[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20250701142223.GA1827@brightrain.aerifal.cx>
Date: Tue, 1 Jul 2025 10:22:25 -0400
From: Rich Felker <dalias@...c.org>
To: Michal Terepeta <michalt@...gle.com>
Cc: musl@...ts.openwall.com, t5y-external <t5y-external@...gle.com>
Subject: Re: Wrong formatting of doubles?
On Tue, Jul 01, 2025 at 07:55:20AM +0200, Michal Terepeta wrote:
> Hi,
>
> We're working on a RISC-V system and using musl as our libc. A recent change
> in musl seems to have caused wrong formatting (printf) of double values:
> https://git.musl-libc.org/cgit/musl/commit/src/stdio/vfprintf.c?id=572a2e2eb91f00f2f25d301cfb50f435e7ae16b3
>
> Using a simple binary to print max double [1] with the current code I get:
>
> ```
> The maximum value of a double (printf %e): 1.348676e+308
> The maximum value of a double (printf %E): 1.348676E+308
> The maximum value of a double (printf %g): 3.13486e+308
> ```
>
> With the patch [2] that reverts most of the above change, I get the expected
> output:
>
> ```
> The maximum value of a double (printf %e): 1.797693e+308
> The maximum value of a double (printf %E): 1.797693E+308
> The maximum value of a double (printf %g): 1.79769e+308
> ```
>
> It'd be great if someone could take a look if they can also repro and perhaps
> revert the change?
>
> Many thanks!
>
> Michal
>
>
>
> [1] Repro program:
>
> ```
> #include <float.h>
> #include <stdio.h>
>
> int main() {
> printf("The maximum value of a double (printf %%e): %e\n", DBL_MAX);
> printf("The maximum value of a double (printf %%E): %E\n", DBL_MAX);
> printf("The maximum value of a double (printf %%g): %g\n", DBL_MAX);
> return 0;
> }
>
> ```
>
>
> [2] Patch to test:
>
> ```
> diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
> index 010041ca6c..01004158bf 100644
> --- a/src/stdio/vfprintf.c
> +++ b/src/stdio/vfprintf.c
> @@ -180,12 +180,8 @@
>
> static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t, int ps)
> {
> - int bufsize = (ps==BIGLPRE)
> - ? (LDBL_MANT_DIG+28)/29 + 1 + // mantissa expansion
> - (LDBL_MAX_EXP+LDBL_MANT_DIG+28+8)/9 // exponent expansion
> - : (DBL_MANT_DIG+28)/29 + 1 +
> - (DBL_MAX_EXP+DBL_MANT_DIG+28+8)/9;
> - uint32_t big[bufsize];
> + uint32_t big[(LDBL_MANT_DIG+28)/29 + 1 // mantissa expansion
> + + (LDBL_MAX_EXP+LDBL_MANT_DIG+28+8)/9]; // exponent expansion
> uint32_t *a, *d, *r, *z;
> int e2=0, e, i, j, l;
> char buf[9+LDBL_MANT_DIG/4], *s;
> ```
Could you try the attached patch?
View attachment "printf_dbl_bug.diff" of type "text/plain" (1208 bytes)
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
