strrstr

[Reuben Thomas]

On Fri, 2 May 2008, Carlos O'Donell wrote:

> On Wed, Apr 30, 2008 at 6:10 PM, Reuben Thomas <rrt@sc3d.org> wrote:
>>  It fills an obvious gap in the API (strchr, strrchr; strstr, strrstr), and
>> has the same semantics and name pretty much everywhere, so there's no
>> contention to resolve in what it should be called or do.
>
> I'm CC'ing libc-help in order to continue this dicussion.
>
> If you are really interested in adding reverse search variants to the
> API you will need to do a little leg work.

I'm only interested in adding this one function.

> 1. What other C libraries implement these functions?

The only one I can find is HP-UX's libc, as I said; the other 
implementations I found were in applications. (I presume by "C library" you 
mean "libc", not "library implemented in C".)

I don't have access to development resources for any proprietary operating 
systems at the moment, other than what I can find on the web, so I can't 
make an exhaustive survey.

> 2. How does relying on these functions effect portability between OSs?

I assume you mean "if an application programmer relies on the existence of 
this function, how does it affect portability between OSs?" Obviously, a 
program that assumes the existence of this function would not work on the 
many OSs that do not support it. Hopefully one day many more OSs will; 
indeed, if only two libc's were to adopt it, glibc and BSD's libc, then it 
within a few years a large proportion of programs would be able to rely on 
its existence on modern systems. However, for the forseeable future I'd 
envisage not relying on it, but, as I do at present in my programs that use 
it, testing for the existence of the function and supplying an 
implementation in ISO C in case it does not exist.

> 3. Are all the strrstr's the same?

So far, I've only looked at the name and prototypes.

> Do you have a test suite which shows they are the same?

No, I don't. I agree that writing a test suite is a good idea. I'll look at 
the glibc sources to see how this is done for other functions.

Since strrstr is to strstr as strrchr is to strchr, there's not much room 
for variation in spec.

> 4. Does your strrstr support all the new character types being proposed by 
> TR 19769 for the C standard?

Yes: variants for different character types can be obtained by replacing 
"char" in strrstr's definition by another fixed-width character type, and 
the call to strlen with a call to an appropriate function for the given 
type.


Here's a suggested definition, based on the info documentation for similar 
functions (specifically, strrchr and strstr):

  -- Function: char * strrstr (const char *HAYSTACK, const char *NEEDLE)
      The function `strrstr' is like `strstr', except that it searches
      backwards from the end of the string HAYSTACK (instead of forwards
      from the front).

      For example,
           strrstr ("hello, world", "l")
               => "ld"
           strrstr ("hello, world", "wo")
               => "world"

-- 
http://rrt.sc3d.org/ | partisan, n.  an adherent without sense (Bierce)