Preprocessor Directives

The C preprocessor is a micro processor that is used by compiler to transform your code before compilation. It is called micro preprocessor because it allows us to add macros.
The C preprocessor is a micro processor that is used by compiler to transform your code before compilation. It is called micro preprocessor because it allows us to add macros.

All preprocessor directives starts with hash # symbol.
Let's see a list of preprocessor directives.
  • #include
  • #define
  • #undef
  • #ifdef
  • #ifndef
  • #if
  • #else
  • #elif
  • #endif
  • #error
  • #pragma
    •

    What is Macros

    A macro is a segment of code which is replaced by the value of macro. Macro is defined by #define directive. There are two types of macros:

  • Object-like Macros
  • Function-like Macros

    • 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 3.14
    Here, PI is the macro name which will be replaced by the value 3.14.

    Function-like Macros

    The function-like macro looks like function call. For example:
    #define MIN(x, y) ((x) < (y)? (x): (y))
    Here, MIN is the macro name.

    Predefined Macros

    ANSI C defines many predefined macros that can be used in c program.

    No.MacroDescription
    1_DATE_represents current date in "MMM DD YYYY" format.
    2_TIME_represents current time in "HH:MM:SS" format.
    3_FILE_represents current file name.
    4_LINE_represents current line number.
    5_STDC_It is defined as 1 when compiler complies with the ANSI standard.

    predefined macros example

    simple.c 
    #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("STDC :%d\n", __STDC__ );  
}  
Output:
File :simple.c
Date :AUG 10 2017
Time :11:10:40
Line :6
STDC :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.
    By the use of #include directive, we provide information to the preprocessor where to look for the header files. There are two variants to use #include directive.

  • #include <filename>
  • #include "filename"

    • #include <filename>

    tells the compiler to look for the directory where system header files are held. In UNIX, it is \user\include directory.

    #include "filename"

    tells the compiler to look in the current directory from where program is running

    #include directive example

    Let's see a simple example of #include directive. In this program, we are including stdio.h file because printf() function is defined in this file. 
    #include <stdio.h>  
main() {  
printf("Hello C program");  
 }  
Output:
Hello C program