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Backtracing segfaults in a daemon in digitalmars D

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Alan Knowles

Posts: 390
Nickname: alank
Registered: Sep, 2004

Alan Knowles is Freelance Developer, works on PHP extensions and PEAR.

Backtracing segfaults in a daemon in digitalmars D

Posted: Jan 31, 2007 2:14 AM

This post originated from an RSS feed registered with PHP Buzz by Alan Knowles.
Original Post: Backtracing segfaults in a daemon in digitalmars D Feed Title: Smoking toooo much PHP Feed URL: http://www.akbkhome.com/blog.php/RSS.xml Feed Description: More than just a blog :)	Latest PHP Buzz Posts Latest PHP Buzz Posts by Alan Knowles Latest Posts From Smoking toooo much PHP

One the projects I'm working on is a SyncML server, written from scratch in D, It's currently in testing mode, and we found that the server was mysteriously crashing. Unfortunatly, since it's threaded, and forked as a daemon, we didn't really want to run it under GDB, (and since GDB segfaults on startup anyway). we where at a bit of a quagmire about how to find the bug.

So after a bit of searching through code.google.com I came across the idea of catching the SIGSEGV signal and calling backtrace and backtrace_symbols

This little trick can product output that looks something like

/path/to/application [0xAAAAAA] << address of code
/path/to/application [0xAAAAAA] << address of code
/path/to/application [0xAAAAAA] << address of code
/path/to/application [0xAAAAAA] << address of code

which initially seemed a bit cryptic, but by putting it together with
addr2line can result in some great debugging information.

This is my little backtrace logger for the deamon logger.

static void print_trace()
{
	
	
	void *btarray[10];
	size_t size;
	char **strings;
	size_t i;
	pid_t pid = getpid();

	//writefln("SEG");
	size = backtrace(cast(void**)btarray, 10);
	strings = backtrace_symbols(cast(void**)btarray, size);

	std.process.system("/bin/echo '----BACKTRACE------' " ~ 
		"> /var/log/myproject/backtrace.log");
	 
	for(i = 0; i < size; i++) {
		
		char[] line = std.string.toString(strings[i]);
		char[][] bits = std.string.split(line, "[");
		char[] left = std.string.strip(bits[0]);
		if (!left.length) {
			continue;
		}
		// skip lines with ( in them...
		if (std.string.find(left,"(") > -1) {
			continue;
		}
		
		char[] addr = bits[1][2..length-1];
		
		 
		std.process.system("/bin/echo '----" ~ addr 
			~ "------' >> /var/log/myproject/backtrace.log");
		std.process.system("/usr/bin/addr2line -f -e " ~ 
			left ~ " " ~ addr ~ " >> /var/log/myproject/myproject.log");
		
		
	}
	free(strings);
}

of course you need to use a few C externs to make this work:

extern (C) {
	int backtrace(void **__array, int __size);
	char** backtrace_symbols(void **__array, int __size);
	pid_t getpid();
	sighandler_t signal(int signum, sighandler_t handler);
 	void  sigsegv(int sig)
	{
		// reset the handler.
		signal(SIGSEGV, cast(sighandler_t) 0);
		print_trace();
		// really die
		exit(SIGSEGV);
	}
	
}

and to add it to you application, stick this in main() somewhere

signal(SIGSEGV, &sigsegv);

testing it is quite simple, just do this in D

void  testSegfault()
{
	
	class SegTest {
		void test() {}
	}
	SegTest a;
	a.test();
}

Now looking at the debug file, you can work out where it failed...

----BACKTRACE------
----805e971------ (THIS IS MY OUTPUT CODE)
_D9myproject7cmdLine6daemon11print_traceFZv
init.c/src/myproject/cmdLine/daemon.d:306
----805e46b------ 
sigsegv
init.c/src/myproject/cmdLine/daemon.d:121
----804db18------ (AND NOW FOR THE LOCATION OF THE SEGFAULT)
_D9myprojectfort7manager7manager7runOptsFZAa
init.c/src/myproject/manager.d:50
----805617a------
_D9myproject10webRequest10webRequest5parseFAaAaKAaZAa
init.c/src/myproject/webRequest.d:89
----8050c3e------
_D9pmyproject14myprojectThread14myprojectThread18dealWithWebRequestFAaAaZv
init.c/src/myproject/myprojectThread.d:331
----80503d0------
_D9myproject14myprojectThread14myprojectThread3runFZi
init.c/src/myproject/myprojectThread.d:111
----8076260------
_D3std6thread6Thread11threadstartUPvZPv
??:0
----a7fd10bd------
??

I'm sure with some more work, you could get it to log to syslog...

Read: Backtracing segfaults in a daemon in digitalmars D

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