Description
Any C-program undergoes a pre-compilation process i.e before the program is compiled, there will be a process to check the program.This process is done by the C Preprocessor.
Preprocessor commands are called as 
- Makes developer to read the program easily.
- Program can be modified without any difficulty.
- Pre-processor makes the program architecture independent.
Directives. Directives start with # symbol. Basically, there are 4 Preprocessor directives in C-compiler.
Description
The files declared in
header files can be used in the main program if once declared.
The file which consists of macro definitions and C-function declarations is called as header file. These can be described more clearly in the lesson Header Files. Description
The
#define is a macro, used to denote the constant values and they can be of any data types.
Eg: #define letter 'A'
Suppose if there is #define max(a,b) in the program, it appears as #define max(a>b ? a:b) after the pre-process commands are compiled. Example
[c]
#include<stdio.h>
#define length 10
void main()
{
printf("value of length is: %d \n", length );
}[/c]
Output:
[c]value of length is: 10[/c]
Description
According to the condition, a set of commands can be involved or removed in the program.
Examples
Below is the example for if,else and endif.
[c]#include<stdio.h>
#define splessons 10
int main()
{
#if (splessons==100)
printf("if is included in the source code\n");
#else
printf("else is included in the source code\n");
#endif
return 0;
}[/c]
Output:
[c]
else is included in the source code
[/c]
Example for ifdef,else,endif
[c]
#include<stdio.h>
#define splessons 100
int main()
{
#ifdef splessons
printf("splessons is defined.It can be added in the C file\n");
#else
printf("splessons is not defined\n");
#endif
return 0;
}[/c]
Output:
[c]
splessons is defined.It can be added in the C file
[/c]
undef is used to un-define the existing macro variable.
[c]
#include<stdio.h>
#define height 100
void main()
{
printf("First defined value for height: %d\n",height);
#undef length
#define length 20
printf("length after undef and redefine:%d",length);
}[/c]
Output:
[c]
First defined value for height: 100
length after undef and redefine:20[/c]
Description
It will allow a directive to be defined and the effects are defined by implementation. If pragma is ignored,if it is not supported.
It halts the compilation and returns the specified error message.
Current line number is allowed by this directive and the apparent name of the current sourcecode filename to be changed.
Key Points
- Pre-processor performs pre-compilation.
- Commands used in preprocessor are called preprocessor directives.
- Macros, conditional directives and file inclusion directives are pre-processor directives.