is strlen()'s read-4-bytes-ahead a standard?

[Giorgos Keramidas]

On Fri, 16 Jul 2010 09:41:23 +0200, "deeptech71 at gmail.com" <deeptech71 at gmail.com> wrote:
>Xin LI wrote:
>>On 2010/07/15 15:38, deeptech71 at gmail.com wrote:
>>> Some C implementations use the read-4-bytes-ahead technique to speed
>>> up strlen(). Does the C standard state anything about strlen() being
>>> allowed to read past the terminating zero?
>>
>> It's not 4-bytes-ahead, but read a whole (aligned) word at one time.
>>
>> I think C standard does not dictate in this detail.
>
> OK, can anyone confirm this?

The only text about strlen()'s behavior in my copy of the ISO/IEC
9899:1999 (E) standard is:

  |     7.21.6.3 The strlen function
  |
  |     Synopsis
  |
  | 1   #include <string.h>
  |     size_t strlen(const char *s);
  |
  |     Description
  |
  | 2   The strlen function computes the length of the string pointed
  |     to by s.
  |
  |     Returns
  |
  | 3   The strlen function returns the number of characters that
  |     precede the terminating null character.

There is no reference to *how* this may be implemented, so you can
safely assume that the usual "as if" rules of the C standard apply.
The underlying code may read a single character at a time, a word at a
time, or may even call system-specific kernel calls that do 'magic'
behind the scenes to compute the string length.

What _really_ matters, as far as the standard is concerned, is that
you get a meaningful return value that matches the current length of
the input string.