[RFC/PATCH v2 3/5] libbb: add ends_with() function

[Tito]

On 08/24/2015 12:51 AM, Jody Bruchon wrote:
> On 2015-08-21 17:26, Tito wrote:
>> what sense would it make to check if str\0 ends_with \0 if all strings
>> do end with \0 ?
>
> In this case, I view this as a problem of handling all possible valid
> inputs in a robust manner. It can also make the code more efficient
> since \0 is just another byte and is guaranteed to be at the end of any
> valid string, thus solving the problem of what to do when passed an
> empty (but properly zero-terminated) string without having to add code
> for a special case and/or add parameters for string lengths.
>
> It does not matter that "ends with <empty>" is a comparison that almost
> certainly serves no practical purpose in the code calling it; what
> matters is that the behavior is consistent if the calling code is silly
> enough to pass empty strings and expect a meaningful response.

Hi Jody,
so you pass the buck to the silly caller to guess what is consistent, 
but even in the few replies on this thread there were different views 
about it.
In my opinion it would be better to cover such corner cases with
meaningful responses by analyzing the semantics of this new function
that checks if a _string_ ends with a specific _string_ or as somebody
suggested a specific _suffix_.
This suffix being non-existent is a anomaly that probably the caller
would like to be informed about.

> If a useless answer for empty strings is a problem, the writer of the calling
> code needs to handle that themselves, and is in a far more flexible
> position to do so than the person writing the called function that only
> answers "yes" or "no."

Why should a function give a useless answer?
Wouldn't it be better for the caller to give him
always useful answers (return different values, set errno, etc.).

> I also say that we're all C programmers if we're here fiddling with the
> guts of BusyBox and we can all easily understand why returning "yes" for
> "ends with \0?" is a technically correct result since all C strings end
> with \0.

This is a somewhat dogmatic argument:
"The people of the inner circle know it better."

SusV3 instead for the strstr function of which ends_with() could be
considered a subset of functionality (just looks for s2 at the end of 
s1) says:

"#include <string.h>

     char *strstr(const char *s1, const char *s2);

DESCRIPTION

     The strstr() function shall locate the first occurrence in the 
string pointed to by s1 of the sequence of bytes (excluding the 
terminating null byte) in the string pointed to by s2.

RETURN VALUE

     Upon successful completion, strstr() shall return a pointer to the 
located string or a null pointer if the string is not found.

     If s2 points to a string with zero length, the function shall 
return s1.

ERRORS

     No errors are defined."

So as you can see they explicitly request a specific action for the case
of a zero length key:

     "If s2 points to a string with zero length, the function shall 
return s1."

Not that I like their solution but still this is a fact and makes me
think that we should also handle this corner case in better way
rather than return a "useless value".

Ciao,
Tito

> Since there is no correct answer from a layman's perspective
> (or even a linguistic interpretation), the technically correct result is
> the only valid way I can see to resolve the stalemate and avoid making
> an arbitrary choice.
>
> -Jody
>