Preprocessors and Macros in C

 

Preprocessors and Macros in C

Pre-processors in C are micro-processor that is basically used by compilers. It converts the code and replaces it at the line where the pre-processor used in your code before the actual compilation of your program. This is the reason the process is known as pre-processing.

It is said to be a micro-pre-processor as it allows you to ass macros in the program.

All the commands used in Pre-processor are called the pre-processor directives. Preprocessor directives should always begin with the “#” symbol and should begin in the first column.

Types of Pre-Processors Used in C

Preprocessor	Syntax/Description
Macro	Syntax: #define This macro defines constant value and can be any of the basic data types.
Header file inclusion	Syntax: #include <file_name> The source code of the file “file_name” is included in the main program at the specified place.
Conditional compilation	Syntax: #ifdef, #endif, #if, #else, #ifndef Set of commands are included or excluded in source program before compilation with respect to the condition.
Other directives	Syntax: #undef, #pragma #undef is used to undefine a defined macro variable. #Pragma is used to call a function before and after the main function in a C program.

 

List of Few Pre-Processor Directives and Its Short Description

Sr.No.	Directive & Description
1	#define Substitutes a preprocessor macro.
2	#include Inserts a particular the header from another file.
3	#undef Undefines a preprocessor macro.
4	#ifdef Returns true if this macro is defined.
5	#ifndef Returns true if this macro is not defined.
6	#if Tests if a compile time condition is true.
7	#else The alternative for #if.
8	#elif #else and #if in one statement.
9	#endif Ends preprocessor conditional.
10	#error Prints error message on stderr.

 

Macros in C:

Macros are generally used to define constant values that are being used repeatedly in programs.

Macros can even accept arguments and such macros are known as function-like macros.

It can be useful if tokens are concatenated into code to simplify some complex declarations.

Macros provide text replacement functionality at pre-processing time.

A macro is a name given to a block of C statements as a pre-processor directive.

Being a pre-processor, the block of code is communicated to the compiler before entering into the actual coding (main () function).

A macro is defined with the preprocessor directive, #define

Types of Macros in C

A macro is a segment of code that is replaced by the value of the macro. Macro is defined by #define directive. There are two types of macros:

1.     Object-like Macros

2.     Function-like Macros

 

Object-Like Macro:

The object-like macro is an identifier that is replaced by value. It is widely used to represent numeric constants. For example:

#define PI 3.14 

Here, PI is the macro name that will be replaced by the value 3.14.

#Define Macro:

In the C Programming Language, the #define directive allows the definition of macros within your source code.

These macro definitions allow constant values to be declared for use throughout your code.

Macro definitions are not variables and cannot be changed by your program code like variables.

You generally use this syntax when creating constants that represent numbers, strings, or expressions.

Constant

#define CNAME value

OR

#define CNAME (expression)

CNAME

The name of the constant. Most C programmers define their constant names in uppercase, but it is not a requirement of the C Language.

value

The value of the constant.

expression

An expression whose value is assigned to the constant. The expression must be enclosed in parentheses if it contains operators.

Note: Do NOT put a semicolon character at the end of #define statements. This is a common mistake.

Number

#define directive to define a numeric constant:

Example: The constant named AGE would contain the value of 10.

#define AGE 10

String

#define directive to define a string constant.

Example: The constant called NAME would contain the value of "C Programming".

#define NAME "C Programming"

Expression

Example: The #define directive to define a constant using an expression.

#define GRADE (20 / 2)

Example 1:

Example 2:

Example 3:

Output:

Example 4:

Output:

 

Example 5:

 

Output:

 

Function Like Macro:

The function-like macro looks like the function call. For example:

#define MIN(a,b) ((a)<(b)?(a):(b))   

Here, MIN is the macro name.

Example :

Output:

 

Predefined Macros in C:

Sr.No.	Macro & Description
1	__DATE__ The current date as a character literal in "MMM DD YYYY" format.
2	__TIME__ The current time as a a character literal in "HH:MM: SS" format.
3	__FILE__ This contains the current filename as a string literal.
4	__LINE__ This contains the current line number as a decimal constant.
5	__STDC__ Defined as 1 when the compiler complies with the ANSI standard.

 

 

Dr. Arpana Chaturvedi

Asst Prof (JIMS-BCA)

Department of Information Technology and Software Development