The Fn strtok function is used to isolate sequential tokens in a null-terminated string, Fa str . These tokens are separated in the string by at least one of the characters in Fa sep . The first time that Fn strtok is called, Fa str should be specified; subsequent calls, wishing to obtain further tokens from the same string, should pass a null pointer instead. The separator string, Fa sep , must be supplied each time, and may change between calls.
The Fn strtok function returns a pointer to the beginning of each subsequent token in the string, after replacing the separator character itself with a NUL character. When no more tokens remain, a null pointer is returned.
The System V Fn strtok , if handed a string containing only delimiter characters, will not alter the next starting point, so that a call to Fn strtok with a different (or empty) delimiter string may return a non- NULL value. Since this implementation always alters the next starting point, such a sequence of calls would always return NULL