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Subject: bug in lrint [was: FW: Printing long int in C program under cygwin64]
To: cygwin AT cygwin DOT com
References: <6f28f46906804c6f8f6b4b861e202492 AT CASMBX02 DOT oslo DOT ngi DOT no>
From: Eric Blake <eblake AT redhat DOT com>
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Message-ID: <d252aaae-b298-6fc8-7e5b-8d8be9f27f21@redhat.com>
Date: Wed, 24 May 2017 07:33:27 -0500
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From: Eric Blake <eblake AT redhat DOT com>
To: cygwin AT cygwin DOT com
Message-ID: <d252aaae-b298-6fc8-7e5b-8d8be9f27f21 AT redhat DOT com>
Subject: bug in lrint [was: FW: Printing long int in C program under cygwin64]
References: <6f28f46906804c6f8f6b4b861e202492 AT CASMBX02 DOT oslo DOT ngi DOT no>
In-Reply-To: <6f28f46906804c6f8f6b4b861e202492 AT CASMBX02 DOT oslo DOT ngi DOT no>


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On 05/24/2017 07:00 AM, Carl Fredrik Forsberg wrote:
> I am experiencing problems printing long int values under cygwin64 instal=
led on a Windows 10 machine.
>=20
> Below is a test program followed by its output to demonstrate the problem=
. The program was initially written to demonstrate the output from lrint(),=
 and developed further to demonstrate to myself how negative integers are t=
ackled by printf type specifiers (e.g. %li, %ld etc).

Are you compiling with -Wall, or even -Wformat?

>=20
> My understanding is that lrint() should return a long int. However I am u=
nable to get printf to print the correct number. Instead its output is trea=
ted as an unsigned integer.
> Any  help or hints would be much appreciated.
>=20
> Regards
> Carl Fredrik
>=20
> #include <stdio.h>      /* printf */
> #include <math.h>       /* lrint */
>=20
> int main ()
> {
>   char text[64];
>   printf ( "int -2 =3D %i\n", -2 );
>   printf ( "int -1 =3D %i\n", -1 );
>   printf ( "int 0 =3D %i\n", 0 );
>   printf ( "int 1 =3D %i\n", 1 );

Okay so far.

>   printf ( "long int -2 =3D %li\n", -2 );
>   printf ( "long int -1 =3D %li\n", -1 );

Both buggy.  You are passing an int through varargs, but then telling
printf to grab a long int.  It may or may not work depending on ABI and
stack sizes and what not, but gcc will warn you that it is bogus.

>   printf ( "type cast -1 =3D %li\n", (long int)-1 );
>   printf ( "type cast lrint(-1.0) =3D %li\n", (long int)lrint(-1.0) );
>   printf ( "lrint(-1.0) =3D %li\n", lrint(-1.0) );
>   printf ( "lrint(1.0) =3D %li\n", lrint(1.0) );

Okay.

>   printf ( "long int 0 =3D %li\n", 0 );
>   printf ( "long int 1 =3D %li\n", 1 );
>   sprintf( text,"long int -1 =3D %li", -1 );

Buggy.

>   printf ( "Via sprintf: %s\n", text);

Okay (well, if you overlook the fact that text was populated in a buggy
manner)

>   printf ( "size of long int: %i\n", sizeof(long int));
>   printf ( "size of int: %i\n", sizeof(int));

Buggy.  size_t should be printed with %zi, not %i (since size_t and int
are not necessarily the same type).

>   return 0;
> }
>=20
>=20
> compiled by:
> gcc lrint_test.c -o lrint_test.exe

Missing -Wall.  Also, some platforms require the use of -lm to actually
link with lrint() (cygwin does not, though).

>=20
> Output:
>=20
> int -2 =3D -2
> int -1 =3D -1
> int 0 =3D 0
> int 1 =3D 1
> long int -2 =3D 4294967294
> long int -1 =3D 4294967295

Evidence of your bugs.

> type cast -1 =3D -1
> type cast lrint(-1.0) =3D 4294967295

Now that's an interesting one - it may be that cygwin1.dll actually has
buggy behavior in lrint().  In the source code, cygwin/math/lrint.c is
dropping down to assembly; it could very well be that the assembly code
is incorrectly truncating things at 32 bits (where it is just fine for
32-bit Cygwin, but wrong for 64-bit):

long lrint (double x)
{
  long retval =3D 0L;
#if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
defined(__i386__)
  __asm__ __volatile__ ("fistpl %0"  : "=3Dm" (retval) : "t" (x) : "st");
#elif defined(__arm__) || defined(_ARM_)
    retval =3D __lrint_internal(x);
#endif
  return retval;
}

But I'm not an assembly coding expert, so perhaps someone else will spot
the fix faster.

> The confidentiality or integrity of this message can not be guaranteed fo=
llowing transmission on the Internet.

Not the worst disclaimer (it is at least not stating something that is
unenforceable), but we do prefer that messages on this list be sent
without company legalese (even if that means sending from a personal
address instead).

--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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