A Little C Primer/C Preprocessor Directives

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.