From: Rich Felker <dalias@libc.org>
To: Peter Ammon <corydoras@ridiculousfish.com>
Cc: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] Fix printf hex float formatting with precision
Date: Fri, 12 Apr 2024 19:33:11 -0400 [thread overview]
Message-ID: <20240412233310.GF4163@brightrain.aerifal.cx> (raw)
In-Reply-To: <20240412195728.GE4163@brightrain.aerifal.cx>
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On Fri, Apr 12, 2024 at 03:57:28PM -0400, Rich Felker wrote:
> On Thu, Apr 11, 2024 at 06:17:25PM -0700, Peter Ammon wrote:
> > if (p>=0 && p<(LDBL_MANT_DIG-1+3)/4) {
> > int re = LDBL_MANT_DIG-1-(p*4);
> > long double round = 1ULL<<re;
>
> This expression overflows. re can be as large as 108 but 1ULL has
> 64-bit type.
>
> There's probably some reasonable way to write it that works for
> reasonable sizes of long double (I think it's safe to assume IEEE quad
> is the max that will ever be supported), but I presume I wrote it with
> the original inefficient loop to be fully general to arbitrary
> precision.
>
> It could just be written to use scalbn, I think. At the time I
> probably was trying to avoid dependency on a libm that could have been
> separate from libc (bad historical thing). IIRC there was a time when
> frexp was not used either.
Does the attached look ok?
Rich
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diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
index 497c5e19..dc648e7e 100644
--- a/src/stdio/vfprintf.c
+++ b/src/stdio/vfprintf.c
@@ -211,18 +211,11 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
if (y) e2--;
if ((t|32)=='a') {
- long double round = 8.0;
- int re;
-
if (t&32) prefix += 9;
pl += 2;
- if (p<0 || p>=LDBL_MANT_DIG/4-1) re=0;
- else re=LDBL_MANT_DIG/4-1-p;
-
- if (re) {
- round *= 1<<(LDBL_MANT_DIG%4);
- while (re--) round*=16;
+ if (p>=0 && p<(LDBL_MANT_DIG-1+3)/4) {
+ double round = scalbn(1, LDBL_MANT_DIG-1-(p*4));
if (*prefix=='-') {
y=-y;
y-=round;
next prev parent reply other threads:[~2024-04-12 23:33 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-12 1:17 Peter Ammon
2024-04-12 2:34 ` Rich Felker
2024-04-12 19:57 ` Rich Felker
2024-04-12 23:33 ` Rich Felker [this message]
2024-04-13 16:00 ` Peter Ammon
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