Directives and Preprocessor.....................

Directive

The instructions to the preprocessor appear in the source as directives and
can be easily spotted in the source code because they all begin with a hash (#) character,
appearing as the first non blank character on a line. The hash character usually appears
on column 1 and is immediately followed by the directive keyword

Example

#define
#include
#ifndef

Preprocessor

The preprocessor reads the source code and responds to directives
embedded in it to produce a modified version of the source, which is fed to the
compiler. The preprocessor is still an important part of C, C++,

Macro

The macro has a name that,when found elsewhere in the text, is replaced with the string of characters defined as the value of the macro. It is possible to specify parameters that are to be used as part of the macro expansion.

Example

#define min(a,b) ((a) < (b) ? (a) : (b))

Directive	Description
#define	Defines a name as a macro that the preprocessor will expand
#elif	Provides an alternative expression to be evaluated by an #if directive.
#else	Provides an alternative set of code to be compiled if an #if, #ifdef, or #ifndef is false.
#error	Produces an error message and halts the preprocessor.
#if	Compiles the code between this directive and its matching #endif only if evaluating an arithmetic expression results in a nonzero value.
#ifdef	Compiles the code between this directive and its matching #endif only if the named macro has been defined.
#ifndef	Compiles the code between this directive and its matching #endif only if the named macro has not been defined.
#include	Searches through a list of directories until it finds the named file; then it inserts the contents of the file just as if it had been inserted by a text editor.
#include_Next	The same as #include, but this directive begins the search for the file in the directory following the one in which the current file was found.
#line	Specifies the line number, and possibly the file name, that is reported to the compiler to be used to create debugging information in the object file.
#pragma	A standard method of providing additional information that may be specific to one compiler or one platform.
#undef	Removes a definition previously created by a #define directive.
#warning	Produces a warning message from the preprocessor.
##	The concatenation operator, which can be used inside a macro to combine two strings into one.