FW: Printing long int in C program under cygwin64

FW: Printing long int in C program under cygwin64

[Carl Fredrik Forsberg]

I am experiencing problems printing long int values under cygwin64 installed on a Windows 10 machine.

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 tackled by printf type specifiers (e.g. %li, %ld etc).

My understanding is that lrint() should return a long int. However I am unable to get printf to print the correct number. Instead its output is treated as an unsigned integer.
Any  help or hints would be much appreciated.

Regards
Carl Fredrik

#include <stdio.h>      /* printf */
#include <math.h>       /* lrint */

int main ()
{
  char text[64];
  printf ( "int -2 = %i\n", -2 );
  printf ( "int -1 = %i\n", -1 );
  printf ( "int 0 = %i\n", 0 );
  printf ( "int 1 = %i\n", 1 );
  printf ( "long int -2 = %li\n", -2 );
  printf ( "long int -1 = %li\n", -1 );
  printf ( "type cast -1 = %li\n", (long int)-1 );
  printf ( "type cast lrint(-1.0) = %li\n", (long int)lrint(-1.0) );
  printf ( "lrint(-1.0) = %li\n", lrint(-1.0) );
  printf ( "lrint(1.0) = %li\n", lrint(1.0) );
  printf ( "long int 0 = %li\n", 0 );
  printf ( "long int 1 = %li\n", 1 );
  sprintf( text,"long int -1 = %li", -1 );
  printf ( "Via sprintf: %s\n", text);
  printf ( "size of long int: %i\n", sizeof(long int));
  printf ( "size of int: %i\n", sizeof(int));
  return 0;
}


compiled by:
gcc lrint_test.c -o lrint_test.exe

Output:

int -2 = -2
int -1 = -1
int 0 = 0
int 1 = 1
long int -2 = 4294967294
long int -1 = 4294967295
type cast -1 = -1
type cast lrint(-1.0) = 4294967295
lrint(-1.0) = 4294967295
lrint(1.0) = 1
long int 0 = 0
long int 1 = 1
Via sprintf: long int -1 = 4294967295
size of long int: 8
size of int: 4



gcc version:
gcc (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.









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bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Eric Blake]

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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 installed on a Windows 10 machine.
> 
> 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 tackled by printf type specifiers (e.g. %li, %ld etc).

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

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

Okay so far.

>   printf ( "long int -2 = %li\n", -2 );
>   printf ( "long int -1 = %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 = %li\n", (long int)-1 );
>   printf ( "type cast lrint(-1.0) = %li\n", (long int)lrint(-1.0) );
>   printf ( "lrint(-1.0) = %li\n", lrint(-1.0) );
>   printf ( "lrint(1.0) = %li\n", lrint(1.0) );

Okay.

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

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

Evidence of your bugs.

> type cast -1 = -1
> type cast lrint(-1.0) = 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 = 0L;
#if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
defined(__i386__)
  __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");
#elif defined(__arm__) || defined(_ARM_)
    retval = __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 following 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).

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


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Erik Bray]

