C preprocessor: Difference between revisions

The most familiar use of the preprocessor is to include header files (containing function declarations, definition of constants etc.):

 

#include <stdio.h>

#include "myheader.h"

 

The effect is that the contents of the given header file are pasted into the source file. The ''technical'' difference between <> and "" is that the compiler is allowed to satisfy <> includes internally, i.e. without actually accessing any on-disk files of that name. None of the prominent compilers do this, to the knowledge of the author, but it has become common practice to use <> for system headers and "" for your own header files.

The preprocessor can ''define'' tokens. It is good custom to write these tokens in ALL CAPS. (See pt. 2 as for why.)

 

#define MYTOKEN

 

Most compilers also allow the definition of preprocessor tokens on the command line, e.g. the "-D MYTOKEN" option for [[GCC]].

The preprocessor can ''conditionally'' select which parts of source code to compile, depending on whether a given token is defined or not (see above).

 

#define MYTOKEN

 

/* This source will be compiled */

#endif

 

Note that such ''#if'' / ''#ifdef'' / ''#ifndef'' - ''#endif'' sections can be nested.

The solution are ''header guards'', a combination of conditional compilation and token definition:

 

/* abc.h */

 

 

#endif

 

== Preprocessor Macros, pt. 2 ==

The ''#if'' statement can be used to base conditional compilation on token values. Note that the preprocessor can only work with compile-time constants. Compiler-evaluated code like `sizeof()` cannot be used in preprocessor directives. On the upside, the preprocessor can natively handle non-numerical values.

 

#define MYTOKEN foo

#define OTHERTOKEN 42

/* Won't be compiled. */

#endif

 

The ''#if'' directive also allows for a simple construct to disable a region of code without having to worry about nested ''/* ... */'' style comments:

 

#if 0

/* disabled code */

#endif

 

Such code can easily be re-enabled temporarily with no more effort than replacing the "0" with a "1". Source comments as to why you disabled code this way are in order.

Assertions are used to catch situations which should never happen, even under error circumstances. If the condition given in the parantheses does not evaluate to "true", a diagnosis is printed which contains source file name, line number, and (since C99) name of the current function; the program then calls abort().

 

#include <assert.h>

 

assert( 1 != 2 );

assert( gdt_ptr != null );

 

For production code, assertions may be turned off by defining NDEBUG:

 

gcc -DNDEBUG ...

 

Note that <assert.h> does not have (or need) a header guard, i.e. can be included multiple times in a source file, and that whether NDEBUG is defined or not is evaluated anew ''at every inclusion of <assert.h>''. You can thus enable / disable assertions at a very fine-grained level if necessary:

 

#include <assert.h>

 

#include <assert.h>

#endif

 

== See also ==

=== Articles ===

* [[C]]

 

=== External Links ===

 

[[Category:C]]