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Re: [PATCH] Implement open-process and related functions on MinGW


From: Mark H Weaver
Subject: Re: [PATCH] Implement open-process and related functions on MinGW
Date: Sat, 22 Feb 2014 17:02:35 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Eli Zaretskii <address@hidden> writes:

>> From: Mark H Weaver <address@hidden>
>> Cc: address@hidden (Ludovic Court├Ęs),  address@hidden
>> Date: Sat, 22 Feb 2014 10:54:16 -0500
>> 
>> Thanks for working on this, but in a multithreaded program, it's no good
>> to change the file descriptors in the main program temporarily before
>> spawning, and then restore them afterwards.  We'll have to find another
>> way of doing this.
>
> Btw, how does the Posix build work reliably when it forks after
> several threads are already running?  I don't see any calls to
> pthread_atfork or any similar machinery in place.  What am I missing?

It's safe to fork a multithreaded program without using pthread_atfork
if only async-signal-safe functions are called before the exec.  In
fact, that's why Andy rewrote 'open-process' in C, as it says in the
comment above that function.

  "If a multi-threaded process calls fork(), the new process shall
   contain a replica of the calling thread and its entire address space,
   possibly including the states of mutexes and other resources.
   Consequently, to avoid errors, the child process may only execute
   async-signal-safe operations until such time as one of the exec
   functions is called."

http://pubs.opengroup.org/onlinepubs/000095399/functions/fork.html

     Mark



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