C – Preprocessors, Header Files and Type Casting

C – Preprocessors

The C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, a C Preprocessor is just a text substitution tool and they instruct compiler to do required pre-processing before actual compilation. We’ll refer to the C Preprocessor as the CPP.

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives:

Directive Description
#define Substitutes a preprocessor macro
#include Inserts a particular header from another file
#undef Undefines a preprocessor macro
#ifdef Returns true if this macro is defined
#ifndef Returns true if this macro is not defined
#if Tests if a compile time condition is true
#else The alternative for #if
#elif #else an #if in one statement
#endif Ends preprocessor conditional
#error Prints error message on stderr
#pragma Issues special commands to the compiler, using a standardized method

Preprocessors Examples

Analyze the following examples to understand various directives.

#define MAX_ARRAY_LENGTH 20

This directive tells the CPP to replace instances of MAX_ARRAY_LENGTH with 20. Use#define for constants to increase readability.

#include <stdio.h>
#include "myheader.h"

These directives tell the CPP to get stdio.h from System Libraries and add the text to the current source file. The next line tells CPP to get myheader.h from the local directory and add the content to the current source file.

#undef  FILE_SIZE
#define FILE_SIZE 42

This tells the CPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE
   #define MESSAGE "You wish!"
#endif

This tells the CPP to define MESSAGE only if MESSAGE isn’t already defined.

#ifdef DEBUG
   /* Your debugging statements here */
#endif

This tells the CPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

Predefined Macros

ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified.

Macro Description
__DATE__ The current date as a character literal in “MMM DD YYYY” format
__TIME__ The current time as a character literal in “HH:MM:SS” format
__FILE__ This contains the current filename as a string literal.
__LINE__ This contains the current line number as a decimal constant.
__STDC__ Defined as 1 when the compiler complies with the ANSI standard.

Let’s try the following example:

#include <stdio.h>

main()
{
   printf("File :%s\n", __FILE__ );
   printf("Date :%s\n", __DATE__ );
   printf("Time :%s\n", __TIME__ );
   printf("Line :%d\n", __LINE__ );
   printf("ANSI :%d\n", __STDC__ );

}

When the above code in a file test.c is compiled and executed, it produces the following result:

File :test.c
Date :Jun 2 2012
Time :03:36:24
Line :8
ANSI :1

Preprocessor Operators

The C preprocessor offers following operators to help you in creating macros:

Macro Continuation (\)

A macro usually must be contained on a single line. The macro continuation operator is used to continue a macro that is too long for a single line. For example:

#define  message_for(a, b)  \
    printf(#a " and " #b ": We love you!\n")

Stringize (#)

The stringize or number-sign operator (‘#’), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list. For example:

#include <stdio.h>

#define  message_for(a, b)  \
    printf(#a " and " #b ": We love you!\n")

int main(void)
{
   message_for(Carole, Debra);
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Carole and Debra: We love you!

Token Pasting (##)

The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example:

#include <stdio.h>

#define tokenpaster(n) printf ("token" #n " = %d", token##n)

int main(void)
{
   int token34 = 40;
   
   tokenpaster(34);
   return 0;
}

When the above code is compiled and executed, it produces the following result:

token34 = 40

How it happened, because this example results in the following actual output from the preprocessor:

printf ("token34 = %d", token34);

This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting.

The defined() Operator

The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows:

#include <stdio.h>

#if !defined (MESSAGE)
   #define MESSAGE "You wish!"
#endif

int main(void)
{
   printf("Here is the message: %s\n", MESSAGE);  
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Here is the message: You wish!

Parameterized Macros

One of the powerful functions of the CPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows:

int square(int x) {
   return x * x;
}

We can rewrite above code using a macro as follows:

#define square(x) ((x) * (x))

Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between and macro name and open parenthesis. For example:

#include <stdio.h>

#define MAX(x,y) ((x) > (y) ? (x) : (y))

int main(void)
{
   printf("Max between 20 and 10 is %d\n", MAX(10, 20));  
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Max between 20 and 10 is 20

C – Header Files

A header file is a file with extension .h which contains C function declarations and macro definitions and to be shared between several source files. There are two types of header files: the files that the programmer writes and the files that come with your compiler.

