Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > linux.debian.user > #275796
| From | Franco Martelli <martellif67@gmail.com> |
|---|---|
| Newsgroups | linux.debian.user |
| Subject | Re: OT: Possible memory leak in an exercise of a C handbook |
| Date | 2024-12-16 21:20 +0100 |
| Message-ID | <JUpJn-2hL-7@gated-at.bofh.it> (permalink) |
| References | <JUkTn-hz8W-9@gated-at.bofh.it> <JUlFM-hzrJ-11@gated-at.bofh.it> <JUmit-hzVD-7@gated-at.bofh.it> <JUmBQ-hA3D-23@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On 16/12/24 at 17:50, Greg Wooledge wrote:
> On Mon, Dec 16, 2024 at 17:34:36 +0100, Franco Martelli wrote:
>>>> void dealloc()
>>>> {
>>>> for ( const DIGIT *p = head; p->next != NULL; p = p->next )
>>>> if ( p->prev != NULL )
>>>> free( p->prev );
>>>> free( last );
>>>> }
>>>
>>> I think you might have an off-by-one error in this function. You stop
>>> the for loop when p->next == NULL, which means you never enter the body
>>> during the time when p == last. Which means you never free the
>>> second-to-last element in the list.
>>
>> It is p->prev not p->next, by doing so free() don't apply for the "last"
>> element so I've to free apart
>>
>>>
>>> Given a list of 5 items, you will free items 1, 2, 3 (during the loop)
>>> and 5 (after the loop), but not 4.
>>
>> OK I'll try some test removing the "if statement"
>
> No, it's not the if statement. It's the for loop's iteration condition.
>
>>>> for ( const DIGIT *p = head; p->next != NULL; p = p->next )
>
> Your iteration condition is p->next != NULL so you stop iterating as
> soon as p->next == NULL.
>
> Given this linked list:
>
> NULL <- [1] <-> [2] <-> [3] <-> [4] <-> [5] -> NULL
> ^ ^
> head last
>
> Your first loop iteration, when p == head, does nothing because of
> the if statement. That's fine.
>
> The second iteration, when p points to [2], frees [1].
>
> The third iteration, when p points to [3], frees [2].
>
> The fourth iteration, when p points to [4], frees [3].
>
> The fifth iteration, when p points to [5], NEVER HAPPENS, because
> p->next == NULL at that point, and the for loop stops. So [4] is
> not freed.
>
> After the loop, you free [5] which is where last points.
>
You're correct! To verify I've added "free( last->prev );" to dealloc():
void dealloc()
{
for ( const DIGIT *p = head; p->next != NULL; p = p->next )
if ( p->prev != NULL )
free( p->prev );
free( last->prev );
free( last );
}
and it worked, now the output of Valgrind is:
$ valgrind --track-origins=yes --leak-check=full -s ./a.out
==114549== Memcheck, a memory error detector
==114549== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==114549== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright
info
==114549== Command: ./a.out
==114549==
12345678
==114549==
==114549== HEAP SUMMARY:
==114549== in use at exit: 0 bytes in 0 blocks
==114549== total heap usage: 9 allocs, 9 frees, 1,216 bytes allocated
==114549==
==114549== All heap blocks were freed -- no leaks are possible
==114549==
==114549== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
I'll change the "for condition" thanks very much again
--
Franco Martelli
Back to linux.debian.user | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 16:10 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Michael Kjörling <c9bc136c6063@ewoof.net> - 2024-12-16 16:50 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 17:30 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Michael Kjörling <c9bc136c6063@ewoof.net> - 2024-12-16 20:50 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 22:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Greg Wooledge <greg@wooledge.org> - 2024-12-16 17:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 17:40 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Greg Wooledge <greg@wooledge.org> - 2024-12-16 18:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 21:20 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Charles Curley <charlescurley@charlescurley.com> - 2024-12-16 19:40 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Jeffrey Walton <noloader@gmail.com> - 2024-12-16 21:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-16 22:40 +0100
Re: OT: Possible memory leak in an exercise of a C handbook songbird <songbird@anthive.com> - 2024-12-17 05:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Anssi Saari <anssi.saari@debian-user.mail.kapsi.fi> - 2024-12-17 12:30 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-17 15:30 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Anssi Saari <anssi.saari@debian-user.mail.kapsi.fi> - 2024-12-18 11:20 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Jean-François Bachelet <jfbachelet@free.fr> - 2024-12-18 05:50 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-17 20:50 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Greg Wooledge <greg@wooledge.org> - 2024-12-17 22:20 +0100
Re: OT: Possible memory leak in an exercise of a C handbook <tomas@tuxteam.de> - 2024-12-18 06:10 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Kevin Chadwick <m8il1ists@gmail.com> - 2024-12-18 11:50 +0100
Re: OT: Possible memory leak in an exercise of a C handbook <tomas@tuxteam.de> - 2024-12-18 12:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Jeffrey Walton <noloader@gmail.com> - 2024-12-17 22:20 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Franco Martelli <martellif67@gmail.com> - 2024-12-18 17:00 +0100
Re: OT: Possible memory leak in an exercise of a C handbook Jeffrey Walton <noloader@gmail.com> - 2024-12-18 17:20 +0100
csiph-web