NULL

NULL is used in several ways.

• As a pointer to address zero. NULL is defined in several ANSI headers as the symbolic constant (void *)0. Zero is often returned from functions that normaly return a pointer to signal an error. It is therefore conveniant to compare a function return code with NULL to catch errors.

  	if ((fp=fopen("/etc/hosts","r") == NULL)
          {
            exit(0);
          }
  

• To mark the end of a character string. A null character is used to terminate the string. For example if you coded:

  	char * text="abc";
  

  You will actually reserve FOUR bytes containing in ASCII hex.

  	61 62 63 00
  	 a  b  c \0
  

  The null at the end can be coded by using the escape sequence '\0'.

\0 is actually an octal escape sequence, strictly speeking it should be written as \000

ANSI headers

• locale.h
• stddef.h
• stdio.h
• stdlib.h
• string.h
• string.h
• time.h

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  Martin Leslie