You request the use of a header file in your program by including it, with the C preprocessing directive #include like you have seen inclusion of stdio.h header file, which comes along with your compiler.

Including a header file is equal to copying the content of the header file but we do not do it because it will be very much error-prone and it is not a good idea to copy the content of header file in the source files, specially if we have multiple source file comprising our program.

A simple practice in C or C++ programs is that we keep all the constants, macros, system wide global variables, and function prototypes in header files and include that header file wherever it is required.

Include Syntax

Both user and system header files are included using the preprocessing directive #include. It has following two forms:

#include <file>

This form is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option while compiling your source code.

#include "file"

This form is used for header files of your own program. It searches for a file named file in the directory containing the current file. You can prepend directories to this list with the -I option while compiling your source code.

Include Operation

The #include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current source file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the #includedirective. For example, if you have a header file header.h as follows:

char *test (void);

and a main program called program.c that uses the header file, like this:

int x;
#include "header.h"

int main (void)
{
   puts (test ());
}

the compiler will see the same token stream as it would if program.c read

int x;
char *test (void);

int main (void)
{
   puts (test ());
}

Once-Only Headers

If a header file happens to be included twice, the compiler will process its contents twice and will result an error. The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like this:

#ifndef HEADER_FILE
#define HEADER_FILE

the entire header file file

#endif

This construct is commonly known as a wrapper #ifndef. When the header is included again, the conditional will be false, because HEADER_FILE is defined. The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice.

Computed Includes

Sometimes it is necessary to select one of several different header files to be included into your program. They might specify configuration parameters to be used on different sorts of operating systems, for instance. You could do this with a series of conditionals as follows:

#if SYSTEM_1
   # include "system_1.h"
#elif SYSTEM_2
   # include "system_2.h"
#elif SYSTEM_3
   ...
#endif

But as it grows, it becomes tedious, instead the preprocessor offers the ability to use a macro for the header name. This is called a computed include. Instead of writing a header name as the direct argument of #include, you simply put a macro name there instead:

 #define SYSTEM_H "system_1.h"
 ...
 #include SYSTEM_H

SYSTEM_H will be expanded, and the preprocessor will look for system_1.h as if the#include had been written that way originally. SYSTEM_H could be defined by your Makefile with a -D option.

C – ┬áType Casting

Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a long value into a simple integer then you can type cast long to int. You can convert values from one type to another explicitly using the cast operatoras follows:

(type_name) expression

Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

#include <stdio.h>

main()
{
   int sum = 17, count = 5;
   double mean;

   mean = (double) sum / count;
   printf("Value of mean : %f\n", mean );

}

When the above code is compiled and executed, it produces the following result:

Value of mean : 3.400000

It should be noted here that the cast operator has precedence over division, so the value ofsum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Integer Promotion

Integer promotion is the process by which values of integer type “smaller” than int orunsigned int are converted either to int or unsigned int. Consider an example of adding a character in an int:

#include <stdio.h>

main()
{
   int  i = 17;
   char c = 'c'; /* ascii value is 99 */
   int sum;

   sum = i + c;
   printf("Value of sum : %d\n", sum );

}

When the above code is compiled and executed, it produces the following result:

Value of sum : 116

Here, value of sum is coming as 116 because compiler is doing integer promotion and converting the value of ‘c’ to ascii before performing actual addition operation.

Usual Arithmetic Conversion

The usual arithmetic conversions are implicitly performed to cast their values in a common type. Compiler first performs integer promotion, if operands still have different types then they are converted to the type that appears highest in the following hierarchy:

Usual Arithmetic Conversion

The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take following example to understand the concept:

#include <stdio.h>

main()
{
   int  i = 17;
   char c = 'c'; /* ascii value is 99 */
   float sum;

   sum = i + c;
   printf("Value of sum : %f\n", sum );

}

When the above code is compiled and executed, it produces the following result:

Value of sum : 116.000000

Here, it is simple to understand that first c gets converted to integer but because final value is double, so usual arithmetic conversion applies and compiler convert i and c into float and add them yielding a float result.