sort of execv command needed

kralizec · 07-02-2010, 12:24 PM

Hi!

I have a problem, I need to launch another binary from my application, but I need to get the control back once the other binary is initialized.

fork/exec doesn't seem to give anything on that side

using pthreads I'm able to wait until the new thread exits, but that's not what I want

at the moment I'm using popen, but it doesn't seem to do the job right...

any help? what am I missing?

harry edwards · 07-02-2010, 05:44 PM

Can't you just run the command in the background using the & character?

E.g.

Code:

 
system( "/mycommand.sh &");

kralizec · 07-02-2010, 05:47 PM

I could, but I'm looking for something at a lower level, like the exec functions family

ntubski · 07-02-2010, 08:57 PM

Quote:

fork/exec doesn't seem to give anything on that side

After forking the parent keeps on executing, you don't have to wait for the child.

ForzaItalia2006 · 07-03-2010, 01:44 AM

Quote:

Originally Posted by kralizec

I have a problem, I need to launch another binary from my application, but I need to get the control back once the other binary is initialized.

Sorry, but I don't totally understand your approach.

(1) Why do you need to get the control back once the new process is forked?

(2) When is the point of initialization reached in your sense? I mean, before main is called, after all the libraries are loaded?

(3) Maybe you could give some more details about what you want to really do. That could possibly help us to tell you alternatives

Quote:

Originally Posted by ntubski

After forking the parent keeps on executing, you don't have to wait for the child.

I think the OP means the child, right?

Andi

kralizec · 07-03-2010, 02:20 AM

yeah, I'm sorry... the fact is that I'm not so fluent in english and I can't explain the problem well.. here's an example:

Code:

int main(int argc, char *argv[]){
	
	char *child_args[] = {"/usr/bin/gcalctool", NULL};
	int cpid = fork()
	if (cpid == 0){
		execv(child_args[0],child_args);
                //perror
	} else if (cpid > 0){
		//parent stuff
	
	} else
		//perror and bla bla bla
}

In this case, I'd like to have the parent to be able to know when the calc has been launched (it's just an example) so, by initialized I think I mean when the main is called.

My parent could need stuff produced by the process called in the child, but sometimes it tries to get them while the process is still launching...

Anyway, thank you all!!

zirias · 07-03-2010, 02:44 AM

This sounds like a scheduling issue where you want to make sure, the execv() call is finished in the child process before the parent process is scheduled again, right?

If that's what you want to do, i doubt it is possible. Of course, there are mechanisms to synchronize processes, but as you have no control over the child's code after the execv(), that doesn't apply here.

Can't you just check somehow in the parent for the things produced by the child (whatever these are) before using them, maybe waiting with some usleep() calls?

kralizec · 07-03-2010, 02:48 AM

Yeah! I think you got it right!

At the moment, I'm using pthread instead of the fork and doing a sleep(1), it works but it's an horrible solution by me... I'd like to make something better... thank you!

bigearsbilly · 07-03-2010, 05:00 AM

well, your explanation isn't good...

if you use pthread you can use a condition variable and
pthread_cond_wait in the main thread. It will wait.
when the child thread is ready you pthread_cond_signal
and the main thread wakes.

You need to be careful using fork in threads too.

maybe you should buy a book.

kralizec · 07-03-2010, 05:07 AM

Quote:

Originally Posted by bigearsbilly

well, your explanation isn't good...

if you use pthread you can use a condition variable and
pthread_cond_wait in the main thread. It will wait.
when the child thread is ready you pthread_cond_signal
and the main thread wakes.

You need to be careful using fork in threads too.

maybe you should buy a book.

yeah, but how could I send the pthread_cond_signal if I still need to make an exec or sort of?

your solution works if I don't need to call an external program and just use internal functions...

bigearsbilly · 07-03-2010, 07:35 AM

it may be you are choosing the wrong method for your problem.

johnsfine · 07-03-2010, 08:24 AM

Quote:

Originally Posted by kralizec

I'd like to have the parent to be able to know when the calc has been launched (it's just an example) so, by initialized I think I mean when the main is called.

I don't think exactly that is possible. If it were possible, I don't think it is what you really want.

Quote:

My parent could need stuff produced by the process called in the child, but sometimes it tries to get them while the process is still launching.

Whether main() has started yet in the child should not be an interesting boundary. I assume whatever the child produces is not yet ready at the instant main() in the child starts.

So you need the parent to wait for the child to produce something. There are several different ways to synchronize work between a parent and child process. Some of those might be usable regardless of the separate method by which whatever the child produces reaches the parent. But it may be better to select a synchronization method specifically appropriate to the data flow:

What does the child "produce" and how does that reach the parent (In a pipe? A file? Shared memory? Or what?).

kralizec · 07-04-2010, 03:48 AM

Quote:

Originally Posted by johnsfine

I don't think exactly that is possible. If it were possible, I don't think it is what you really want.

yes, that was just an example, even if now, I would be interested in knowing how to do that too... just so

Quote:

What does the child "produce" and how does that reach the parent (In a pipe? A file? Shared memory? Or what?).

it's a file that could already exists before the child start, my problem is that I'm not the owner of the other process and I just have the exec file, not the source... thank you for your patience and help

zirias · 07-04-2010, 09:44 AM

First, some nitpicking

Quote:

Originally Posted by kralizec

it's a file that could already exists before the child start, my problem is that I'm not the owner of the other process and I just have the exec file, not the source... thank you for your patience and help

Sure you are the owner of the PROCESS, you started it

But then, if it's some file change you're waiting for, have a look at inotify. Never tried it myself, but it should be the solution to your synchronization problem.

johnsfine · 07-04-2010, 10:13 AM

Quote:

Originally Posted by zirias

Sure you are the owner of the PROCESS, you started it

I expect kralizec meant "owner" in the software development sense, meaning kralizec does not have the ability/right to modify the source code of the executable that will be run in that child process.

Quote:

But then, if it's some file change you're waiting for, have a look at inotify. Never tried it myself, but it should be the solution to your synchronization problem.

That looks like it could be a very elegant solution (I didn't read it carefully enough to be sure). But the crude solution of just stat'ing the file multiple times until it changes might be simpler.

I don't seem to have the right dev package installed for man 2 stat to work for me (I ought to fix that). But man 2 stat should give you basic doc on the function(s) you need for the crude approach.