How do you implement c program for conditional compilation?

Answer 1

Conditional compilation is achieve through preprocessor directives. First, define the preprocessor symbols upon which conditional compilation depends, then test them using #if and #else preprocessor directives. A #endif directive indicates the end of the nearest enclosing conditional block, thus conditional blocks may be nested.

The following example demonstrates how we can conditionally define debug or production code based upon the absence or existence of the NDEBUG symbol:

#ifdef NDEBUG

/* all C code within this block is compiled when NDEBUG is defined (production code) */

#else

/* all C code within this block is compiled when NDEBUG is not defined (debug code) */

#endif

Note that the NDEBUG symbol is typically defined via the command line, however symbols can also be defined or undefined via the source using the #define and #undefine directives. For instance, header files typically require guards to protect against being included more than once in a compilation and preprocessor directives provide the conventional means of ensuring that is the case:

// myheader.h

#ifndef _MYHEADER_H_

#define _MYHEADER_H_

// all header code goes here...

#endif

By convention, preprocessing symbols (macros) are defined with all uppercase and are intentionally ugly to avoid any confusion with C identifiers. Header guards must be unique to each header thus they are typically based upon the header file name itself.