CODEWITHSUNDEEP
cprocessforkblockingwaitpid
Derek
Derek

Reputation: 12378

Running/pausing child processes in C?

I'm running child processes in C and I want to pause and then run the same child process. Not really sure how to describe my problem in a better way since I'm new at this but here's a shot.

So I know that you can run a process after another process exits by using waitpid. But what if the process I'm waiting on doesn't exist at the creation of the process that does the waiting. So in this case, I'm thinking of pausing the process that does the waiting and when the process that is waited is created and then finishes, it would call on the process that does the waiting to run again. So how would you do this? Again, I'm not familiar with this, so I don't know if this is the proper way to do this.

edit: What I'm trying to do

I'm using child processes to run command via execvp() in parallel so if I have a sequence sleep 1; sleep 1;, the total sleep time will be 1 second. However there are cases where I try to parallel echo blah > file; cat < file; in which case I'm assuming cat reads the file after echo inputs blah into file. Therefore, I have to wait for echo to finish to do cat. There are more specifics to this, but generally assume that for any command with an output to a file must be waited on by any command that reads the file later in the script.

Upvotes: 2

Views: 2457

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 753695

Any application (process) can only wait with waitpid() on its own direct children. It can't wait on grandchildren or more distant descendants, and it can wait on neither siblings nor ancestors nor on unrelated processes.

If your application is single-threaded, you can't wait on a process that will be created after the waitpid() call starts because there is nothing to do the necessary fork() to create the child.

In a multi-threaded process, you could have one thread waiting for dying children and another thread could be creating the children. For example, you could then have the waitpid() call in thread 1 start at time T0, then have thread 2 create a child at T1 (T1 > T0), and then the child dies at T2, and the waitpid() would pick up the corpse of the child at T3, even though the child was created after the waitpid() started.

Your higher level problem is probably not completely tractable. You can't tell which processes are accessing a given file just by inspecting the command lines in a 'shell script'. You can see those that probably are using it (because the file name appears on the command line); but there may be other processes that have the name hardwired into them and you can't see that by inspecting the command line.

Upvotes: 0

perreal
perreal

Reputation: 97948

In Linux: You can set an alarm() before you waitpid() so you can wakeup after a certain number of seconds and waitpid() should return EINTR so you would know the situation and can kill the misbehaving one. Another way would be to use a mutex and having a block like this in the waiting process:

if (pthread_mutex_trylock(&mutex) {
  sleep(some seconds);
  if (pthread_mutex_trylock(&mutex) {
    kill the process
  }
}

and the process that is monitored:

ENTRY-POINT:

pthread_mutex_lock(&mutex);
do_stuff();
pthread_mutex_unlock(&mutex);

Upvotes: 1

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