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Date: Fri, 26 May 2023 16:33:59 -0400
From: Rich Felker <dalias@...c.org>
To: Jens Gustedt <Jens.Gustedt@...ia.fr>
Cc: musl@...ts.openwall.com
Subject: Re: [C23 printf 3/3] C23: implement the wfN length modifiers
for printf
On Fri, May 26, 2023 at 09:41:04PM +0200, Jens Gustedt wrote:
> Musl only has a difference between fixed-width and fastest-width
> integer types for N == 16. And even here all architectures have made
> the same choice, namely mapping to 32 bit types.
> ---
> src/stdio/vfprintf.c | 9 ++++++++-
> src/stdio/vfwprintf.c | 9 ++++++++-
> 2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
> index 1a516663..9f02a594 100644
> --- a/src/stdio/vfprintf.c
> +++ b/src/stdio/vfprintf.c
> @@ -529,9 +529,16 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
> ps=st;
> st=states[st]S(*s++);
> if (st == WPRE) {
> + // See if "fast" is requested. Difference is only
> + // relevant for a fast type of minimum width 16.
> + int fast16 = HPRE;
> + if (*s == 'f') {
> + fast16 = BARE;
> + ++s;
> + }
> switch (getint(&s)) {
> case 8: st = HHPRE; goto wpre;
> - case 16: st = HPRE; goto wpre;
> + case 16: st = fast16; goto wpre;
> case 32: st = BARE; goto wpre;
> #if UINTPTR_MAX >= UINT64_MAX
> case 64: st = LPRE; goto wpre;
> diff --git a/src/stdio/vfwprintf.c b/src/stdio/vfwprintf.c
> index 4320761a..4e9ce63a 100644
> --- a/src/stdio/vfwprintf.c
> +++ b/src/stdio/vfwprintf.c
> @@ -244,9 +244,16 @@ static int wprintf_core(FILE *f, const wchar_t *fmt, va_list *ap, union arg *nl_
> ps=st;
> st=states[st]S(*s++);
> if (st == WPRE) {
> + // See if "fast" is requested. Difference is only
> + // relevant for a fast type of minimum width 16.
> + int fast16 = HPRE;
> + if (*s == 'f') {
> + fast16 = BARE;
> + ++s;
> + }
> switch (getint(&s)) {
> case 8: st = HHPRE; goto wpre;
> - case 16: st = HPRE; goto wpre;
> + case 16: st = fast16; goto wpre;
> case 32: st = BARE; goto wpre;
> #if UINTPTR_MAX >= UINT64_MAX
> case 64: st = LPRE; goto wpre;
> --
> 2.34.1
It may just work to have w8, w16, wf8, and wf16 all resolve to BARE
unconditionally, as the default promitions for variadic functions
force these all to be passed as int. In the current code, for h and hh
prefixes we cast down and discard any high bits, but unless the caller
invoked UB by passing an argument with mismatched type, the cast is
guaranteed not to change the value.
Rich
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