C compiler preprocessors

Preprocessor commands are executed before the compiler compiles the source code. These commands will change the original code usually to suit the operating environment and/or to add code that will be required by calls to library functions. Preprocessors are recognised by the leading # in their names.

Preprocessors
#include Insert a source file.

#define Define a preprocessor constant.

#if Branch based on an expression.

#ifdef Branch if preprocessor constant has been defined?

#ifndef Branch if a preprocessor constant has NOT been defined.

#line Specify the number of the next source line.

#undef Remove a preprocessor constant.

#pragma Perform an implementation dependent action. See specific compiler documentation

#else Executed if #if #ifdef or #ifndef fails.

#error Write an error message.

#elif Executed when an #if fails.

#endif Close #if #ifdef or #ifndef

Preprocessors
#include	Insert a source file.
#define	Define a preprocessor constant.
#if	Branch based on an expression.
#ifdef	Branch if preprocessor constant has been defined?
#ifndef	Branch if a preprocessor constant has NOT been defined.
#line	Specify the number of the next source line.
#undef	Remove a preprocessor constant.
#pragma	Perform an implementation dependent action. See specific compiler documentation
#else	Executed if #if #ifdef or #ifndef fails.
#error	Write an error message.
#elif	Executed when an #if fails.
#endif	Close #if #ifdef or #ifndef

Notes:

Preprocessors should start in column 1.
The -E option tells gcc to stop after the preprocessor stage and send the O/P to STDOUT (usually the screen).

Martin Leslie