CODEWITHSUNDEEP
clinux
sammy
sammy

Reputation: 45

System call is blocking (program wont end) while creating a new process

I am trying to use the clone system call and sometimes the program won't end. This program just creates a new process using clone and tries to execute the cat command. Here is the program:

#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <unistd.h>

int childFunc(void*arg)
{
   char* paramlist[] = {"/bin/cat", "test.txt", NULL};
   execv("/bin/cat",paramlist);
   _exit(1);
}

int main(void) {
  int STACKSIZE = 65536;
  void* stack;
  stack = malloc(STACKSIZE);
  
  int ret = clone(childFunc, stack + STACKSIZE, 0, NULL);

  waitpid(ret, NULL, 0);
  free(stack);
  return 0;
}

Upvotes: 1

Views: 77

Answers (1)

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48572

When you use clone to create a new process that you later wait on, you need to either pass SIGCHLD in flags to clone (recommended, unless you want to change what signal gets sent on purpose) or pass __WALL or __WCLONE in options to waitpid. See man 2 clone and man 2 waitpid for more details. Since you weren't doing this, the waitpid syscall was failing due to not having anything to wait on instead of waiting for the child process to complete. This resulted in the contents of test.txt being printed after your shell prompt, which made you think it was still running, even though it wasn't. You would have noticed this if you checked the result waitpid returned.

Upvotes: 2

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