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How to use this C conditional compilation statements

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neoaspilet11

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Hello Friends,

This might sound simple:

How to use the following conditional statements

1.) #ifdef

2.) #ifundef


IF I have this
#define DEBUG_STRING 1

is

#if(DEBUG_STRING)
//... blah
#endif


equal to

#ifdef(DEBUG_STRING)
//..blah
#endif
 

They are not the same. You can use '#if' with any expression, and you use '#ifdef' with macros.

#if expression
If the value of expression is true, then the code that immediately follows the command will be compiled.

#ifdef macro
If the macro has been defined by a #define statement, then the code immediately following the command will be compiled.

The #ifdef and #ifndef directives are short forms of '#if defined(defined-value)' and '#if !defined(defined-value)' respectively. defined(identifier) is valid in any expression evaluated by the preprocessor, and returns true (in this context, equivalent to 1) if a preprocessor variable by the name identifier was defined with #define and false (in this context, equivalent to 0) otherwise. In fact, the parentheses are optional, and it is also valid to write defined identifier without them.
 
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