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Date: Wed, 16 Jul 2025 17:36:42 +0200
To: cygwin AT cygwin DOT com
Subject: Re: Calling system() in multi-threads.
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Jul 16 23:52, Takashi Yano via Cygwin wrote:
> Hi Corinna,
> 
> On Wed, 18 Jun 2025 20:31:27 +0900
> Takashi Yano wrote:
> > On Tue, 17 Jun 2025 15:42:26 -0700
> > Mark Geisert wrote:
> > > Hi Takashi,
> > > 
> > > On 6/17/2025 5:54 AM, Takashi Yano via Cygwin wrote:
> > > > Hi,
> > > > 
> > > > If system() is called in parallel in threads, system()
> > > > fails with exit code 127.
> > > > 
> > > > Just compile pthread_system.c attached and run.
> > > > 
> > > > I believe system() is multi-thread safe so the STC
> > > > should work.
> > > 
> > > I regret to inform that system() is not currently MT-safe.  I discovered 
> > > this a few years ago when porting YAFU (Yet Another Factoring Utility).
> > > 
> > > I ended up supplying a wrapper that serialized the system() call itself 
> > > but allowed multiple wrappers (and thus child processes) to be running 
> > > at the same time.  I never got around to supplying a fix for Cygwin. 
> > > I've attached the code.
> > > HTH somehow,
> > 
> > Thanks for the information. I guess the cause is that accessing
> > child_info_spawn NO_COPY ch_spawn;
> > in spwan.cc conflicts with other threads. I wonder ch_spawn should
> > be thread-specific. Or should be guarded by lock.
> 
> Do you have any idea?

Locking would be super-simple.

But theoretically it should be possible to use a local child_info_spawn
variable at this point.  The ch_spawn child_info_spawn instance is not
copied to the child anyway, so that should be safe.  The same goes for
posix_spawn() then, btw.

I checked the sources and I don't see any dependency to ch_spawn
from a spawning process, in contrast to an exec'ing process.  That
doesn't mean there is none, just that I didn't find any.


Corinna

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