C Preprocessor Directives

A Little C Primer

We've already seen the "#include" and "#define" preprocessor directives. The C preprocessor supports several other directives as well. All such directives start with a "#" to allow them to be distinguished from C language commands.

As explained in the first chapter, the "#include" directive allows the contents of other files to be included in C source code:

#include <stdio.h>

Notice that the standard header file "stdio.h" is specified in angle brackets. This tells the C preprocessor that the file can be found in the standard directories designated by the C compiler for header files. To include a file from a nonstandard directory, use double quotes:

#include "\home\mydefs.h"

Include files can be nested. They can call other include files.

Also as explained in the first chapter, the "#define" directive can be used to specify symbols to be substituted for specific strings of text:

#define PI 3.141592654
...
a = PI * b;

In this case, the preprocessor does a simple text substitution on PI throughout the source listing. The C compiler proper not only does not know what PI is, it never even sees it.

The "#define" directive can be used to create function-like macros that allow parameter substitution. For example:

#define ABS(value)  ( (value) >=0 ? (value) : -(value) )

This macro could then be used in an expression as follows:

printf( "Absolute value of x = %d\n", ABS(x) );

Beware that such function-like macros don't behave exactly like true functions. For example, suppose "x++" is as an argument for the macro above:

val = ABS(x++);

This would result in "x" being incremented twice because "x++" is substituted in the expression twice:

val = ( (x++) >=0 ? (x++) : -(x++) )

Along with the "#define" directive, there is also an "#undef" directive that undefines a constant that has been previously defined:

#undef PI

Another feature supported by the C preprocessor is conditional compilation, using the following directives:

#if
#else
#elif
#endif

These directives can test the values of defined constants to define which blocks of code are passed on to the C compiler proper:

#if WIN == 1
  #include "WIN.H"
#elif MAC == 1
  #include "MAC.H"
#else
  #include "LINUX.H"

#endif

These directives can be nested if needed. The "#if" and "#elif" can also test to see if a constant has been defined at all, using the "defined" operator:

#if defined( DEBUG )
   printf( "Debug mode!\n");
#endif

—or test to see if a constant has not been defined:

#if !defined( DEBUG )
   printf( "Not debug mode!\n");
#endif

Finally, there is a "#pragma" directive, which by definition is a catch-all used to implement machine-unique commands that are not part of the C language. Pragmas vary from compiler to compiler, since they are by definition nonstandard.

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