Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date: Wed, 25 Sep 2019 13:11:03 -0500
From: "Anastasios" <>
To: "Rich Felker" <>
Cc: "" <>
Subject: Re: Bug report: strtod drops LSB

For the record, this is the program I used to confirm that it's a duplicate:

    #include <stdlib.h>
    #include <stdio.h>

    void main() {
        unsigned short control_word = 0x37f;
        asm ("fldcw %0" : : "m" (control_word));
        printf("%lf\n", strtod("283686952306183", NULL));

Perhaps it can help if anyone on WSL reports this again.
On 9/25/2019 12:56:35 PM, Anastasios <> wrote:
Having looked into it more, I found this is due to being on WSL, and it is a duplicate of [], which, I see, you are already well aware of. Please excuse the noise.
On 9/25/2019 10:51:53 AM, Rich Felker <> wrote:
On Wed, Sep 25, 2019 at 09:32:35AM -0500, Anastasios wrote:
> Hello,
> Consider this program, strtod.c:
>     #include
>     #include
>     int main()
>     {
>         printf("%lf\n", strtod("283686952306183", NULL));
>     }
> With current musl master from Git:
>     $ musl-gcc -static strtod.c -o a.musl
>     $ ./a.musl
>     283686952306176.000000
> By comparison, with glibc:
>     $ gcc -static strtod.c -o a.glibc
>     $ ./a.glibc
>     283686952306183.000000
> The correct binary representation of this float is
>     0x42f0203040506070
> but musl strtod produces
>     0x42f0203040506000
> i.e., it fails to set the LSB. I examined this while ruling out printf as the cause.

I can't reproduce this. My test program for strtod shows, for the
input "283686952306183":

d: 283686952306183 [0x1.020304050607p+48] [42f0203040506070]

I suspect you miscompiled musl, possibly by passing in CFLAGS (perhaps
from defaults in your environment?) that break floating point
semantics. We test for and refuse to build if __FAST_MATH__ is
defined, but GCC only defines it if you use -ffast-math, not if you
manually enable one or more of the individual broken options that
-ffast-math enables.

Alternatively, it's possible that you have a broken compiler version
that miscompiles floating point code.


Content of type "text/html" skipped

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.