Using C to send an exec process to the background?

Question

My question sounds the same as this but it isn't:

Start a process in the background in Linux with C

I know how to do fork() but not how to send a process to the background. My program should work like a simple command unix shell that supports pipes and background processes. I could do pipe and fork but I don't know how to send a process to the background with & like the last line of the program:

~>./a.out uname
SunOS
^C
my:~>./a.out uname &

How to achieve the background process?

#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#define TIMEOUT (20)

int main(int argc, char *argv[])
{
  pid_t pid;

  if(argc > 1 && strncmp(argv[1], "-help", strlen(argv[1])) == 0)
    {
      fprintf(stderr, "Usage: Prog [CommandLineArgs]\n\nRunSafe takes as arguments:\nthe program to be run (Prog) and its command line arguments (CommandLineArgs) (if any)\n\nRunSafe will execute Prog with its command line arguments and\nterminate it and any remaining childprocesses after %d seconds\n", TIMEOUT);
      exit(0);
    }

  if((pid = fork()) == 0)        /* Fork off child */
    {
      execvp(argv[1], argv+1);
      fprintf(stderr,"Failed to execute: %s\n",argv[1]);
      perror("Reason");
      kill(getppid(),SIGKILL);   /* kill waiting parent */
      exit(errno);               /* execvp failed, no child - exit immediately */
    }
  else if(pid != -1)
    {
      sleep(TIMEOUT);
      if(kill(0,0) == 0)         /* are there processes left? */
    {
      fprintf(stderr,"\Attempting to kill remaining (child) processes\n");
      kill(0, SIGKILL);      /* send SIGKILL to all child processes */
    }
    }
  else
    {
      fprintf(stderr,"Failed to fork off child process\n");
      perror("Reason");
    }
}

The solution in plain English appears to be here: How do I exec() a process in the background in C?

Catch SIGCHLD and in the the handler, call wait().

Am I on the right track?

PS: Here's a great example of a SIG_CHLD handler, from the most excellent Beej's Guide to Network Programming. It uses sigaction() and waitpid(), and the main program is doing other useful work (waiting for new connections) in addition to handling SIGCHLD. — paulsm4
– paulsm4, Commented Jun 11, 2012 at 18:49

Community · Accepted Answer · 2017-05-23 11:58:07Z

5

Q: How do I send a process to the background?

A: In general, exactly what you're already doing: fork()/exec().

Q: What's not working as you expect?

I suspect maybe you also want a "nohup" (to completely disassociate the child from the parent).

The key to doing this is to run "setsid()" in the child process:

edited May 23, 2017 at 11:58

CommunityBot

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answered Jun 11, 2012 at 18:33

paulsm4

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