Missing optimization: mempcpy(3) vs memcpy(3)

Alejandro Colomar alx.manpages@gmail.com
Mon Dec 12 17:15:18 GMT 2022

Hi Jakub,

On 12/12/22 17:09, Jakub Jelinek wrote:
> On Mon, Dec 12, 2022 at 04:56:27PM +0100, Alejandro Colomar wrote:
>> "Names beginning with ‘str’, ‘mem’, or ‘wcs’ followed by a lowercase letter
>> are reserved for additional string and array functions. See String and Array
>> Utilities."
> It is not that simple.
> mem*, str* and wcs* are just potentially reserved identifiers, they are only
> reserved if the implementation provided them.

To clarify:
While ISO C up to C17 had them fully reserved, ISO C23 will make them 
potentially reserved identifiers.  POSIX further fully reserves them again 
(maybe next POSIX aligns with C23 on that; I don't know).

>  And what we discuss here
> is how to reliably find out if it was an implementation that provided them,
> because in case of gcc the implementation is GCC and the C library and
> perhaps some other libraries too.
> gcc can be used with lots of different C libraries, and many don't implement
> mempcpy.

Well, if GCC can't know what the implementation provides, then we're in big 
trouble.  Me, being just a user-space programmer, only know of _GNU_SOURCE for 
determining if the function is available at compile-time.  :)

Any of the POSIX or ISO C feature_test_macro(7)s prior to C23 should also be 
enough to tell the compiler that mem* identifiers are reserved, and therefore 
possibly provided by libc.

mempcpy(3)                 Library Functions Manual                 mempcpy(3)

        mempcpy, wmempcpy  - copy memory area

        Standard C library (libc, -lc)

        #define _GNU_SOURCE         /* See feature_test_macros(7) */
        #include <string.h>

        void *mempcpy(void dest[restrict .n], const void src[restrict .n],
                      size_t n);

        #define _GNU_SOURCE         /* See feature_test_macros(7) */
        #include <wchar.h>

        wchar_t *wmempcpy(wchar_t dest[restrict .n],
                      const wchar_t src[restrict .n],
                      size_t n);



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