On Wed, May 24, 2017 at 2:33 PM, Eric Blake wrote:
> On 05/24/2017 07:00 AM, Carl Fredrik Forsberg wrote:
>> I am experiencing problems printing long int values under cygwin64 installed on a Windows 10 machine.
>>
>> 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 tackled by printf type specifiers (e.g. %li, %ld etc).
>
> Are you compiling with -Wall, or even -Wformat?
>
>>
>> My understanding is that lrint() should return a long int. However I am unable to get printf to print the correct number. Instead its output is treated as an unsigned integer.
>> Any  help or hints would be much appreciated.
>>
>> Regards
>> Carl Fredrik
>>
>> #include <stdio.h>      /* printf */
>> #include <math.h>       /* lrint */
>>
>> int main ()
>> {
>>   char text[64];
>>   printf ( "int -2 = %i\n", -2 );
>>   printf ( "int -1 = %i\n", -1 );
>>   printf ( "int 0 = %i\n", 0 );
>>   printf ( "int 1 = %i\n", 1 );
>
> Okay so far.
>
>>   printf ( "long int -2 = %li\n", -2 );
>>   printf ( "long int -1 = %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 = %li\n", (long int)-1 );
>>   printf ( "type cast lrint(-1.0) = %li\n", (long int)lrint(-1.0) );
>>   printf ( "lrint(-1.0) = %li\n", lrint(-1.0) );
>>   printf ( "lrint(1.0) = %li\n", lrint(1.0) );
>
> Okay.
>
>>   printf ( "long int 0 = %li\n", 0 );
>>   printf ( "long int 1 = %li\n", 1 );
>>   sprintf( text,"long int -1 = %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;
>> }
>>
>>
>> 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).
>
>>
>> Output:
>>
>> int -2 = -2
>> int -1 = -1
>> int 0 = 0
>> int 1 = 1
>> long int -2 = 4294967294
>> long int -1 = 4294967295
>
> Evidence of your bugs.
>
>> type cast -1 = -1
>> type cast lrint(-1.0) = 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 = 0L;
> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
> defined(__i386__)
>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");
> #elif defined(__arm__) || defined(_ARM_)
>     retval = __lrint_internal(x);
> #endif
>   return retval;
> }
>
> But I'm not an assembly coding expert, so perhaps someone else will spot
> the fix faster.

I took a look at this.  The code in Cygwin looks fine, I think.  But
the assembly is wrong:

000000018015cd40 <lrint>:
   18015cd40:   48 83 ec 28             sub    $0x28,%rsp
   18015cd44:   f2 0f 11 44 24 08       movsd  %xmm0,0x8(%rsp)
   18015cd4a:   dd 44 24 08             fldl   0x8(%rsp)
   18015cd4e:   db 5c 24 18             fistpl 0x18(%rsp)
   18015cd52:   48 8b 44 24 18          mov    0x18(%rsp),%rax
   18015cd57:   48 83 c4 28             add    $0x28,%rsp
   18015cd5b:   c3                      retq

That last `mov` should be a `movq`.  The result in $rsp+0x18 is
correct (I checked in gdb), but the `mov` truncates it.  Not sure if
that's a gcc bug or what.

Best,
Erik

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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Erik Bray]

On Wed, May 24, 2017 at 5:55 PM, Erik Bray <erik.m.bray@gmail.com> wrote:
> On Wed, May 24, 2017 at 2:33 PM, Eric Blake wrote:
>> On 05/24/2017 07:00 AM, Carl Fredrik Forsberg wrote:
>>> I am experiencing problems printing long int values under cygwin64 installed on a Windows 10 machine.
>>>
>>> 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 tackled by printf type specifiers (e.g. %li, %ld etc).
>>
>> Are you compiling with -Wall, or even -Wformat?
>>
>>>
>>> My understanding is that lrint() should return a long int. However I am unable to get printf to print the correct number. Instead its output is treated as an unsigned integer.
>>> Any  help or hints would be much appreciated.
>>>
>>> Regards
>>> Carl Fredrik
>>>
>>> #include <stdio.h>      /* printf */
>>> #include <math.h>       /* lrint */
>>>
>>> int main ()
>>> {
>>>   char text[64];
>>>   printf ( "int -2 = %i\n", -2 );
>>>   printf ( "int -1 = %i\n", -1 );
>>>   printf ( "int 0 = %i\n", 0 );
>>>   printf ( "int 1 = %i\n", 1 );
>>
>> Okay so far.
>>
>>>   printf ( "long int -2 = %li\n", -2 );
>>>   printf ( "long int -1 = %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 = %li\n", (long int)-1 );
>>>   printf ( "type cast lrint(-1.0) = %li\n", (long int)lrint(-1.0) );
>>>   printf ( "lrint(-1.0) = %li\n", lrint(-1.0) );
>>>   printf ( "lrint(1.0) = %li\n", lrint(1.0) );
>>
>> Okay.
>>
>>>   printf ( "long int 0 = %li\n", 0 );
>>>   printf ( "long int 1 = %li\n", 1 );
>>>   sprintf( text,"long int -1 = %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;
>>> }
>>>
>>>
>>> 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).
>>
>>>
>>> Output:
>>>
>>> int -2 = -2
>>> int -1 = -1
>>> int 0 = 0
>>> int 1 = 1
>>> long int -2 = 4294967294
>>> long int -1 = 4294967295
>>
>> Evidence of your bugs.
>>
>>> type cast -1 = -1
>>> type cast lrint(-1.0) = 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 = 0L;
>> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
>> defined(__i386__)
>>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");
>> #elif defined(__arm__) || defined(_ARM_)
>>     retval = __lrint_internal(x);
>> #endif
>>   return retval;
>> }
>>
>> But I'm not an assembly coding expert, so perhaps someone else will spot
>> the fix faster.
>
> I took a look at this.  The code in Cygwin looks fine, I think.  But
> the assembly is wrong:
>
> 000000018015cd40 <lrint>:
>    18015cd40:   48 83 ec 28             sub    $0x28,%rsp
>    18015cd44:   f2 0f 11 44 24 08       movsd  %xmm0,0x8(%rsp)
>    18015cd4a:   dd 44 24 08             fldl   0x8(%rsp)
>    18015cd4e:   db 5c 24 18             fistpl 0x18(%rsp)
>    18015cd52:   48 8b 44 24 18          mov    0x18(%rsp),%rax
>    18015cd57:   48 83 c4 28             add    $0x28,%rsp
>    18015cd5b:   c3                      retq
>
> That last `mov` should be a `movq`.  The result in $rsp+0x18 is
> correct (I checked in gdb), but the `mov` truncates it.  Not sure if
> that's a gcc bug or what.

