how to tell when jobs are finished

Nicholas Henke henken at
Wed Aug 1 11:58:26 PDT 2001

Thanks guys for the input--
	I think the signal handler with a polling mechanism should work
very nice.


> Hi,
> I agree with Sean about the use of waitpid(). About the daemon, well, I think
> is not necesary. If I not  misunderstood you, what you want to do is execute
> a certain number of programs and know when anyone of those programs exited.
> Here is my proposal, in  the form of a pseudo-shell
> somehow the list of programs to execute
> 2. For each program to run, create a child using fork() (the master
> creates all the childs)
> 3. (optional) you may want to redirect each child output to some file
> 4. get from the childs its pid via some IPC mechanism (a pipe will do) and
> store the in an array or something (i would use a linked list, or a
> search-tree table if you will have lots of pids)
> 5. Finally, each child calls exec*() and replaces its memory image with the
> program desired - that is - executes the program
> 6. Now, you have to know when each of the programs you executed has exited.
> For simplicity, lets assume you whant to printf something like "Hey!, PID
> XXXX finished!". You can do this in two forms (to mi knowledge):
> 	a) loop until all the programs have exited. You can use waitpid with WNOHANG
> to poll each pid. Advantage: Simple.  Disadvantage: You can't do other
> productive things while waiting
> 	b) Set a signal handler to the alarm signal, and test say each second for
> completion of one of the pids in your list. If completion, print message and
> remove that pid from the list. Disable signal callback when list is empty,
> and re-enable when list has at leat one element. Advantages: You can do other
> productive things, like launching more processes. Disadvantages: A little
> more complicated. If you use this option, see sigaction(2) and signal(2)

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