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.

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:

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 ==