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Date: Thu, 9 Mar 2017 20:39:59 +0900
From: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
To: cygwin AT cygwin DOT com
Subject: fork() fails if it is called recursively from a child thread.
Message-Id: <20170309203959.1e7e32d20bf7ec06c705d3ba@nifty.ne.jp>
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Hello,

I found fork() fails if it is called recursively from a child thread.

Simple test case, attached (fk.c), reproduces this problem.

Expected result:
Parent 0 [22034] exit.
Child 0 [22036] works.
Parent 1 [22036] exit.
Child 1 [22038] works.
Parent 2 [22038] exit.
Child 2 [22039] works.
Parent 3 [22039] exit.
Child 3 [22040] works.
Parent 4 [22040] exit.
Child 4 [22041] works.

Result in cygwin 2.7.0:
Child 0 [4668] works.
Parent 0 [7188] exit.
      0 [main] a 4668 fork: child -1 - forked process 8456 died unexpectedly, retry 0, exit code 0xC0000142, errno 11
fork(): Resource temporarily unavailable

Strictly speaking, the test case is not safe because it calls functions
which are not async-signal-safe from forked child process, i.e. printf()
and perror(), in spite of multi-thread. However the same happens even
without printf() and perror().

This is the cause of which iperf 2.0.5 with option -s -D fails to start
as daemon.

Is this the known issue?

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

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#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <pthread.h>

void MoveToChild(int n)
{
	pid_t pid;

	if ( (pid = fork()) == -1 ) {
		perror("fork()");
		_exit(1);
	} else if ( pid != 0 ) {
		printf("Parent %d [%d] exit.\n", n, getpid());
		_exit(0);
	}

	printf("Child %d [%d] works.\n", n, getpid());
}

void *thread_main(void *args)
{
	int i;
	for (i=0; i<5; i++) MoveToChild(i);
	return NULL;
}

int main()
{
	pthread_t t;
	pthread_create(&t, NULL, thread_main, NULL);
	pthread_join(t, NULL);
	return 0;
}


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