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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: on gdb's sizeof versus gcc's sizeof
Date: Thu, 26 Sep 2019 13:08:31 -0700
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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 */