# 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

The following headers define NULL.

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

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