Preprocessor Directives - C Programming

The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual compilation (Proprocessor direcives are executed before compilation.). It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs. A macro is a segment of code which is replaced by the value of macro. Macro is defined by #define directive.

Preprocessing directives are lines in your program that start with #. The # is followed by an identifier that is the directive name. For example, #define is the directive that defines a macro. Whitespace is also allowed before and after the #.

The # and the directive name cannot come from a macro expansion. For example, if foo is defined as a macro expanding to define, that does not make #foo a valid preprocessing directive.

All preprocessor directives starts with hash # symbol.

List of preprocessor directives :

  1. #include
  2. #define
  3. #undef
  4. #ifdef
  5. #ifndef
  6. #if
  7. #else
  8. #elif
  9. #endif
  10. #error
  11. #pragma

1. #include

The #include preprocessor directive is used to paste code of given file into current file. It is used include system-defined and user-defined header files. If included file is not found, compiler renders error. It has three variants:

#include <file>

This variant is used for system header files. It searches for a file named file in a list of directories specified by you, then in a standard list of system directories.

#include "file"

This variant is used for header files of your own program. It searches for a file named file first in the current directory, then in the same directories used for system header files. The current directory is the directory of the current input file.

#include anything else

This variant is called a computed #include. Any #include directive whose argument does not fit the above two forms is a computed include.

2. Macro's (#define)

Let's start with macro, as we discuss, a macro is a segment of code which is replaced by the value of macro. Macro is defined by #define directive.

Syntax

#define token value  

There are two types of macros:

  1. Object-like Macros
  2. Function-like Macros

1. Object-like Macros

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

#define PI 3.1415  

Here, PI is the macro name which will be replaced by the value 3.14. Let's see an example of Object-like Macros :

#include <stdio.h>  
#define PI 3.1415 
main() 
{  
   printf("%f",PI);  
}  

Output:

3.14000

2. Function-like Macros

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

#define MIN(a,b) ((a)<(b)?(a):(b))    

Here, MIN is the macro name. Let's see an example of Function-like Macros :

#include <stdio.h>  
#define MIN(a,b) ((a)<(b)?(a):(b))  
void main() {  
   printf("Minimum between 10 and 20 is: %d\n", MIN(10,20));    
}  

Output:

Minimum between 10 and 20 is: 10

Preprocessor Formatting

A preprocessing directive cannot be more than one line in normal circumstances. It may be split cosmetically with Backslash-Newline. Comments containing Newlines can also divide the directive into multiple lines.

for example, you can split a line cosmetically with Backslash-Newline anywhere:

/*
*/ # /*
*/ defi\
ne FO\
O 10\
20

is equivalent into #define FOO 1020.

3. #undef

To undefine a macro means to cancel its definition. This is done with the #undef directive.

Syntax:

#undef token  

define and undefine example

#include <stdio.h>  
#define PI 3.1415  
#undef PI  
main() {  
   printf("%f",PI);  
}  

Output:

Compile Time Error: 'PI' undeclared

4. #ifdef

The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code.

Syntax:

#ifdef MACRO  
//code  
#endif

5. #ifndef

The #ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code.

Syntax:

#ifndef MACRO  
//code  
#endif  

6. #if

The #if preprocessor directive evaluates the expression or condition. If condition is true, it executes the code.

Syntax:

#if expression  
//code  
#endif  

7. #else

The #else preprocessor directive evaluates the expression or condition if condition of #if is false. It can be used with #if, #elif, #ifdef and #ifndef directives.

Syntax:

#if expression  
//if code  
#else  
//else code  
#endif

Syntax with #elif

#if expression  
//if code  
#elif expression  
//elif code  
#else  
//else code  
#endif  

Example

#include <stdio.h>  
#include <conio.h>  
#define NUMBER 1  
void main() {  
#if NUMBER==0  
printf("Value of Number is: %d",NUMBER);  
#else  
print("Value of Number is non-zero");  
#endif         
getch();  
}  

Output

Value of Number is non-zero

8. #error

The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.

C #error example

#include<stdio.h>  
#ifndef __MATH_H  
#error First include then compile  
#else  
void main(){  
    float a;  
    a=sqrt(7);  
    printf("%f",a);  
}  
#endif

9. #pragma

The #pragma preprocessor directive is used to provide additional information to the compiler. The #pragma directive is used by the compiler to offer machine or operating-system feature. Different compilers can provide different usage of #pragma directive.

Syntax:

#pragma token 

Example

#include<stdio.h>  
#include<conio.h>  
  
void func() ;  
  
#pragma startup func  
#pragma exit func  
  
void main(){  
printf("\nI am in main");  
getch();  
}  
  
void func(){  
printf("\nI am in func");  
getch();  
}  

Output

I am in func
I am in main
I am in func

Next - Simple and Static Assertion (assert)


Discussion

Read Community Guidelines
You've successfully subscribed to Developer Insider
Great! Next, complete checkout for full access to Developer Insider
Welcome back! You've successfully signed in
Success! Your account is fully activated, you now have access to all content.