Last Minute C Programming Preprocessor Directives Tutorial

A Preprocessor in a C programming language is a Program that processes source code of a C program, expanding Macro Definitions, File Inclusions and more.

Only through Preprocessor directives, you are able to include library files in your C program.

Preprocessing in C happens before compilation. Part of the Source code is replaced with Preprocessing expressions where ever encountered. So the size of Total code increases before passing to the Compiler.

C Preprocessor Directives

All preprocessor directives start with a Pound Symbol #. There are 3 types of Preprocessor Directives

1. Macro Expansion
2. File Inclusion
3. Conditional Compilation and Run

1. Macro Expansion Preprocessor Directives

Macro is like a function or shortcut that can be used to achieve code reuse. Syntax of defining a macro is as follows.

#define NAME EXPRESSION

Example 1:

#define AREA(x) x*x
int main()
{
  int side=20;
  int area = AREA(side);
  printf("AREA=%d", area);

  return 0;
}
// AREA(side) replaced with side * side

Example 2: Proper use of Parantheses avoids unexpected results.

#define AREA(x) x*x
int main()
{
  int side=2;
  float total = 20 / AREA(side);
  printf("AREA=%d", area);

  return 0;
}
// AREA(side) replaced with side * side
// 20 / side * side
// (20/side) * side
// 10 * 2
// 20
//What you expected ???
// 20 / (2*2)
// 5
// Correction: ???
//#define AREA(x) (x*x)

2. File Inclusion Preprocessor Directives

As the name suggests, using a C Precessor directives of File Inclusion type, you are including either Library header files or User defined files. We usually include STDIO.H in our C program to use Printf and Scanf functions.

Note: Preprocessor copies all code of the included file into our C program and sends the combined single copy of code to the compiler.

C editor like TURBO C allows defining list of directories that can be searched for files in a later time. These directories fall into two types.

1. Current Directory
2. Extra Specified Directories

Syntax:

#include "filename"
(or)
#include <filename>

There are two ways of including a file using preprocessor directive #include.

1. #include "filename"

This notation searches file in the CURRENT directory and Extra Specified Directory list.

2. #include <filename>

This notation searches files in only CURRENT directory. If the file is not available no error is raised.

Example: Including a header file

#include<math.h>
int main()
{
  float a = 10.3f;
  float b = sin(a);
  printf("SIN(%f)=%f", a, b);
  return 9;
}

3. Conditional Compilation Preprocessor Directives

Unlike Macro Expansion and File Inclusion preprocessor directives, these Conditional Compilation preprocessor directives are written and used inside our C Program itself.

Condition check is done using IF, ELSE and ELSE IF statements. For Conditional Compilation, directives like #ifdef, #endif, #else, #elif and #ifndef are defined is C language. These help in implementing multiple source codes and version control management in Hardware and Software environments.

The conditional directives check for EXISTENCE of a MACRO and compiles code accordingly. Find examples below for good understanding.

define AREA printf("SUCCESS");
int main()
{
  #ifdef AREA
    printf("EUROPE");
  #else
    printf("AMAZON");
  #endif
  return 9;
}

In the above example, if the MACRO AREA exists, then EUROPE is printed. Otherwise, AMAZON is printed.

#IFNDEF is the reverse of #IFDEF. IFNDEF works if the MACRO does not exist.

#UNDEF Preprocessor Directive

#undef is a special preprocessor directive in C language to Nullify or Remove Definition of existing Macros.

#define AREA printf("SUCCESS");
int main()
{
  #undef AREA

  #ifdef AREA
    printf("EUROPE");
  #else
    printf("AMAZON");
  #endif
  return 9;
}
//OUTPUT
//AMAZON
//Macro AREA is undefined. So #ifdef fails.

Online Test on C Preprocessor Directives