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 LINUX

preap

preap(1)                                                           User Commands                                                          preap(1)

NAME

       preap - force a defunct process to be reaped by its parent

SYNOPSIS

       preap [-F] pid...

DESCRIPTION

       A  defunct  (or  zombie)  process  is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
       waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
       happen  if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
       to PID 1. See init(1M), which periodically reaps such processes.

       An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
       nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
       do consume a small amount of system memory.

       preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.

       preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

         o  The process is a child of init(1M).

         o  The parent process is stopped and might wait on the child when it is again allowed to run.

         o  The process has been defunct for less than one minute.

OPTIONS

       The following option is supported:

       -F       Forces the parent to reap the child, overriding safety checks.

OPERANDS

       The following operand is supported:

       pid      Process ID list.

EXIT STATUS

       The following exit values are returned by preap, which prints the exit status of each target process reaped:

       0        Successfully operation.

       non-zero Failure, such as no such process, permission denied, or invalid option.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWesu (32-bit)             |
       +-----------------------------+-----------------------------+
       |                             |SUNWesxu (64-bit)            |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)

WARNINGS

       preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes  will
       not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.

SunOS 5.10                                                          26 Mar 2001                                                           preap(1)

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