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Preprocessor

Preprocessor

Preprocessor: An instruction that takes action before compilation. It may expand source code, include/exclude source code, or optimise code in a particular way.

Preprocessor directives: Lines which start with #, which will be processed by the preprocessor.

4 Types of Preprocessor Directives

  1. File Inclusion
    Tells the compiler to include a file using the #include directive.

    • standard / header file: files provided by the (standard) library. Contains pre-defined functions like printf(). These files are included with <>. Example: #include <stdio.h>
    • user-defined file: header files defined/written by the user. It's a good practice to divide up functions into separate files and include only when needed. These files are included with "". Example: #include "mylibrary.h"

  2. Macro
    Associates a name with a piece of code. During preprocessing, the compiler will replace all encounters of the name with the corresponding code. Macros are defined with the #define directive.

    #define SIZE 5

//  All occurrences of `SIZE` will be replaced by `5`.
int array[SIZE];
for (int i = 0; i < SIZE; i++) {
	scanf(" %d", &array[i]);
}
for (int i = 0; i < SIZE; i++) { 
    printf("%d \n", array[i]); 
}

    Macros can take arguments and can be used like functions.

    Sample Code

    #include <stdio.h>

#define PI 3.14159265
#define CIRCLE_AREA(r) (r*r*PI)

int main() {
    float radius = 10.0, area;
    
    area = CIRCLE_AREA(radius);
    printf("Area of the circle : %ld", area);
    
    return 0;
}

  3. Conditional Compilation
    Tells C to compile a specific portion of the program or to skip compilation of some specific part of the program based on conditions, enclosed with #ifdef, and #endif.

    #ifdef macro_name
    // code to be compiled if a macro named
    // `macro_name` is defined.
    // if it's not defined, compiler will just 
    // skip this chunk of code.
#endif

    You can also use #if,#elif and #else for conditional compilation.

    #if defined macro_name
    // code to be compiled if a macro named
    // `macro_name` is defined.
#elif defined another_macro
    // code to be compiled if the macro 
    // above is not defined.
    // and a macro named `another_macro` is defined.
#else
    // code to be compiled if above two macros
    // are not defined.
#endif

  4. Others

  • #undef: used to undefine existing macros.

    #define X 10
// after this, X will be replaced with 10
#undef X
// after this, X is not defined
#define X 20
// after this, X will be replaced with 20

  • #pragma: a subset of special commands for the compiler. Some pragma directives are unique to certain compilers. Examples:

    #pragma once
#pragma GCC optimize ("O3")
#pragma forceinline