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Re: Issues with process forking when 'multithreaded'.

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Sander Smeenk
March 4, 2013 18:52
Re: Issues with process forking when 'multithreaded'.
Message ID:
Quoting Dave Mitchell (

> > So, it's not a fork() from the main process (which would perhaps
> > copy all threads to the fork()ed process, or maybe not, who knows).
> fork() on linux creates a single thread in the child process, but copies
> the memory and state of *all* threads. Which is why you can get in a right
> mess. For example, if one of the other threads had something locked, that
> will be still be locked in the new process, but without a corresponding
> thread still running that can unlock it. So the forked process might
> deadlock or otherwise behave strangely.

Thanks. This is a clear explanation on why you should take care with
threads and forks. As you might have gathered already i'm not that
experienced with the internals of threads and fork...

This mail thread (haha!) made clear to me why removing 'lock($sharedfoo)'
calls from the threaded code improved upon the stability. It did indeed
no longer 'lock up' randomly, the above quoted must be exactly why.
I reworked the code so i no longer needed to lock() things and thought
to be done with it.

> This is why people say not to mix threads and forks.

I honestly swear i will rewrite my production code using fork() only. :-)

Still, out of curiosity, i would like to keep discussing this (and
fiddle with the thread+fork mix some more). I'm fairly certain that my
threaded code is not creating locks anymore. I've ran the code
uncountable times now and it has never locked up (yet).

Locks aren't really my problem, it is waitpid() that is 'acting up' and
i am unable to find an explanation for that.

Is it true that even though each thread has it's own 'safe' memory to
work with, fork()ing from two threads at separate times
changes (reinitialises?) memorystructures in either thread?

For example, can a succesfull fork() (or waitpid()?) in thread2 cause
the waitpid() in thread1 to return too? Even though the childprocess
'in' thread1 is still running?

Thanks a bundle so far!
Still learning every day. :)

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