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Date: Tue, 7 Mar 2017 20:43:18 +0900
From: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
To: cygwin AT cygwin DOT com
Subject: Changing behaviour of pthread_cond_wait().
Message-Id: <20170307204318.8dda1744403eafdb79e2cf06@nifty.ne.jp>
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Hello,

I would like to propose chaging behaviour of pthread_cond_wait().

POSIX states as follows about pthread_cond_wait():
If a signal is delivered to a thread waiting for a condition variable,
upon return from the signal handler the thread resumes waiting for the
condition variable as if it was not interrupted, or it returns zero
due to spurious wakeup.

cygwin 2.7.0 employs the latter behaviour, while Debian GNU/Linux and
FreeBSD employ the former one. Of course, this is not a cygwin bug.
However, IMHO, it is better to have the compatibility with the other
major platforms.

Because of this difference, iperf 2.0.5 is not terminated normally in
cygwin 2.7.0. In client mode, iperf 2.0.5 does not stop after measurement
automatically. It needs ^C to stop it. In server mode, iperf 2.0.5 can
not stop even by ^C.  Simple test case, attached (pt.c), reproduces this
problem. I know this test case (and iperf 2.0.5) is essentially unsafe
because pthread functions are called from signal handler. But, anyway it
works in Debian GNU/Linux and FreeBSD.

The test case acts as follows in Debian GNU/Linux and FreeBSD.
% gcc pt.c -lpthread; ./a.out
Thread 1
Alarm 2
Thread 3
%

However, in cygwin, it acs as:
% gcc pt.c; ./a.exe
Thread 1
(Deadlock: ^C is needed to terminate.)
%

I would like to propose a patch attached (pthread.patch), for the above
reason. With this patch, iperf 2.0.5 as well as the test case works fine.

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

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 name="pt.c"
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#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t c;
int flag = 0;

#define N 2

void sig_handler(int sig)
{
	pthread_mutex_lock(&m);
	flag ++;
	pthread_cond_signal(&c);
	printf("Alarm %d\n", flag); fflush(stdout);
	pthread_mutex_unlock(&m);
}

void *thread_main(void *args)
{
	int i;
	for (i=0; i<N; i++) {
		pthread_mutex_lock(&m);
		flag ++;
		pthread_cond_signal(&c);
		printf("Thread %d\n", flag); fflush(stdout);
		pthread_mutex_unlock(&m);
		usleep(2000000);
	}
	return NULL;
}

int main()
{
	pthread_t t;
	signal(SIGALRM, sig_handler);

	pthread_cond_init(&c, NULL);
	pthread_create(&t, NULL, thread_main, NULL);

	alarm(1);

	pthread_mutex_lock(&m);
	while (flag < N+1) {
		pthread_cond_wait(&c, &m);
	}
	pthread_mutex_unlock(&m);

	return 0;
}

--Multipart=_Tue__7_Mar_2017_20_43_18_+0900_WHN_arJRDl+j3mFs
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Content-Disposition: attachment;
 filename="pthread.patch"
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