= 0; i<n; ++i) {
if ((pids[i] = fork()) <0) {
perror("fork");
abort();
}
else if (pids[i] == 0) {
...
}
}
i in child process is the same as it was in parent before fork, so for loop continue to execute and spawn a lot of grand children processes.
After loop is executed all spawned processes call wait(). Processes which are spawned on last iteration (with i==9) have no children, so wait() indicates error by returning -1 (and errno==ECHILD). Then you call kill() with pid==-1, which is:
If pid equals -1, then sig is sent to every process for which the calling process has permission to send signals, except for process 1 (init)
If you want child processes to terminate by themself then you should exit explicitly, e.g. with _exit() function. In this case you don't need to send TERM signal:
int main(void)
{
pid_t pids[10];
int i;
int n = 10;
for (i = 0; i<n; ++i)
{
if ((pids[i] = fork()) < 0)
{
perror("fork");
abort();
// abort() never return, "else" is not needed
}
if (pids[i] == 0)
{
struct timeval tv;
time_t t;
struct tm*info;
gettimeofday(&tv,NULL);
t = tv.tv_sec;
info = localtime(&t);
sleep(1);
printf("CHILD PID: %d and the time is %s\n",getpid(),asctime(info));
_exit(0); // terminate process explicitly
}
}
int status;
pid_t pid;
sleep(5);
while (n > 0)
{
pid = wait(&status);
// No kill needed for already exited process
// kill((long)pid, SIGTERM);
if (pid == -1)
{
perror("wait");
abort();
} else {
printf("CHILD %d killed.\n", pid);
}
--n;
}
}
If you want to terminate child processes from parent process then children should wait until terminated, but you should send signal before waiting for child process termination:
int main(void)
{
pid_t pids[10];
int i;
int n = 10;
for (i = 0; i<n; ++i)
{
if ((pids[i] = fork()) < 0)
{
perror("fork");
abort();
// abort() never return, "else" is not needed
}
if (pids[i] == 0)
{
struct timeval tv;
time_t t;
struct tm*info;
gettimeofday(&tv,NULL);
t = tv.tv_sec;
info = localtime(&t);
sleep(1);
printf("CHILD PID: %d and the time is %s\n",getpid(),asctime(info));
// wait until terminated
for (;;)
pause();
}
}
int status;
pid_t pid;
sleep(5);
for (i = 0; i<n; ++i) {
if (kill(pids[i], SIGTERM) == -1) {
perror("kill");
}
}
while (n--)
{
pid = wait(&status);
if (pid == -1)
{
perror("wait");
abort();
} else {
printf("CHILD %d killed.\n", pid);
}
}
}