Actually, I take it back.  gdb/objdump (and presumably binutils in
general) was being deceptive to me about the nature of that mov
instruction.  And in fact the fistpl should be fistpq.  That fixes it.

Erik

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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Eric Blake]

[-- Attachment #1.1: Type: text/plain, Size: 995 bytes --]

On 05/24/2017 11:53 AM, Erik Bray wrote:

>>> 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 = 0L;
>>> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
>>> defined(__i386__)
>>>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");

> Actually, I take it back.  gdb/objdump (and presumably binutils in
> general) was being deceptive to me about the nature of that mov
> instruction.  And in fact the fistpl should be fistpq.  That fixes it.

Is fistpq right on 32-bit, or is this a case where we need different
assembly for 32-bit (fistpl) vs. 64-bit (fistql) to match the size of
long that we are converting to?

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


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RE: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Carl Fredrik Forsberg]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="utf-8", Size: 2936 bytes --]

Further to my first question, I also tried llrint() as below. It gives the same output as lrint(). This may not be a big surprise as long long int and long int have the same length.

#include <stdio.h>      /* printf */
#include <math.h>       /* lrint, llrint, rint */

int main ()
{
  char text[64];
  printf ( "type cast -1 = %li\n", (long int)-1 );
  printf ( "type cast lrint(-1.0) = %li\n", (long int)lrint(-1.0) );
  printf ( "rint(-1.0) = %f\n", rint(-1.0) );
  printf ( "lrint(-1.0) = %li\n", lrint(-1.0) );
  printf ( "llrint(-1.0) = %lli\n", llrint(-1.0) );
  printf ( "lrint(1.0) = %li\n", lrint(1.0) );
  printf ( "llrint(1.0) = %lli\n", llrint(1.0) );
  sprintf( text,"int -1 = %i", -1 );
  printf ( "Via sprintf: %s\n", text);
  printf ( "size of long long int: %zi\n", sizeof(long long int));
  printf ( "size of long int: %zi\n", sizeof(long int));
  printf ( "size of int: %zi\n", sizeof(int));
  return 0;

compiled by
gcc -Wall lrint_test.c -o lrint_test.exe

output:
type cast -1 = -1
type cast lrint(-1.0) = 4294967295
rint(-1.0) = -1.000000
lrint(-1.0) = 4294967295
llrint(-1.0) = 4294967295
lrint(1.0) = 1
llrint(1.0) = 1
Via sprintf: int -1 = -1
size of long long int: 8
size of long int: 8
size of int: 4
-----Original Message-----
From: cygwin-owner@cygwin.com [mailto:cygwin-owner@cygwin.com] On Behalf Of Eric Blake
Sent: 24 May 2017 18:57
To: cygwin@cygwin.com
Subject: Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

On 05/24/2017 11:53 AM, Erik Bray wrote:

>>> 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 = 0L;
>>> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
>>> defined(__i386__)
>>>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) :
>>> "st");

> Actually, I take it back.  gdb/objdump (and presumably binutils in
> general) was being deceptive to me about the nature of that mov
> instruction.  And in fact the fistpl should be fistpq.  That fixes it.

