Zombie process and fork

Question

i have a code like this...

c = fork();
if(c==0) {
    close(fd[READ]);

    if (dup2(fd[WRITE],STDOUT_FILENO) != -1) 
        execlp("ssh", "ssh", host, "ls" , NULL);
    _exit(1);

}   
close(fd[WRITE]);

fd[READ] and fd[WRITE] are pipe file descriptors.

when i run it continuously, there are a lot of zombie processes when i use ps ax. How to rectify this? Is this because i am not using the parent to wait for the exit status of the child process...

Answer 1

If you have no intention to wait for your child processes, set the SIGCHLD handler to SIG_IGN to have the kernel automatically reap your children, eg.

signal(SIGCHLD, SIG_IGN);

Answer 2

Yes, the parent must wait for the child return status. You can do it asynchronously by catching SIGCHILD in the parent process and then call waitpid in the capture method.

Answer 3

Yes, waitpid() should be called from parent. waitpid() will clean-up any child process of the parent process, which is currently in terminated state.

You can add below code to your program :

if(c>0)
{
while(1){ 
ret = waitpid(-1,&status,0);

if(ret>0){
if(WIFEXITED(status)){
   if(WEXITSTATUS(status) == 0){ 
       printf("child process terminated normally and successfully\n");
   }
   else{

       printf("child process terminated normally and unsuccessfully\n");
   }
 }
else{ 

       printf("child process terminated abnormally and unsuccessfully\n");
    }
}
if(ret<0) { 
     break; 
    }
  }
}

FYI : more on waitpid.

First parameter is set to -1 such that waitpid() will clean-up any child process of this parent process, which is currently in terminated state.The first parameter can also be +ve - in this case, waitpid() will cleanup only the specific child process.Most common use is to set first parameter to -1 also refer to manual page of waitpid(). Second parameter is used to extract the termination/exit status code of the child process - waitpid() system call API fills the status field when the system call API is invoked. Last field is the flags field - currently unused - in most cases, flags field will be set to 0 - meaning, default behaviour of the system call API !!! if you really need to use flags, refer to manual page of waitpid().

Note: In the code you submitted, _exit(1) will be called iff execlp() fails. so you can put a condition for execlp() fail and that condition _exit() can be called. The Reason is, execlp() functions only return if an error has occurred.

Modified code can be like below :

c = fork();
if(c==0) {
close(fd[READ]);

if (dup2(fd[WRITE],STDOUT_FILENO) != -1) 
    ret_execlp = execlp("ssh", "ssh", host, "ls" , NULL);
if(ret_execlp == -1 ) {
      printf("execlp is failed");
      _exit(1);
   }
}
close(fd[WRITE]);

I appreciate the above 2 answers. Wish this answer may give more clarity. Thank you.