In C language, what does conditional compilation mean?

In the C programming language, there are several instructions that control the selective compilation of program code. They are as follows −

• #if
• #else
• #elif
• #endif

# The general form of if is as follows −

#if constant_expression
   statement sequence
#endif

#else works similarly to the C keyword else.

#elif means "else if" and establishes an if else-if compilation chain.

Among other things, #if provides an alternative to "commenting out" code.

For example,

#if 0
   printf("#d", total);
#endif

Here, the compiler will ignore printf("#d", total);

#ifdef and #ifndef

#ifdef means "if defined" and ends with #endif.

#ifdef means "if not defined".

#undef

#undef deletes a previously defined definition.

#line

#line changes the contents of __LINE__, which contains the line number of the currently compiled code, and __FILE__, which is a source file containing the source file being compiled A string of names. Both of these are identifiers predefined in the compiler.

#pragma

#The pragma directive is an implementation-defined directive that allows various directives to be provided to the compiler.

Example

The following is a C programExample demonstrating #ifdef, #ifndef, #else and #endif-

Live Demo

# include <stdio.h>
# define a 10
void main(){
   #ifdef a
   printf("</p><p> Hello I am here..");
   #endif
   #ifndef a
   printf("</p><p> Not defined ");
   #else
   printf("</p><p> R u There ");
   #endif
}

Output

When the above program is executed, it produces the following results −

Hello I am here..
R u There

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