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Date:	Fri, 14 Jan 2022 09:22:27 +0900
From:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
To:	cygwin AT cygwin DOT com
Subject:	Re: proc_waiter: error on read of child wait pipe 0x0, Win32 error 6
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On Thu, 13 Jan 2022 16:11:04 -0800
Mark Geisert wrote:
> Takashi Yano wrote:
> > On Wed, 12 Jan 2022 07:41:41 +0100
> > Marco Atzeri wrote:
> >> On 12.01.2022 07:27, Jay K wrote:
> >>> Ok, here is a small demonstration of the problem.
> >>>
> >>> #include <stdlib.h>
> >>> #include <stdio.h>
> >>> #include <windows.h>
> >>>
> >>> unsigned __stdcall thread(void* p)
> >>> {
> >>>     unsigned i;
> >>>     for (i = 0; i < 100; ++i)
> >>>      system("./a.exe");
> >>>     return 0;
> >>> }
> >>>
> >>> int main()
> >>> {
> >>> unsigned i;
> >>> HANDLE threads[100] = {0};
> >>> FILE* f = fopen("a.c", "w");
> >>> fprintf(f, "int main() { return 0; }\n");
> >>> fclose(f);
> >>
> >>
> >> so you are mixing Cygwin and Windows calls ?
> >> That is looking for trouble.
> >>
> >> Or it is a tentative to produce a test case ?
> > 
> > I found that the same happens even with pthread rather than
> > win32 thread functions.
> > 
> > #include <stdlib.h>
> > #include <pthread.h>
> > 
> > void *thread(void *p)
> > {
> > 	system("true");
> > 	return NULL;
> > }
> > 
> > int main()
> > {
> > 	int i;
> > 	pthread_t threads[2];
> > 
> > 	for (i = 0; i < 2; i++)
> > 		pthread_create(&threads[i], NULL, thread, NULL);
> > 
> > 	for (i = 0; i < 2; i++)
> > 		pthread_join(threads[i], NULL);
> > 
> > 	return 0;
> > }
> > 
> > Executing above code results in hang with message:
> >        0 [waitproc] a 786 proc_waiter: error on read of child wait pipe 0x0, Win32 error 6
> > 
> 
> POSIX does not require system() to be thread-safe.  On Cygwin, it isn't.  When I 
> ran into this a while back, I implemented an application wrapper around system() 
> to serialize calls.  It's tricky because you want to serialize just the mechanism 
> of system(), not the programs that the multiple system()s call.

Ah, indeed.
https://pubs.opengroup.org/onlinepubs/9699919799/functions/system.html
says:
  "The system() function need not be thread-safe."
while Linux's system() is MT-safe.

Thanks.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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