Is fistpq right on 32-bit, or is this a case where we need different assembly for 32-bit (fistpl) vs. 64-bit (fistql) to match the size of long that we are converting to?

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

The confidentiality or integrity of this message can not be guaranteed following transmission on the Internet. The addressee should be aware of this before using the contents of this message.
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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Erik Bray]

On Wed, May 24, 2017 at 6:57 PM, Eric Blake wrote:
> On 05/24/2017 11:53 AM, Erik Bray wrote:
>
>>>> 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 = 0L;
>>>> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
>>>> defined(__i386__)
>>>>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");
>
>> Actually, I take it back.  gdb/objdump (and presumably binutils in
>> general) was being deceptive to me about the nature of that mov
>> instruction.  And in fact the fistpl should be fistpq.  That fixes it.
>
> Is fistpq right on 32-bit, or is this a case where we need different
> assembly for 32-bit (fistpl) vs. 64-bit (fistql) to match the size of
> long that we are converting to?

Right.  I didn't read that pre-processer line past the first couple
defines.  Really this should be fistpq on 64-bit and fistpl on 32-bit.
And if llrint has the same bug then it should be fixed there too.

The discussion about printf format specifiers is orthogonal to this.
I can submit a patch for l(l)rint if you'd like.

Erik

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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Steven Penny]

On Wed, 24 May 2017 07:33:27, Eric Blake wrote:
> Buggy.  size_t should be printed with %zi, not %i (since size_t and int
> are not necessarily the same type).

Aren’t both wrong? By definition %i is a signed integer, and size_t is unsigned.
So %zu or %llu would be more correct:

http://wikipedia.org/wiki/C_data_types

They all seem to do the job though:

    $ cat alfa.c
    #define __USE_MINGW_ANSI_STDIO 1
    #include <stdio.h>
    int main() {
      size_t bravo = 1;
      printf("%zi %zu %llu\n", bravo, bravo, bravo);
    }

    $ x86_64-w64-mingw32-gcc -o alfa alfa.c

    $ ./alfa
    1 1 1


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Steven Penny]

On Wed, 24 May 2017 16:36:03, Steven Penny wrote:
> Aren’t both wrong? By definition %i is a signed integer, and size_t is unsigned.
> So %zu or %llu would be more correct:
> 
> http://wikipedia.org/wiki/C_data_types
> 
> They all seem to do the job though:

Correcting myself. Here is why you cannot use %zi:

    $ cat alfa.c
    #define __USE_MINGW_ANSI_STDIO 1
    #include <stdio.h>
    int main() {
      printf("%zi %zu %llu\n", __SIZE_MAX__, __SIZE_MAX__, __SIZE_MAX__);
    }

    $ x86_64-w64-mingw32-gcc -o alfa alfa.c

    $ ./alfa
    -1 18446744073709551615 18446744073709551615


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Brian Inglis]

On 2017-05-24 19:31, Steven Penny wrote:
> On Wed, 24 May 2017 16:36:03, Steven Penny wrote:
>> Aren’t both wrong? By definition %i is a signed integer, and size_t is
>> unsigned.
>> So %zu or %llu would be more correct:
>> http://wikipedia.org/wiki/C_data_types
>> They all seem to do the job though:
> Correcting myself. Here is why you cannot use %zi:
>	$ cat alfa.c
>	#define __USE_MINGW_ANSI_STDIO 1
>	#include <stdio.h>
>	int main() {
>		printf("%zi %zu %llu\n", __SIZE_MAX__, __SIZE_MAX__, __SIZE_MAX__);
>	}
>	$ x86_64-w64-mingw32-gcc -o alfa alfa.c
>	$ ./alfa
>	-1 18446744073709551615 18446744073709551615

