C Preprocessor

C Preprocessor is not a part of the compiler in the C program, but it is a separate function in the compilation process. Also, we can say it’s a text substitution tool and it instructs the compiler to do the required preprocessing before the actual compilation process. C Pre-processor refers as CPP in the program.

All the pre-processors commands begin with a hash symbol (#). It must be the first non-blank character and for readability. It directive should begin in the first column.

These are the important preprocessor directives −

These are basic file operations to perform.

• #define: It substitutes a pre-processor macro.
• #include: It inserts a particular header from another file.
• #undef: It undefines a preprocessor macro.
• #ifdef: It returns a true if this macro is defined.
• #ifndef: It returns a true if this macro is not defined.
• #if: It tests if a compile time condition is true.
• #else: It’s alternative for #if directive.
• #elif: #else and #if should be in one statement.
• #endif: It ends a preprocessor conditional.
• #error: It prints an error message on stderr.
• #pragma: It issues the special command to the compiler, using the standardized method.

Example:

#define MAX_ARRAY_LENGTH 5

In this example the CPP is to replace the instances of MAX_ARRAY_LENGTH with 5