C Preprocessor

What are preprocessors in C?

• Preprocessors are programs that process our source code before compilation.
• It is also called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.

What are preprocessor directives?

• Preprocessor programs provide preprocessor directives that tell the compiler to preprocess the source code before compiling. 
• All of these preprocessor directives begin with a ‘#’ (hash) symbol. The statements starting with a ‘#’ symbol will be executed by the preprocessor. 
• Examples of some preprocessor directives are: #include, #define, #ifndef etc.

Types of Preprocessor Directives: 

• File Inclusion
• Macros
• Conditional Compilation 
• Other directives

1. File Inclusion:

The #include directive is a preprocessor directive that is used to include a file in the source program. 

There are two ways of including files using #include :

1.1 Using angular brackets:

If the filename is enclosed within angle brackets, the file is searched for in the standard compiler include paths.

    #include <file_name>

1.2 Using double quotes:

If the filename is enclosed within double quotes, the search path is expanded to include the current source file directory.

    

    #include "file_name"

2. Macros

• A macro is a piece of code that is mapped to a name or identifier. 
• Whenever the name is used, it is replaced by the contents of the macro.
• The #define directive is used to define a macro.

There are two types of macros: 

2.1 Object-like macros : They do not take any parameters.

    

    #define <identifier> <macro-content> 

An example of a object-like macro is:

    

    #define PI 3.14 

2.2 Function-like macros : They take parameters. (although the list of parameters may be empty). 

    

    #define <identifier>(<parameter list>) <macro-content>

An example of a function-like macro is:

    

    #define SUM(X, Y)   (X+Y)

How to remove a macro definition?

A macro definition can be removed with #undef:

Syntax:

    

    #undef <identifier> 

Example:

    

    #undef PI 

Predefined Macros:

The preprocessor defines a number of predefined macros. 

Some of the important predefined macros are 

Predefined Macro	Description
__FILE__	It expands to the name of the current input file, in the form of a C string constant.
__LINE__	It expands to the current input line number, in the form of a decimal integer constant.
__DATE__	It expands to a string constant that describes the date on which the preprocessor is being run.
__TIME__	It expands to a string constant that describes the time at which the preprocessor is being run.
__STDC__	It expands to the constant 1, to signify that this compiler conforms to ISO Standard C.
__STDC_VERSION__	It expands to the C Standard's version number, a long integer constant of the form yyyymmL where yyyy and mm are the year and month of the Standard version.

Reference: https://gcc.gnu.org/onlinedocs/gcc-3.2.3/cpp/Standard-Predefined-Macros.html

Note:  Each of the above names is prefixed and suffixed by double underscore characters. 

#include <stdio.h>
 int main(){    
   printf("File Name :%s\n", __FILE__ );    
   printf("Line Number :%d\n", __LINE__ );
   printf("Current Date :%s\n", __DATE__ );    
   printf("Current Time :%s\n", __TIME__ );
   printf("STDC :%d\n", __STDC__ ); 
   printf("STDC VERSION :%d\n", __STDC_VERSION__ ); 
   return 0;  
 }  

OUTPUT:

File Name :/tmp/BXFX45KmyX.c

Line Number :5

Current Date :Oct  4 2022

Current Time :13:02:17

STDC :1

STDC VERSION :201710