I think either 10+ digits or a negative value are adequate indicators
that something may have gone wrong, and -1 is a more compact format for
*_MAX, unless you need to see actual values.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Eric Blake]

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On 05/24/2017 06:36 PM, Steven Penny wrote:
> On Wed, 24 May 2017 07:33:27, Eric Blake wrote:
>> Buggy.  size_t should be printed with %zi, not %i (since size_t and int
>> are not necessarily the same type).
> 
> Aren’t both wrong? By definition %i is a signed integer, and size_t is
> unsigned.
> So %zu

Correct.  Newer gcc's -Wformat-signedness will flag the discrepency.

> or %llu would be more correct:

NO. Do NOT use %llu with size_t, because it is not portable to 32-bit
platforms.  That's WHY %zu exists.

> They all seem to do the job though:

Yes. On all modern platforms, you can freely mix signed and unsigned
integers and get correct (when the number is positive and does not
exceed the signed maximum) or at least sane results (2s complement
counterpart for all other values) regardless of which direction you mess
with incorrect signedness.  However, such behavior is technically not
required by the C standard, which is why gcc added -Wformat-signedness.

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


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Steven Penny]

On Thu, 25 May 2017 13:17:30, Eric Blake wrote:
> Correct.  Newer gcc's -Wformat-signedness will flag the discrepency.

Uh, have you actually tried this? It doesnt do anything:

    $ cat alfa.c
    #define __USE_MINGW_ANSI_STDIO 1
    #include <stdio.h>
    int main() {
      printf("%zi %zu %llu\n", __SIZE_MAX__, __SIZE_MAX__, __SIZE_MAX__);
    }

    $ x86_64-w64-mingw32-gcc -Wformat-signedness -o alfa alfa.c

    $ ./alfa
    -1 18446744073709551615 18446744073709551615


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Steven Penny]

On Thu, 25 May 2017 18:43:58, Steven Penny wrote:
> Uh, have you actually tried this? It doesnt do anything:
> 
>     $ cat alfa.c
>     #define __USE_MINGW_ANSI_STDIO 1
>     #include <stdio.h>
>     int main() {
>       printf("%zi %zu %llu\n", __SIZE_MAX__, __SIZE_MAX__, __SIZE_MAX__);
>     }
> 
>     $ x86_64-w64-mingw32-gcc -Wformat-signedness -o alfa alfa.c

Correcting myself: you need 2 flags to get this working:

    $ x86_64-w64-mingw32-gcc -Wformat -Wformat-signedness alfa.c
    alfa.c: In function ‘main’:
    alfa.c:11:10: warning: format ‘%zi’ expects argument of type
    ‘signed size_t’, but argument 2 has type ‘long long unsigned int’
    [-Wformat=]
       printf("%zi %zu %llu\n", __SIZE_MAX__, __SIZE_MAX__, __SIZE_MAX__);
              ^


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Re: bug in lrint [was: FW: Printing long int in C program under cygwin64]

[Corinna Vinschen]

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On May 24 07:33, Eric Blake wrote:
> On 05/24/2017 07:00 AM, Carl Fredrik Forsberg wrote:
> > type cast lrint(-1.0) = 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 = 0L;
> #if defined(_AMD64_) || defined(__x86_64__) || defined(_X86_) ||
> defined(__i386__)
>   __asm__ __volatile__ ("fistpl %0"  : "=m" (retval) : "t" (x) : "st");
> #elif defined(__arm__) || defined(_ARM_)
>     retval = __lrint_internal(x);
> #endif
>   return retval;
> }
> 
> But I'm not an assembly coding expert, so perhaps someone else will spot
> the fix faster.

I just applied a patch to fix this.  Using fistpl is fine and dandy for
Mingw because sizeof(long) == 4, but on 64 bit Cygwin this function
has to use fistpll to account for sizeof(long) == 8.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
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Red Hat

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