CODEWITHSUNDEEP
c++linuxwaitpid
bob
bob

Reputation: 41

run background process linux simple shell. C++

I'm creating a simple shell in linux and I'm having trouble getting a command to run in the background.

here is my code so far:

create command,argv and check if it is to be ran in the background &
enter this code:

if(strcmp(command,"cd")==0)
{        
  chdir(argv[1]);

}
else if(strcmp(command,"clr") == 0)
{
  if ((pid = fork()) == 0) 
  {        
     execvp("clear",argv);
  }
  wait(&status);
}
else
{      
   if ((pid = fork()) == 0)
{        
  execvp( prog, argv );        
}
  wait(&status);

}

command and argv are what I get from the user.

I need to run a command in the background if '&' is the last part of the command. I have already checked for that with a boolean var of bg. but I'm having trouble with the WAITPID() function im not sure where it goes.

Upvotes: 0

Views: 1715

Answers (1)

jxh
jxh

Reputation: 70402

I guess your issue is if you end up calling waitpid() directly, you will block, and the process is not really running in the background. You can handle this in two ways.

  • Choice 1: Avoid the issue. After calling fork once, have the child fork one more time. Let the grandchild execute the command, and let the child exit. The parent waits on the child like before, but the grandchild is executing the command in the background. The grandchild process will be reaped by the init process.

  • Choice 2: Notice the asynchronous notification that the child is done. When a child process dies, the parent process will receive the SIGCHLD signal. Have a signal handler notice this signal and reap the child. This might require that you restructure your code so that the synchronous waiters actually receive a synchronous notification from your signal handler. You would then need a mechanism for the signal handler to distinguish between foreground and background children, and only send synchronous notifications for the foreground children.

Upvotes: 1

Related Questions