C Preprocessor and Macros
Excerpt
The C preprocessor is a macro preprocessor (allows you to define macros) that transforms your program before it is compiled. In this tutorial, you will be introduced to c preprocessors, and you will learn to useinclude,define and conditional compilation with the help of examples.
Working of C Preprocessor
The C preprocessor is a macro preprocessor (allows you to define macros) that transforms your program before it is compiled. These transformations can be the inclusion of header files, macro expansions, etc.
All preprocessing directives begin with a # symbol. For example,
<span>#<span>define</span> PI 3.14</span>
Some of the common uses of C preprocessors are:
The #include preprocessor is used to include header files to C programs. For example,
<span>#<span>include</span> <span><stdio.h></span></span>
Here, stdio.h is a header file. The #include preprocessor directive replaces the above line with the contents of stdio.h header file.
Thatâs the reason why you need to use #include <stdio.h> 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.
<span>#<span>include</span> <span>"my_header.h"</span></span>
Visit this page to learn more about using header files.
Macros usingdefine
A macro is a fragment of code that is given a name. You can define a macro in C using the #define preprocessor directive.
Hereâs an example.
<span>#<span>define</span> c 299792458 <span>// speed of light</span></span>
Here, when we use c in our program, it is replaced with 299792458.
Example 1:define preprocessor
<span>#<span>include</span> <span><stdio.h></span></span>
<span>#<span>define</span> PI 3.1415</span>
<span><span>int</span> <span>main</span><span>()</span>
</span>{
<span>float</span> radius, area;
<span>printf</span>(<span>"Enter the radius: "</span>);
<span>scanf</span>(<span>"%f"</span>, &radius);
<span>// Notice, the use of PI</span>
area = PI*radius*radius;
<span>printf</span>(<span>"Area=%.2f"</span>,area);
<span>return</span> <span>0</span>;
}
Function like Macros
You can also define macros that work in a similar way as a function call. This is known as function-like macros. For example,
<span>#<span>define</span> circleArea(r) (3.1415*(r)*(r))</span>
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(<span>5</span>) <span>expands <span>to</span> <span>(<span>3.1415</span>*<span>5</span>*<span>5</span>)</span></span>
Example 2: Usingdefine preprocessor
<span>#<span>include</span> <span><stdio.h></span></span>
<span>#<span>define</span> PI 3.1415</span>
<span>#<span>define</span> circleArea(r) (PI*r*r)</span>
<span><span>int</span> <span>main</span><span>()</span> </span>{
<span>float</span> radius, area;
<span>printf</span>(<span>"Enter the radius: "</span>);
<span>scanf</span>(<span>"%f"</span>, &radius);
area = circleArea(radius);
<span>printf</span>(<span>"Area = %.2f"</span>, area);
<span>return</span> <span>0</span>;
}
Visit this page to learn more about macros anddefine preprocessor.
Conditional Compilation
In C programming, you can instruct the preprocessor whether to include a block of code or not. To do so, conditional directives can be used.
Itâs similar to a if statement with one major difference.
The if statement is tested during the execution time to check whether a block of code should be executed or not whereas, the conditionals are used to include (or skip) a block of code in your program before execution.
Uses of Conditional
- use different code depending on the machine, operating system
- compile the same source file in two different programs
- to exclude certain code from the program but to keep it as a reference for future purposes
How to use conditional?
To use conditional, #ifdef, #if, #defined, #else and #elif directives are used.
ifdef Directive
<span>#<span>ifdef</span> MACRO </span>
<span>// conditional codes</span>
<span>#<span>endif</span></span>
Here, the conditional codes are included in the program only if MACRO is defined.
if,elif andelse Directive
<span>#<span>if</span> expression</span>
<span>// conditional codes</span>
<span>#<span>endif</span></span>
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.
<span>#<span>if</span> expression</span>
conditional codes <span>if</span> expression is non-zero
<span>#<span>else</span></span>
conditional <span>if</span> expression is <span>0</span>
<span>#<span>endif</span></span>
You can also add nested conditional to your #if...#else using #elif
<span>#<span>if</span> expression</span>
<span>// conditional codes if expression is non-zero</span>
<span>#<span>elif</span> expression1</span>
<span>// conditional codes if expression is non-zero</span>
<span>#<span>elif</span> expression2</span>
<span>// conditional codes if expression is non-zero</span>
<span>#<span>else</span></span>
<span>// conditional if all expressions are 0</span>
<span>#<span>endif</span></span>
defined
The special operatordefined is used to test whether a certain macro is defined or not. Itâs often used withif directive.
<span>#<span>if</span> defined BUFFER_SIZE && BUFFER_SIZE >= 2048</span>
<span>// codes</span>
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 time. |
Example 3: Get current time using __TIME__
The following program outputs the current time using __TIME__ macro.
<span>#<span>include</span> <span><stdio.h></span></span>
<span><span>int</span> <span>main</span><span>()</span>
</span>{
<span>printf</span>(<span>"Current time: %s"</span>,__TIME__);
}
Output
<samp>Current time: 19:54:39</samp>