@(#)Is the process alive 30 AUG 2000 Rob Thomas robt@cymru.com
How to check the state of another process from within a C program.

Is the process still alive?

From within a program, you may wish to determine if a corollary process 
is running. Within a shell script or Perl, this can be accomplished 
with the use of ps(1). However, it is less than efficient to use popen() 
or (the infamous and evil) system() within a C program. Here is a simple 
solution using, of all things, kill():

/*
 * @(#)isit.c Rob Thomas robt@cymru.com http://www.cymru.com/~robt
 * A program that determines if a given PID is still receiving signals,
 * e.g. "alive."
 * Usage: ./isit <PID>
 *
 */

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

extern int errno;

int
main(int argc, char **argv)
{
  int ret_val;
  pid_t mypid;

  mypid = atoi(argv[1]);

  ret_val = kill(mypid, 0);

  if (0 != ret_val) {
    /* Look for ESRCH */
    if (3 == errno) {
      fprintf(stderr, "Proc %d not alive.\n", mypid);
      exit(1);
    } else {
      perror("kill");
    }
  }

  fprintf(stdout, "Proc %d still active.\n", mypid);
  exit(0);
}

By simply sending a signal 0 to the process, we can determine if the 
process still exists. Remember that permissions still count with 
signals, and only root can send a signal to any process. This snippet of 
code can be cut and pasted into any application for which process 
monitoring is necessary.

--
Rob Thomas, robt@cymru.com
http://www.cymru.com/~robt