From: Alejandro Colomar <alx.manpages@gmail.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: "Jonathan Wakely" <jwakely.gcc@gmail.com>,
"Martin Liška" <mliska@suse.cz>,
gcc@gcc.gnu.org, "GNU C Library" <libc-alpha@sourceware.org>
Subject: Re: Missing optimization: mempcpy(3) vs memcpy(3)
Date: Mon, 12 Dec 2022 18:15:18 +0100 [thread overview]
Message-ID: <00d9ee65-9d75-bc13-93a4-e16e80b31962@gmail.com> (raw)
In-Reply-To: <Y5dSV6oL7Uipp1QG@tucnak>
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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)
NAME
mempcpy, wmempcpy - copy memory area
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
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next prev parent reply other threads:[~2022-12-12 17:15 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-09 17:11 Alejandro Colomar
2022-12-12 13:37 ` Martin Liška
2022-12-12 13:44 ` Alejandro Colomar
2022-12-12 13:56 ` Jakub Jelinek
2022-12-12 14:05 ` Alejandro Colomar
2022-12-12 14:48 ` Jonathan Wakely
2022-12-12 14:53 ` Jakub Jelinek
2022-12-12 15:56 ` Alejandro Colomar
2022-12-12 16:09 ` Jakub Jelinek
2022-12-12 17:15 ` Alejandro Colomar [this message]
2022-12-12 17:42 ` Jonathan Wakely
2022-12-12 14:34 Wilco Dijkstra
2022-12-12 14:57 ` Cristian Rodríguez
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