Re: [PATCH v5 2/5] stpecpy.3, stpecpyx.3, ustpcpy.3, ustr2stp.3, zustr2stp.3, zustr2ustp.3: Add new links to string_copy(7)

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 12/16/22 20:03, Alejandro Colomar wrote:
> On 12/16/22 19:47, Stefan Puiu wrote:
>> On Thu, Dec 15, 2022 at 2:46 AM Alejandro Colomar
>> <alx.manpages@gmail.com> wrote:
>>>          An implementation of these functions might be:
>>>
>>>              char *
>>>              stpcpy(char *restrict dst, const char *restrict src)
>>>              {
>>>                  char  *end;
>>>
>>>                  end = mempcpy(dst, src, strlen(src));
>>>                  *end = '\0';
>>>
>>>                  return end;
>>>              }
>>>
>>>              char *
>>>              strcpy(char *restrict dst, const char *restrict src)
>>>              {
>>>                  stpcpy(dst, src);
>>>                  return dst;
>>>              }
>>>
>>>              char *
>>>              strcat(char *restrict dst, const char *restrict src)
>>>              {
>>>                  stpcpy(dst + strlen(dst), src);
>>>                  return dst;
>>>              }
>>
>> Are you sure this section adds any value? I think good documentation
>> should explain how a function works without delving into the
>> interpretation.
> 
> To be honest, this page doesn't benefit too much from it.  strcpy(3)/strcat(3) 
> are dead simple, and the explanations above should be enough.
> 
> However, the same thing in strncpy(3) and strncat(3) is very helpful, IMO.  For 
> consistency I just showed trivial implementations in all of the pages.  (And in 
> fact, there was an example implementation in the old strncat(3) and maybe a few 
> others, IIRC.)
> 
>> Also, people might get confused and think this is the
>> actual implementation.
> 
> I don't think there's any problem if one believes this is the implementation. 
> Except for stpcpy(3), in which I preferred readability, they are actually quite 
> good implementations.  A faster implementation of stpcpy(3) might be done in 
> terms of memccpy(3).
> 
> Funnily enough, I just checked what musl libc does, and it's the same as shown 
> here:
> 
> 
> alx@debian:~/src/musl/musl$ grepc -tfd strcpy
> ./src/string/strcpy.c:3:
> char *strcpy(char *restrict dest, const char *restrict src)
> {
>      __stpcpy(dest, src);
>      return dest;
> }
> alx@debian:~/src/musl/musl$ grepc -tfd strcat
> ./src/string/strcat.c:3:
> char *strcat(char *restrict dest, const char *restrict src)
> {
>      strcpy(dest + strlen(dest), src);
>      return dest;
> }
> 
> 

And considering memccpy(3) is defined in terms of memchr(3) and mempcpy(3) in 
glibc, I don't feel so bad about my own stpcpy(3) :).  See:


alx@debian:~/src/gnu/glibc$ grepc -tfd __memccpy
./string/memccpy.c:30:
void *
__memccpy (void *dest, const void *src, int c, size_t n)
{
   void *p = memchr (src, c, n);

   if (p != NULL)
     return __mempcpy (dest, src, p - src + 1);

   memcpy (dest, src, n);
   return NULL;
}


Cheers,

Alex


>>
>>>
>>> RETURN VALUE
>>>          stpcpy()
>>>                 This  function returns a pointer to the terminating null byte at
>>>                 the end of the copied string.
>>>
>>>          strcpy()
>>>          strcat()
>>>                 These functions return dest.
>>>
>>> ATTRIBUTES
>>>          For an explanation of the terms  used  in  this  section,  see  attrib‐
>>>          utes(7).
>>>          
>>> ┌────────────────────────────────────────────┬───────────────┬─────────┐
>>>          │Interface                                   │ Attribute     │ 
>>> Value   │
>>>          
>>> ├────────────────────────────────────────────┼───────────────┼─────────┤
>>>          │stpcpy(), strcpy(), strcat()                │ Thread safety │ 
>>> MT‐Safe │
>>>          
>>> └────────────────────────────────────────────┴───────────────┴─────────┘
>>>
>>> STANDARDS
>>>          stpcpy()
>>>                 POSIX.1‐2008.
>>>
>>>          strcpy()
>>>          strcat()
>>>                 POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.
>>>
>>> CAVEATS
>>>          The strings src and dst may not overlap.
>>>
>>>          If  the  destination  buffer is not large enough, the behavior is unde‐
>>>          fined.  See _FORTIFY_SOURCE in feature_test_macros(7).
>>>
>>> BUGS
>>>          strcat()
>>>                 This function can be  very  inefficient.   Read  about  Shlemiel
>>>                 the      painter     ⟨https://www.joelonsoftware.com/2001/12/11/
>>>                 back-to-basics/⟩.
>>
>> I'm not sure this is a bug, rather a design limitation. Maybe it
>> belongs in NOTES or CAVEATS?
> 
> Yeah, I had been thinking of downgrading it.  I'll do it.
> 
>> Also, I think this can be summarized
>> along the lines of 'strcat needs to walk the destination buffer to
>> find the null terminator, so it has linear complexity with respect to
>> the size of the destination buffer up to the terminator' (hmm, I'm
>> sure this can be expressed more concisely), so the page is more self
>> contained. Outside links sometimes go dead, like on Wikipedia, so I
>> think just in case, it helps to make explicit the point that you want
>> the reader to study further in the URL.
> 
> I wasn't inspired to write it short enough to not be too verbose.  Maybe I'll 
> write something based on your suggestion.
> 
>>
>> Regards,
>> Stefan.
> 
> Thanks for the review!
> 
> Cheers,
> 
> Alex
> 
>>
>>>
>>> EXAMPLES
>>>          #include <stdio.h>
>>>          #include <stdlib.h>
>>>          #include <string.h>
>>>
>>>          int
>>>          main(void)
>>>          {
>>>              char    *p;
>>>              char    buf1[BUFSIZ];
>>>              char    buf2[BUFSIZ];
>>>              size_t  len;
>>>
>>>              p = buf1;
>>>              p = stpcpy(p, "Hello ");
>>>              p = stpcpy(p, "world");
>>>              p = stpcpy(p, "!");
>>>              len = p - buf1;
>>>
>>>              printf("[len = %zu]: ", len);
>>>              puts(buf1);  // "Hello world!"
>>>
>>>              strcpy(buf2, "Hello ");
>>>              strcat(buf2, "world");
>>>              strcat(buf2, "!");
>>>              len = strlen(buf2);
>>>
>>>              printf("[len = %zu]: ", len);
>>>              puts(buf2);  // "Hello world!"
>>>
>>>              exit(EXIT_SUCCESS);
>>>          }
>>>
>>> SEE ALSO
>>>          strdup(3), string(3), wcscpy(3), string_copy(7)
>>>
>>> Linux man‐pages (unreleased)        (date)                           strcpy(3)
>>>
>>> -- 
>>> <http://www.alejandro-colomar.es/>
> 

-- 
<http://www.alejandro-colomar.es/>

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