Question about background processes Post: 302165683

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Operating Systems Linux Question about background processes Post 302165683 by neimaD on Friday 8th of February 2008 12:12:18 PM

02-08-2008

Registered User

Question about background processes

Hi!

First of all, let me warn you I'm quite new to the world of LINUX and Operating Systems understanding, so that's why I pose these newbie and stupid qustions...

Anyway, I'm trying to build my own simple shell in C and I'm getting some problems in implementing the background process ('&') functionality.

The parsing of the input command is taken care of, the thing is, when forking a new process after a previous forking of a background process, the new child process shows a parent PID as the PID of that background process and not the PID of the shell process...

I think the problem resides in the fact that i actually DON'T KNOW what a background process really is... I just tell a background process from a 'normal' one by not calling/ calling the wait() system call:

Code:

int main(void)
{

(...)

    while (! done) {   

(...)
	
		int pid;
		
		pid = fork();				
				
		if(pid == 0){
			execvp(path, elmntpointer);		

		}else if(pid > 0){
			if(command->background)
				printf("%d Child Process Done\n", pid);
			else{
				wait(NULL);
				printf("%d Child Process Done\n", pid);
			}
			
			exit(0);
		}else{
			fprintf(stderr,"Fork Failed\n");
			printf("Fork Failed\n");
			exit(-1);
		}	
	    }
	}
	
	if(line)
	    free(line);
    }
    return 0;
}

I also get some problems with the exit() system call... When I call it, the process that is terminated is the one of the shell!

Can you help me with something here?

Thanks!
Damiao

Last edited by Perderabo; 02-08-2008 at 01:38 PM.. Reason: Switch quote tags to code tags

neimaD

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LEARN ABOUT ULTRIX

setpgid

setpgid(3)						     Library Functions Manual							setpgid(3)

Name
       setpgid - set process group (POSIX)

Syntax
       #include <sys/types.h>
       int
       setpgid(pid, pgrp)
       pid_t pid, pgrp;

Description
       The  function is used to either join an existing process group or create a new process group within the session of the calling process (see
       Upon successful completion, the process group ID of the process that has a process ID which matches pid is set to pgrp.	If  pid  is  zero,
       then the call applies to the current process. In addition, if pgrp is zero, the process ID of the indicated process is used.

       This function is available only in the POSIX environment.

Return Values
       The  function  returns 0 when the operation is successful.  If the request fails, -1 is returned and the global variable indicates the rea-
       son.

Diagnostics
       The function fails and the process group is not altered if one of the following occurs:

       [EACCES]       The value of the pid argument matches the process ID of a child process of the calling process and  the  child  process  has
		      successfully executed an function.

       [EINVAL]       The value of the pgrp argument is less than zero or is not a supported value.

       [EPERM]	      The process indicated by the pid argument is a session leader.

		      The  value of the pid argument matches the process ID of a child process of the calling process and the child process is not
		      in the same session as the calling process.

		      The value of the pgrp argument does not match the process ID of the process indicated by the pid argument and  there  is	no
		      process with a process group ID that matches the value of the pgrp argument in the same session as the calling process.

       [ESRCH]	      The  value  of  the  pid	argument  does	not  match the process ID of the calling process of a child process of the calling
		      process.

See Also
       getpgrp(2), setsid(2)

																	setpgid(3)

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