dirname and basename break a null-terminated pathname string into directory and filename components. In the usual case, dirname returns the string up to, but not including, the final '/', and basename returns the component following the final '/'. Trailing '/' characters are not counted as part of the pathname.
If path does not contain a slash, dirname returns the string basename returns a copy of path. If path is the string dirname and basename return the string path is a NULL pointer or points to an empty string, then both dirname and basename return the string __
Concatenating the string returned by dirname, a basename yields a complete pathname.
Both dirname and basename may modify the contents of path, so if you need to preserve the pathname string, copies should be passed to these functions. Furthermore, dirname and basename may return pointers to statically allocated memory which may overwritten by subsequent calls.
The following list of examples (taken from SUSv2) shows the strings returned by dirname and basename for different paths: