Get child value (exit code) from father process in C

Question

I would like to know if there's some way to get inside the while statement (printf("Death of child: %d with exit code %d\n",i, WEXITSTATUS(status));) the integer i from the child.

I'm getting the output from the execlp (this program has a timeout inside that returns a random number) but I would like to get which is the identification i of each returned response.

int main (int argc, char *argv[])
{
  int i;
  int pid;
  int status;
  int pro= atoi (argv[argc - 1]);
  char programName[] = "./child";

  if (argc < 2 || argc > 2){
    printf("Only one argument is allowed\n"); 
    exit (0);
  }


  for (i = 0; i < pro; i++)  
  { 
    pid = fork();
    if(pid == 0) 
    { 
      printf("Created child %d with pid=%d\n",i, getpid());
      char child[10];
      snprintf(child, 10, "%d", i);
      if (execlp(programName, programName, child, NULL) < 0){
        printf("Error running: %s\n",programName);
      }
      exit(2); 
    }    
  }
  while(wait(&status)>0)
  {
    if ( WIFEXITED(status) ){
      printf("Death of child: %d with timeout %d\n",i, WEXITSTATUS(status));
      if (WEXITSTATUS(status) > 5)
      {
        printf("Time exceeded\n");
      }
    }
  } 
  exit (0);
}

Maybe my approach is not correct. Any tip is welcome. I'm completely new in c.

It is possible to reach into the memory of another process, but almost certainly not what you want, and would likely require 100+ lines of code. What are you trying to achieve here? — PiRocks, Dec 05 '19 at 08:03
Does this answer your question? [return value from child process c](https://stackoverflow.com/questions/43554978/return-value-from-child-process-c) — Jaideep Shekhar, Dec 05 '19 at 08:06

selbie · Accepted Answer · 2019-12-05T08:23:07.723

wait will return the pid of the process that changed. To map that back to i, save all your pids returned from fork into a table and reference back as a lookup.

pid_t table[pro] = {0};
pid_t child=0;

for (i = 0; i < pro; i++)  
  { 
    pid = fork();
    if(pid == 0) 
    { 
      printf("Created child %d with pid=%d\n",i, getpid());
      char child[10];
      snprintf(child, 10, "%d", i);
      if (execlp(programName, programName, child, NULL) < 0){
        printf("Error running: %s\n",programName);
      }
      exit(2); 
    }
    else
    {
      table[i] = pid;
    }    
  }
  while((child=wait(&status)) > 0)
  {
    if ( WIFEXITED(status) ){

      i = -1;
      for (int j = 0; j < pro; j++)
      {
        if (table[j] == child))
        {
          i = j;
          break;
        }
      }

      printf("Death of child: %d with timeout %d\n",i, WEXITSTATUS(status));

Get child value (exit code) from father process in C

1 Answers1