Preprocessing in C

C's preprocessor provides a facility for defining constant and substitution, which are commonly called macros.

• #include
  

The include directive tells the compiler to include all the contents of a specified file in the source file before giving the source file for compiling.

eg: #define TRUE 1

• #undef

Nullify the effect of define directive.
eg: #undef TRUE
#define TRUE correct

#include
Angle brackets Searches in the default directories of include files.

Quotes specifies search in current directory.

• #ifdef

ifdef is used to make a substitution depending on whether a certain identifier is defined. If defined returns true else false.

e.g: #ifdef TRUE
#define FALSE 0
#endif
Note: we can easily add multiple macros in between the two macros.

• #ifndef

eg: #ifndef FALSE
#define WRONG 0

• #if

#if allows you to define more generalized conditions. Multiple conditions, which are connected by relational operators such as AND(&&), OR(||), are allowed.

• #error

Used to display error message by preprocessor.

#if !define FALSE
#error "Specify FALSE condition"

• #line

Mainly used for debugging purpose.
eg: #line100 indicates it is in line 100

C has provided two special identifiers: __FILE__ and __LINE__, which indicate the file name of the source file and the current line number, respectively.

Macros allow replacement of the identifier by using a statement or expression.
eg: #define CUBE(x) x*x*x