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From: Nicholas Sherlock <n DOT sherlock AT gmail DOT com>
Subject: Signal support under Cygwin
Date: Wed, 28 Apr 2010 04:25:27 +0000 (UTC)
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Hi everybody,

I've got some code on Linux which attempts to take periodic samples of my multi-
threaded program to find out what task each thread is working on. For this I am 
installing a signal handler for each thread, and periodically sending SIGUSR1 to 
each thread I want to check with pthread_kill. This works flawlessly on Linux, 
but crashes after a short interval on Cygwin on Windows 7 64-bit. My code is:

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <signal.h>

#include <unistd.h>
#include <pthread.h>

volatile sig_atomic_t pauses = 0;

static void profiling_sample(int signum) {
	(void) signum;

	sig_atomic_t old_value = pauses;

	++old_value;

	/* Might lose concurrent updates to 'pauses' here. Not a big deal. */
	pauses = old_value;
}

static void * childthread(void * arg) {
	(void) arg;

	for (;;) {
		sleep(1000);
	}
}

#define NUM_THREADS 8

int main(void) {
	int i;

	pthread_t handles[NUM_THREADS];

	struct sigaction action;

	action.sa_handler = profiling_sample;
	action.sa_flags = 0;
	sigemptyset(&action.sa_mask);

	assert(sigaction(SIGUSR1, &action, NULL) == 0);

	for (i = 0; i < NUM_THREADS; i++) {
		assert(pthread_create(&handles[i], NULL, childthread, NULL) == 
0);
	}

	printf("Delivered signals is at least:\n");
	for (;;) {
		for (i = 0; i < NUM_THREADS; i++) {
			int err = pthread_kill(handles[i], SIGUSR1);

			if (err) {
				fprintf(stderr, "Error from pthread_kill 
(%d)\n", err);
				exit(-1);
			}
		}

		printf("%d\n", pauses);
	}

	return EXIT_SUCCESS;
}

My Cygwin is:

$ uname -a
CYGWIN_NT-6.1-WOW64 NickLaptop 1.7.5(0.225/5/3) 2010-04-12 19:07 i686 Cygwin

I am building with:

gcc -std=gnu99 -g3 -W -Wall -o sigtest.exe sigtest.c -lpthread  

A typical run on Cygwin shows the "pauses" value counting upwards to about 4000 
before the program either hangs without printing anything or causes a segfault 
(without a useful looking stacktrace):

(gdb) thread apply all where

Thread 10 (thread 4444.0x179c):
#0  0x1b4bdb2c in ?? ()
#1  0x000003e8 in ?? ()
#2  0x1b4bce64 in ?? ()
#3  0x1b4bcd98 in ?? ()
#4  0x610df995 in pthread::thread_init_wrapper () from /usr/bin/cygwin1.dll
Backtrace stopped: frame did not save the PC

Thread 9 (thread 4444.0x14fc):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000001e4 in ?? ()
#4  0x00000000 in ?? ()

Thread 8 (thread 4444.0x15a4):
#0  0x77bef8dd in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef8dd in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x7748d232 in WriteFile ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x00000124 in ?? ()
#4  0x00000000 in ?? ()

Thread 7 (thread 4444.0x1610):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000000f0 in ?? ()
#4  0x00000000 in ?? ()

Thread 6 (thread 4444.0x144c):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000000f0 in ?? ()
#4  0x00000000 in ?? ()

Thread 5 (thread 4444.0x125c):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000000f0 in ?? ()
#4  0x00000000 in ?? ()

Thread 4 (thread 4444.0x414):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000000f0 in ?? ()
#4  0x00000000 in ?? ()

Thread 3 (thread 4444.0x804):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000001dc in ?? ()
#4  0x00000000 in ?? ()

Thread 2 (thread 4444.0x150c):
#0  0x77bef8a5 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef8a5 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x7748d0c5 in ReadFile () from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x00000120 in ?? ()
#4  0x00000000 in ?? ()

Thread 1 (thread 4444.0xa8):
#0  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#1  0x77bef871 in ntdll!RtlUpdateClonedSRWLock ()
   from /cygdrive/c/Windows/system32/ntdll.dll
#2  0x77490816 in WaitForSingleObjectEx ()
   from /cygdrive/c/Windows/syswow64/KernelBase.dll
#3  0x000001cc in ?? ()
#4  0x00000000 in ?? ()
#0  0x1b4bdb2c in ?? ()

Is this supposed to work?

Cheers,
Nicholas Sherlock


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