Thread: Shell Programming - Process Table

Hello,

I'm writing a shell application that makes use of the basic system calls used to manage processes. While most things seem to work I'm having a peculiar problem:

Everytime a new process is started it's correctly added to a list. Once it's killed it's removed.

The problem is that the moment I kill a process the list is no longer updated and will keep on showing only the processes that were in the list, and not add anymore to them.

Hence my question:

1. Does this already hint something that I may be doing wrong, before posting the code?
2. I know I may avoid using my own list all together querying the system process table for children of my shell process (which I know). However all I found was that I could access /proc/pid files to make the enquiries, but that looks over complicated (open, read and close file). Besides I don't know how to know all my children all at one time. Is there a quicker way to gather the information I need?

I'm using Unix (on Mac OS X).

My guess is, since once you try to remove something from the list your list no longer works, that your "remove from the list" bit is broken.

that would be really humiliating..here's the code for it:

Code:

void delJob(pid_t pid){
	jobs* temp = job;
	bool found = false;
	if (temp->pid == pid){
		free(temp);
		temp = NULL;
		job = job->next;
		job->last = job;
		found = true;
	}
	else{
		while (temp != NULL && temp->next != NULL){
			if (temp->next->pid == pid){
				jobs* toDel = temp->next;
				temp->next = temp->next->next;
				toDel = NULL;
				if (job->last == NULL){
					job->last = temp->next;
				}
				found = true;
				printf("%d deleted from list.\n",pid);
				return;
			}
			temp = temp->next;
		}
		if(!found) printf("%d has not been found in the job list.\n",pid);
	}
}

job is a global variable in the h file containing this function. Memory is allocated to it in the addJob function.

1. If you delete the head of the list, given that temp == job, you probably shouldn't be trying to access job->next after you free temp. (It will almost certainly still "work", but theoretically job->next doesn't exist after free(temp).
2. I don't see you setting the backwards link after deleting a node? (I'm assuming that's what ->next is supposed to mean.)
3. I also don't see you doing free(toDel).
4. What is if (job->last == NULL) supposed to check? (Note that setting toDel = NULL does not change every other pointer that used to point there to NULL also.)

gosh..it was the code for deleting the list!

Why do you say that free(toDel) is the problem? What's wrong with it? (And I'm really hoping that you're just trying to show us a snippet and that's not actually your function.)

Well, it's the first part of it, the other part just iterates. The problem is that I get a *** glibc detected ** free() invalid next size (fast) error when executing that line. Checking in eclipse at that point of time temp points to exactly what I want to free.

I also didn't write the local variable jobs* temp in the snippet above.

what I posted is the actual code of concern! The other portion is just iteration not reached.

Originally Posted by simpatico_qa

what I posted is the actual code of concern! The other portion is just iteration not reached.

You don't walk the list? Set all the links around the to-be-deleted item so that your list doesn't shatter into a million pieces? (Hint: there are no errors in the code you posted in and of itself, only (possibly) in the way it interacts with the rest of your program.)