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


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 <>
#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 ) {
	} else if ( pid != 0 ) {
		printf("Parent %d [%d] exit.\n", n, getpid());

	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;

Problem reports:
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