[dtrace-discuss] Newbie Question

[Adam Leventhal]

Hi Daniel,

It sounds like what you want is is-enabled USDT probes:
    http://blogs.sun.com/ahl/entry/user_land_tracing_gets_better

Specifically, I suggest you do something like this:

---8<--- dprintf.d ---8<---
provider debug {
	probe debug(char *);
};
--->8--- dprintf.d --->8---

---8<--- dprintf.h ---8<---
#define dprintf(...) \
	{ if (DEBUG_DEBUG_ENABLED()) _dprintf(__VA_ARGS__); }
--->8--- dprintf.h --->8---

---8<--- dprintf.c ---8<---
void
_dprintf(const char *fmt, ...)
{
	va_list ap;
	char buf[512], *bufp = buf;
	int n;

	va_start(ap, fmt);
	n = vsnprintf(buf, sizeof (buf), fmt, ap);
	va_end(ap);

	if (n >= sizeof (buf)) {
		bufp = alloca(n + 1);
		va_start(ap, fmt);
		n = vsprintf(bufp, fmt, ap);
		va_end(ap);
	}

	DEBUG_DEBUG(bufp);
}
--->8--- dprintf.c --->8---

Then from your code you can call dprintf() and incur basically no overhead.
You can use a very simple D script to format the output:

---8<---

#!/usr/sbin/dtrace -s

#pragma D option quiet

debug*:::debug
{
	printf("%s\n", copyinstr(arg0));
}

--->8---

I'd actually like to see something like this ship as a Solaris library...

Adam

On Mon, Jul 23, 2007 at 12:37:57PM -0700, Daniel Templeton wrote:
> Hi,
> 
> I'm working on a project where I would like to use DTrace to plug into 
> printf* calls being made by a deamon and print out what those printfs 
> would be printing were the daemon's output stream not getting dumped 
> into the bit bucket.  I can connect to the daemon and grab the printf 
> calls and print the format string, but how do I deal with the printf 
> varargs?  Not only do I not know how many args there are, I don't know 
> which ones are numbers (and can be used directly), and which ones are 
> pointers/strings (and must be copyin'ed or copyinstr'ed).  I don't see 
> any way in D to process the format string to find the answers.  Any 
> clever tricks for handling something like this?
> 
> * The printf calls aren't actually printf() calls.  They are calls to an 
> internal routine that functions similarly to printf(), with a caveat.  
> Unless debugging is turned on (by setting an env var), the contents of 
> the printf's won't be executed.  It does a cheap test and returns if no 
> debug output is requested.  My real objective is to use DTrace to be 
> able to dynamically turn on debugging by plugging into the printf calls 
> themselves, so that restarting the daemon to get debug output is no 
> longer required.  This is an old and large source base, so I'm trying to 
> avoid massive code overhauls to reach my goal.
> 
> Thanks,
> Daniel
> _______________________________________________
> dtrace-discuss mailing list
> dtrace-discuss at opensolaris.org

-- 
Adam Leventhal, Solaris Kernel Development       http://blogs.sun.com/ahl