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on gdb's sizeof versus gcc's sizeof

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  on gdb's sizeof versus gcc's sizeof Louis Valence <lvalence@example.com> - 2019-09-25 22:03 -0300
    Re: on gdb's sizeof versus gcc's sizeof Keith Thompson <kst-u@mib.org> - 2019-09-25 19:22 -0700
    Re: on gdb's sizeof versus gcc's sizeof scott@slp53.sl.home (Scott Lurndal) - 2019-09-26 13:26 +0000
      Re: on gdb's sizeof versus gcc's sizeof Keith Thompson <kst-u@mib.org> - 2019-09-26 13:08 -0700
        Re: on gdb's sizeof versus gcc's sizeof Louis Valence <lvalence@example.com> - 2019-09-28 16:57 -0300

#146345 — on gdb's sizeof versus gcc's sizeof

Can you explain this?  I don't get it.

%cat sizeof-long.c
#include <stdio.h>

int main() {
  printf("sizeof unsigned long: %ld\n", sizeof (unsigned long));
  printf("sizeof long: %ld\n", sizeof (long));
}

%./sizeof-long
sizeof unsigned long: 8
sizeof long: 8

%gdb
GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1

[...]

(gdb) p sizeof (unsigned long)
$1 = 4
(gdb) p sizeof (long)
$2 = 4
(gdb)

%uname -a
Linux e 4.13.0-21-generic #24-Ubuntu SMP Mon Dec 18 17:29:16 UTC 2017 \
  x86_64 x86_64 x86_64 GNU/Linux

%lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 17.10
Release:        17.10
Codename:       artful
%

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#146346

Louis Valence <lvalence@example.com> writes:
> Can you explain this?  I don't get it.
>
> %cat sizeof-long.c
> #include <stdio.h>
>
> int main() {
>   printf("sizeof unsigned long: %ld\n", sizeof (unsigned long));
>   printf("sizeof long: %ld\n", sizeof (long));
> }

Use "%zu" to print values of type size_t.

> %./sizeof-long
> sizeof unsigned long: 8
> sizeof long: 8
>
> %gdb
> GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1
>
> [...]
>
> (gdb) p sizeof (unsigned long)
> $1 = 4
> (gdb) p sizeof (long)
> $2 = 4
> (gdb)
>
> %uname -a
> Linux e 4.13.0-21-generic #24-Ubuntu SMP Mon Dec 18 17:29:16 UTC 2017 \
>   x86_64 x86_64 x86_64 GNU/Linux
>
> %lsb_release -a
> No LSB modules are available.
> Distributor ID: Ubuntu
> Description:    Ubuntu 17.10
> Release:        17.10
> Codename:       artful
> %

I get the same result (Ubuntu 18.04 x86_64).

gdb is not a C implementation, and it can debug programs written in
languages other than C.  If you invoke it without having it control
a program, I don't know how it decides what assumptions to make.
(It does seem to assume C, as indicated by "show language").

If you compile your C program with "-g" and then invoke gdb on that
program, you should get sizeof (long) == 8, as I did.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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#146348

Louis Valence <lvalence@example.com> writes:
>Can you explain this?  I don't get it.
>
>%cat sizeof-long.c
>#include <stdio.h>
>
>int main() {
>  printf("sizeof unsigned long: %ld\n", sizeof (unsigned long));
>  printf("sizeof long: %ld\n", sizeof (long));
>}
>
>%./sizeof-long
>sizeof unsigned long: 8
>sizeof long: 8
>
>%gdb
>GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1
>
>[...]
>
>(gdb) p sizeof (unsigned long)
>$1 = 4

You may want to use a 64-bit build of GDB instead of the 32-bit:

(gdb) print sizeof(unsigned long)
$1 = 0x8
(gdb) quit

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#146363

scott@slp53.sl.home (Scott Lurndal) writes:
> Louis Valence <lvalence@example.com> writes:
>>Can you explain this?  I don't get it.
>>
>>%cat sizeof-long.c
>>#include <stdio.h>
>>
>>int main() {
>>  printf("sizeof unsigned long: %ld\n", sizeof (unsigned long));
>>  printf("sizeof long: %ld\n", sizeof (long));
>>}
>>
>>%./sizeof-long
>>sizeof unsigned long: 8
>>sizeof long: 8
>>
>>%gdb
>>GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1
>>
>>[...]
>>
>>(gdb) p sizeof (unsigned long)
>>$1 = 4
>
> You may want to use a 64-bit build of GDB instead of the 32-bit:
>
> (gdb) print sizeof(unsigned long)
> $1 = 0x8
> (gdb) quit

I don't believe that's the issue.

Were you debugging a program when you got that result, or did you invoke
gdb with no arguments as Louis and I both did?

On my system:

    $ echo 'print sizeof (long)' | gdb
    GNU gdb (Ubuntu 8.1-0ubuntu3.1) 8.1.0.20180409-git
    [...]
    This GDB was configured as "x86_64-linux-gnu".
    [...]
    (gdb) $1 = 4
    (gdb) quit
    $

As far as I can tell, gdb uses the environment of the program it's
debugging, falling back to some default environment if it's not given
a program to debug.  In that default -- at least on my system --
long is 4 bytes.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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#146399

Keith Thompson <kst-u@mib.org> writes:

> scott@slp53.sl.home (Scott Lurndal) writes:
>> Louis Valence <lvalence@example.com> writes:
>>>Can you explain this?  I don't get it.
>>>
>>>%cat sizeof-long.c
>>>#include <stdio.h>
>>>
>>>int main() {
>>>  printf("sizeof unsigned long: %ld\n", sizeof (unsigned long));
>>>  printf("sizeof long: %ld\n", sizeof (long));
>>>}
>>>
>>>%./sizeof-long
>>>sizeof unsigned long: 8
>>>sizeof long: 8
>>>
>>>%gdb
>>>GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1
>>>
>>>[...]
>>>
>>>(gdb) p sizeof (unsigned long)
>>>$1 = 4
>>
>> You may want to use a 64-bit build of GDB instead of the 32-bit:
>>
>> (gdb) print sizeof(unsigned long)
>> $1 = 0x8
>> (gdb) quit
>
> I don't believe that's the issue.
>
> Were you debugging a program when you got that result, or did you invoke
> gdb with no arguments as Louis and I both did?
>
> On my system:
>
>     $ echo 'print sizeof (long)' | gdb
>     GNU gdb (Ubuntu 8.1-0ubuntu3.1) 8.1.0.20180409-git
>     [...]
>     This GDB was configured as "x86_64-linux-gnu".
>     [...]
>     (gdb) $1 = 4
>     (gdb) quit
>     $
>
> As far as I can tell, gdb uses the environment of the program it's
> debugging, falling back to some default environment if it's not given
> a program to debug.  In that default -- at least on my system --
> long is 4 bytes.

Thanks for the investigation, Keith.  I think you're correct here.
Here's some evidence.

--8<---------------cut here---------------start------------->8---
%gdb
GNU gdb (Ubuntu 8.0.1-0ubuntu1) 8.0.1
[..]
(gdb) p sizeof (unsigned long)
$1 = 4
(gdb) set architecture
Requires an argument. Valid arguments are i386, i386:x86-64,
i386:x64-32, i8086, i386:intel, i386:x86-64:intel, i386:x64-32:intel,
i386:nacl, i386:x86-64:nacl, i386:x64-32:nacl, auto.
(gdb) set architecture i386:x86-64
The target architecture is assumed to be i386:x86-64
(gdb) p sizeof (unsigned long)
$2 = 8
(gdb) quit
%
--8<---------------cut here---------------end--------------->8---

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