If *stringp is NULL, the strsep() function returns NULL and does nothing else. Otherwise, this function finds the first token in the string *stringp, where tokens are delimited by symbols in the string delim. This token is terminated with a \0 character (by overwriting the delimiter) and *stringp is updated to point past the token. In case no delimiter was found, the token is taken to be the entire string *stringp, and *stringp is made NULL.
The strsep() function returns a pointer to the token, that is, it returns the original value of *stringp.
The strsep() function was introduced as a replacement for strtok(), since the latter cannot handle empty fields. However, strtok() conforms to ANSI-C and hence is more portable.
This function suffers from the same problems as strtok(). In particular, it modifies the original string. Avoid it.