#include <strings.h>char *index(const char *s, int c);
The index() and rindex() functions operate on null-terminated strings.
The index() function returns a pointer to the first occurrence of character c in string s .
The rindex() function returns a pointer to the last occurrence of character c in string s .
Both index() and rindex() return a null pointer if c does not occur in the string. The null character terminating a string is considered to be part of the string.
On most modern computer systems, you can not use a null pointer to indicate a null string. A null pointer is an error and results in an abort of the program. If you wish to indicate a null string, you must use a pointer that points to an explicit null string. On some machines and with some implementations of the C programming language, a null pointer, if dereferenced, would yield a null string. Though often used, this practice is not always portable. Programmers using a null pointer to represent an empty string should be aware of this portability issue. Even on machines where dereferencing a null pointer does not cause an abort of the program, it does not necessarily yield a null string.