C Preprocessor and Macros

C preprocessor and Macros: In this tutorial, you will be acquainted with c preprocessors, and you will learn to use #include, #define, and conditional compilation with the help of examples.

The C preprocessor is a macro preprocessor (allow you to characterize macros) that changes your program before it is gathered. These changes can be the incorporation of header file, macro expansions etc.

All preprocessing mandates start with a # symbol. For instance,

#define PI 3.14

Some of the common uses of C preprocessor are:

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Including Header Files: #include

The #include preprocessor is used to include header files to C programs. For example,

#include <stdio.h>

Here, stdio.h is a header file. The #include preprocessor directive replaces the above line with the contents of stdio.h header file.

That’s the reason why you need to use #include before you can use functions like scanf() and printf().

You can also create your own header file containing function declaration and include it in your program using this preprocessor directive.

#include "my_header.h"

Visit this page to learn more about using header files.

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Macros using #define

A macro is a fragment of code that is given a name. You can define a macro in C using the #define preprocessor directive.

Here’s an example.

#define c 299792458  // speed of light

Here, when we use c in our program, it is replaced with 299792458.

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Example 1: #define preprocessor

#include <stdio.h>
#define PI 3.1415

int main()
{
    float radius, area;
    printf("Enter the radius: ");
    scanf("%f", &radius);

    // Notice, the use of PI
    area = PI*radius*radius;

    printf("Area=%.2f",area);
    return 0;
}

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Functions like Macros

You can likewise define macros that work along these lines like a capacity call. This is known as capacity like macros. For instance,

#define circleArea(r) (3.1415*(r)*(r))

Every time the program encounters circleArea(argument), it is replaced by (3.1415(argument)(argument)).

Suppose, we passed 5 as an argument then, it expands as below:

circleArea(5) expands to (3.1415*5*5)

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Example 2: Using #define preprocessor

#include <stdio.h>
#define PI 3.1415
#define circleArea(r) (PI*r*r)

int main() {
    float radius, area;

    printf("Enter the radius: ");
    scanf("%f", &radius);
    area = circleArea(radius);
    printf("Area = %.2f", area);

    return 0;
}

Visit this page to learn more about macros and #define preprocessor.

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Conditional Compilation

In C programming, you can instruct preprocessor whether to incorporate a square of code or not. To do as such, contingent orders can be used.

It’s similar an if statement with one significant contrast.

The if the statement is tried during the execution time to check whether a square of code ought to be executed or not while the conditionals are used to incorporate (or skip) a block of code in your program before execution

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Uses of Conditional

use different code depending on the machine, operating system
compile same source file in two different programs
to exclude certain code from the program but to keep it as reference for future purpose

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How to use conditional?

To use conditional, #ifdef, #if, #defined, #else and #elseif directives are used.

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#ifdef Directive

#ifdef MACRO     
   // conditional codes
#endif

Here, the conditional codes are included in the program only if MACRO is defined.

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#if, #elif and #else Directive

#if expression
   // conditional codes
#endif

Here, expression is an expression of integer type (can be integers, characters, arithmetic expression, macros and so on).

The conditional codes are included in the program only if the expression is evaluated to a non-zero value.

The optional #else directive can be used with #if directive.

#if expression
   conditional codes if expression is non-zero
#else
   conditional if expression is 0
#endif

You can also add nested conditional to your #if…#else using #elif

#if expression
    // conditional codes if expression is non-zero
#elif expression1
    // conditional codes if expression is non-zero
#elif expression2
    // conditional codes if expression is non-zero
#else
    // conditional if all expressions are 0
#endif

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#defined

The special operator #defined is used to test whether a certain macro is defined or not. It’s often used with #if directive.

#if defined BUFFER_SIZE && BUFFER_SIZE >= 2048
  // codes

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Predefined Macros

Here are some predefined macros in C programming.

Macro	Value
__DATE__	A string containing the current date
__FILE__	A string containing the file name
__LINE__	An integer representing the current line number
__STDC__	If follows ANSI standard C, then the value is a nonzero integer
__TIME__	A string containing the current date.

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Example 3: Get current time using TIME

The following program outputs the current time using TIME macro.

#include <stdio.h>
int main()
{
   printf("Current time: %s",__TIME__);   
}

Output

Current time: 19:54:39

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Recommended Readings

Line control

Pragmas

Preprocessor Output

Other directives

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Please feel free to give your comment if you face any difficulty here.

Additional Topics