Help with #ifdef DEBUG #endif

Hi All,
I am very new to C (but have programmed in other languages) and have been asked to look at some program to modify. I have downloaded codeblocks-10.05mingw-setup.exe (windows). I am trying to break the program up so that I can understand it better.

There are some sections in the code like

#ifdef DEBUG
fprintf(stderr, "Entering module\n");
#endif

How do I access the DEBUG block to see what it does through the CodeBlocks compiler? I tried DEBUG mode but it just runs and I do not see anything. Where is the stderr supposed to show? on the console?

Thanks
Lefti

DEBUG is not a block of anything... it's a defined symbol...

If you #define DEBUG ... the code between #if and #endif is executed.
If you don't it isn't.

That's all there is to it.

The C Preprocessor

This has nothing to do with "debug mode". In your code, DEBUG is a macro specified by whoever wrote your code. The preprocessor does text substitution in a pre-compilation phase.

In CodeBlocks, go to Project->Build Options....

A window will pop up with several tabs. One of those is labelled "Compiler Settings". Select that. Within that, there will be another tab labelled #defines. Select that.

That will show you a text edit pane. In that, define each macro you want to define.

In your case (since using #ifdef DEBUG) enter "DEBUG" (without the quotes, case sensitive) on a line of its own. This is equivalent, at the top of your code, adding a line of the form "#define DEBUG".

If you want the preprocessor to expand DEBUG to a value (for example, code has #if DEBUG == 2) then enter "DEBUG=2". This is equivalent, at the top of your code, to adding a line of the form "#define DEBUG 2".

It would help if you read up on the C or C++ preprocessing. Any basic text on either language will describe enough for you to make sense of the above.

Quote:

---

Originally Posted by CommonTater

DEBUG is not a block of anything... it's a defined symbol...

If you #define DEBUG ... the code between #if and #endif is executed.
If you don't it isn't.

That's all there is to it.

The C Preprocessor

---

#if ( #if #ifndef #ifdef ) are compile time directives not run time directives so the code between #if and #endif is compiled if DEBUG is defined, and will execute like any other code.