A lot of functions takes pointers to data and returns pointers. It would be good to mark 'const' pointers if a function does not change the data or a caller should not change the returned data.

Why:

1. It will be a good hint for a user - a person can easily see if a function changes his/her data or function just reads them

2. A compiler can be a real helper and warn a user if the user tries to change the data(returned by a function) that should be changed at all

A good example for #2 is function 'hdutypestring'. It returns 'char*' but really it returns a pointer to read-only data, so a client code will likely to crash if, e.g, the client code tries to change the returned value in place. A compiler warns the user in this case about casting a 'const char*' to 'char *'.