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Full Discussion: Question about background processes
Operating Systems Linux Question about background processes Post 302166900 by otheus on Wednesday 13th of February 2008 04:05:07 AM
Old 02-13-2008
Quote:
Originally Posted by neimaD
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...
Looks to me you're doing quite fine...


Quote:
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 don't see how that's happening in the code below. But there is a bug...

Quote:
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:
Pretty much you have it right. The confusion may be what fork() does. fork() clones the process; the new process has a pid of 0. You do not (normally) want to exit() after a fork() call. The exception is when exec() fails in the new process.

There are also some other considerations in creating a background process. Which process will have control of the terminal? Where will the background process receive "stdin" from? Normally, you should close stdin (fd[0]) before spawning the background process.

See my comments below.

Code:
pid = fork();				
				
		if(pid == 0){
			execvp(path, elmntpointer);		
                        /* YOU NEED ERROR HANDLING HERE -- if path is not found, for instance. Then you need an exit().  */

		}else if(pid > 0){
			if(command->background)
				printf("%d Child Process Done\n", pid);
			else{
				wait(NULL);
				printf("%d Child Process Done\n", pid);
			}
			/* THIS EXIT IS WRONG */
			exit(0);
		}else{
			fprintf(stderr,"Fork Failed\n");
			printf("Fork Failed\n");
                        /* THIS EXIT IS WRONG */
			exit(-1);
		}	
	    }

 

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LEARN ABOUT V7

fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME

       fork  -	spawn new process

SYNOPSIS

       fork( )

DESCRIPTION

       Fork  is  the only way new processes are created.  The new process's core image is a copy of that of the caller of fork.  The only distinc-
       tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process,  while	the  value
       returned in the child is 0.  Process ID's range from 1 to 30,000.  This process ID is used by wait(2).

       Files open before the fork are shared, and have a common read-write pointer.  In particular, this is the way that standard input and output
       files are passed and also how pipes are set up.

SEE ALSO

       wait(2), exec(2)

DIAGNOSTICS

       Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or  the
       system's process table is full.	Only the super-user can take the last process-table slot.

ASSEMBLER

       (fork = 2.)
       sys fork
       (new process return)
       (old process return, new process ID in r0)

       The return locations in the old and new process differ by one word.  The C-bit is set in the old process if a new process could not be cre-
       ated.

																	   FORK(2)
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