Preprocessor

The preprocessor is a program that edits C programs prior to compilation. It uses preprocessing directives which are commands that begin with a # character. For example, the #include directive is used to include header files in a C program. This directive tells the preprocessor to include the contents of the specified header file in the program.

Preprocessing Directives

Preprocessing directives fall into three categories:

Macro definition. #define is used to define macros. #undef is used to undefine macros.
File inclusion. #include causes the contents of a specified file to be included in the program.
Conditional compilation. #if, #ifdef, #ifndef, #elif, #else and #endif are used to conditionally compile parts of a program.

Macro Definitions

We'll look at macro definitions first. A macro is a name that represents a sequence of characters. When the preprocessor encounters a macro name in a program, it replaces the name with the sequence of characters that the macro represents. This process is called macro expansion.

Simple Macros

Simple macros are defined using the #define directive. The syntax is:

#define name replacement

The name is the name of the macro and the replacement is the sequence of characters that the macro represents. For example, the following macro defines the name PI to represent the value 3.14159:

#define PI 3.14159
#define PRINT printf("Hello, world!\n")

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Notice the lack of a semicolon at the end of the PRINT macro. In C, macros defined using the #define preprocessor directive do not require a semicolon at the end. In fact, if you put a semicolon at the end of a macro definition, the macro will include that semicolon when it's expanded, which might not be what you want.

When the preprocessor encounters the name PI in a program, it replaces it with the value 3.14159. Consider the following program:

#include <stdio.h>
 
#define PI 3.14159
#define PRINT printf("Hello, world!\n")
 
int main(void) {
    printf("PI = %f\n", PI);
    PRINT;
 
    return 0;
}

The preprocessor replaces the name PI with the value 3.14159 before the program is compiled. The compiler sees the following program:

#include <stdio.h>
 
int main(void) {
    printf("PI = %f\n", 3.14159);
    printf("Hello, world!\n");
    return 0;
}

Parameterized Macros

Parameterized macros are function-like macros. The syntax is:

#define name(parameters) replacement

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There must be no whitespace between a parameterized macro name and the left parenthesis.

The name is the name of the macro and the replacement is the sequence of characters that the macro represents. The parameters are the parameters of the macro. For example, the following macro defines the name MAX to represent the maximum of two values:

#define MAX(a, b) ((a) > (b) ? (a) : (b))

When the preprocessor encounters the name MAX in a program, it replaces it with the sequence of characters ((a) > (b) ? (a) : (b)). Consider the following program:

#include <stdio.h>
 
#define MAX(a, b) ((a) > (b) ? (a) : (b))
 
int main(void) {
    printf("MAX(3, 5) = %d\n", MAX(3, 5)); // prints MAX(3, 5) = 5
    return 0;
}