Re: on gdb's sizeof versus gcc's sizeof

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: on gdb's sizeof versus gcc's sizeof
Date	2019-09-25 19:22 -0700
Organization	None to speak of
Message-ID	<lntv8z4wzq.fsf@kst-u.example.com> (permalink)
References	<867e5vkgw2.fsf@example.com>

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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 */

Thread

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

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