Interrupt a process calling the function popen

Question

I need to implement a child process that will execute a file and send the execution result, the 2 process will communicate with a shared memory segment.

My problem is that i want to kill the child process calling popen after 10 seconds but the function popen ignores signals.

Here is my code (shared memory segment not included) :

void kill_child(int sig)
{
 kill(child_pid,SIGKILL);
printf("processus killed \n");

}

/*code....*/

signal(SIGALRM,(void (*)(int))kill_child);

if(fork()==0){
                res.buffer=true;
                FILE * fd;
                char cmd[BUFFER_SIZE],output[BUFFER_SIZE];
                strcpy(cmd,"./");
                strcat(cmd,res.filepath);
                system(cmd);
                if((fd=popen(cmd,"r"))== NULL)
                    exit(1);
                else 
                    res.status=200;


                strcpy(output,"");
                while(fgets(buf,sizeof(buf)-1,fd))
                    strcat(output,buf);

                if(pclose(fd))
                    exit(1);

                strcat(res.customHTML,output);
                res.buffer=true;


                int err = sendResponse(res,args->client_fd);
                if (err < 0) on_error("failed!\r\n");



                exit(0);


 } 

 else{

               int status;
               alarm(10);
               waitpid(-1,&status,0);
               printf("status %d _n);
}

How can make the child process interruptible?

thanks

Answer 1

First off, you need to actually store the child PID into child_pid. It's returned from fork for the parent process so changing your fork call to

child_pid = fork();
if(child_pid == 0)
  {
...

otherwise your call to kill is being passed a random value. Luckily it seems to be defaulting to 0, which kill takes to mean kill all processes in the same process group so your child process is being killed.

Secondly, rather than calling popen() call the executable yourself with (for example) execvp() and have the parent read the output using a pipe you create yourself...

int fds[2];

pipe(fds);
child_pid = fork();
if(child_pid == 0)
  {
  char *cmd[]={"mycmd",NULL};

  /* Replace stdout with the output of the pipe and close the original */
  dup2(fds[1],1);
  close(fds[0]);
  close(fds[1]);
  execvp(cmd[0],cmd);
  }
else
  {
  close(fds[1]);
  alarm(10);
  while(...)
     {
     read(fds[0],....);
     if(waitpid(child_pid,&status,WNOHANG))
         {
         ....
         }
     }
  }

This way you've only got the one child process which is running your executable and you've got visibility on when and how it exits.