Printf Debugging Revisited

gsasha writes "After long nights spent in debugging, w e have developed a C++ logging facility geared for debugging - and an article that describes our debugging methodology. The article consists of two parts: the first one describes the basics of the method, and the second one presents advanced techniques (to be completed if there is enough reader interest).
Happy debugging!"

Comments

C++ makes it hard

[YGingras]

It's hard to do a good printing debugger in C++. The lack of introspection throws most of the work in the hands of the pre-processor/developper.

When I learned Common Lisp, the first macro I did was for printing debugging. It reads the expresions it is debugging, prints it (and shortens it with "..." if needed), evaluates it, prints the results and returns the results.

What a monster you might say. Lets fist see an example of it's use:

CL-USER> (+ 16 34 (- 6 7 8) (/ (* 3 (expt 4 1/2)) 2 3))
42.0
CL-USER> (dbg (+ 16 34 (dbg (- 6 7 8)) (dbg (/ (* 3 (dbg (expt 4 (/ 1 2)))) 2 3))))
(- 6 7 8): -9
(EXPT 4 (/ 1 2)): 2.0
(/ (* 3 (DBG (EXPT 4 (/ 1 2)))) 2 3): 1.0
(+ 16 34 (DBG (- 6 7 [...] 4 (/ 1 2)))) 2 3))): 42.0
42.0

It's done like that (and it's actually readable when indented properly):

(defmacro dbg (expr)
`(let ((val ,expr)
(repr (format nil "~A" ',expr)))
(if (> (length repr) 50)
(format t "~A [...] ~A: ~A~%"
(subseq repr 0 20)
(subseq repr (- (length repr) 20))
val)
(format t "~A: ~A~%" repr val))
val))

Most of the hard work is taken away by the ability of the program to read itself, by dynamic typing and by the notion that there are no statements, only expressions. That being said, I don't claim that you should never use C++, just that it lacks introspection and that it makes printing debuging a lot harder.

Why is this on Slashdot at all?

[Anonymous Coward]

As many have said, this has been done before. One of their main macros isn't even correct. Ie.,

#define LOG(logger) if ((logger).is_active()) (logger).os()

This will break if one writes:

if (value == 0)
LOG(xxx) "hello" endl;
else ...

The "else" gets interpreted as being attached to the "if" inside the LOG macro when it shouldn't.

It should be written as:

#define LOG(logger) if (!(logger).is_active()) ; else (logger).os()

For a much more expansive trace/log system see OSE at:

http://ose.sourceforge.net

and specifically

http://ose.sourceforge.net/browse.php?group=librar y-manual&entry=logger.htm

and

http://ose.sourceforge.net/browse.php?group=librar y-manual&entry=tracer.htm

The OSE library has had this stuff for over ten years now.