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How to implement a leveled debug system?

Answer 1

+4 A:

Sure, there are more elegant ways to do this, of course, but this works just fine

#ifdef DEBUG
 extern int g_debuglevel;
 #define printd(level, x) (level <= g_debuglevel) ? 0 : printf x
#else
 #define printd(level, x)
#endif

Although personally I prefer this

#ifdef DEBUG
 extern void printdf(level, fmt, ...);
 #define printd printfd
#else
 #define printd
#endif

where printdf is a function that tests the level and then calls vprintf passing along the fmt and va_args.

John Knoeller 2010-02-02 02:15:19

There are sound maintenance reasons for ensuring that the code is always expanded so that the compiler always compiles the debug code. Use `if (0) { ...debug printing...}` or equivalents. It greatly reduces the chance of the debugging being broken when you next activate it - which might be years after the last time.

Jonathan Leffler 2010-02-02 02:46:13

@Johnathon: Actually, I simplified a bit to match the code he posted. my actual debug macro expands to some code whether debug is defined or not. one lesson at a time.

John Knoeller 2010-02-02 02:56:17

@Johnathon: Also, building debug and retail on every checking solves the maintenance issue without the cost of bloating the retail code with debug information.

John Knoeller 2010-02-02 03:10:28

There is no code bloat in the 'retail' code unless the compiler is broken; it will optimize away the code after 'if (0)'. I can agree with one lesson at a time - though it doesn't hurt to point out that the current lesson is not the final word on the subject.

Jonathan Leffler 2010-02-02 04:02:35

@Johnathon Leffler: that has not been my experience, the optimizer can _usually_ discard the debug code, but it sometimes has trouble when it references variables outside of the debug block.

John Knoeller 2010-02-02 04:07:22

Answer 2

+1 A:

See the answers to:

These will give you a lot of pointers on what to do.

Jonathan Leffler 2010-02-02 02:44:40

Yes, you can do fancy new things with a C99 compliant compiler, but most of us don't have that yet.

John Knoeller 2010-02-02 02:58:36

@John Knoeller: As I point out in my answer to the first of the two questions listed, there is a perfectly viable solution for C89 which I used until a couple of years ago. It isn't as neat and tidy as the C99 solution using the `__VA_ARGS__` macro arguments, but it most certainly works.

Jonathan Leffler 2010-02-02 03:59:52

Thank you! I've done something similar in the end, and now I now it's better to allow the compiler see debug code in release builds.

Carla Álvarez 2010-02-02 14:15:26

Answer 3

+1 A:

If you want to go the extra step and log your level debug statements they you could try log4c. http://log4c.sourceforge.net/

in the printfd function described above, you can also check for an environment variable say DEBUG_LOG_LEVEL to dynamically switch on and off your logging.

I used a similar technique to implement a lightweight leveled logging library for an embedded linux environment.

Also for an example of log4c - http://log4c.sourcearchive.com/documentation/1.2.1/helloworld_8c-source.html Thanks to google search :)

ka05 2010-02-02 21:22:11

Thanks! But at least for now (it's kind of a small project) the simple logging suggested is enough for me.

Carla Álvarez 2010-02-03 10:01:56

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