From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Mikael Magnusson <mikachu@gmail.com>
Cc: gregkh@linuxfoundation.org, jslaby@suse.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tty: n_hdlc: Use flexible-array member
Date: Tue, 21 Jan 2020 09:14:15 -0600 [thread overview]
Message-ID: <995ff17c-81e2-15ad-c776-f07c8d61385b@embeddedor.com> (raw)
In-Reply-To: <20200121150053.31457-1-mikachu@gmail.com>
On 1/21/20 09:00, Mikael Magnusson wrote:
> Gustavo Silva wrote:
>> On 1/20/20 23:54, Jiri Slaby wrote:
>>> On 21. 01. 20, 0:45, Gustavo A. R. Silva wrote:
>>>> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
>>>> index 98361acd3053..b5499ca8757e 100644
>>>> --- a/drivers/tty/n_hdlc.c
>>>> +++ b/drivers/tty/n_hdlc.c
>>>> @@ -115,7 +115,7 @@
>>>> struct n_hdlc_buf {
>>>> struct list_head list_item;
>>>> int count;
>>>> - char buf[1];
>>>> + char buf[];
>>>> };
>>>>
>>>> #define N_HDLC_BUF_SIZE (sizeof(struct n_hdlc_buf) + maxframe)
>>>
>>> Have you checked, that you don't have to "+ 1" here now?
>>>
>>
>> Yep. That's not necessary.
>>
>> _In terms of memory allocation_, zero-length/one-element arrays and flexible-array
>> members work exactly the same way.
>
> This is not true, but maybe it's still not necessary in this particular code, I didn't examine it.
>
I should have said _in terms of dynamic memory allocation_.
Your example is correct:
"... a one-element array always occupies at least as much space as a single object of the type."[1]
But the above does not affect on the current code.
[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Thanks
--
Gustavo
> Consider the following:
> #include <stdio.h>
>
> struct flex {
> int count;
> char buf[PRE];
> char flex[];
> };
>
> struct one {
> int count;
> char buf[PRE];
> char one[1];
> };
>
> void main() {
> printf("%ld %ld\n", sizeof(struct flex), sizeof(struct one));
> }
>
> --snip--
>
> % gcc -o siz siz.c -std=c99 -DPRE=7 && ./siz
> 12 12
> % gcc -o siz siz.c -std=c99 -DPRE=8 && ./siz
> 12 16
>
> Since all the preceding stuff in the struct in the patch is aligned, then the [1] will definitely add something to the sizeof count.
>
next prev parent reply other threads:[~2020-01-21 15:34 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-20 23:45 [PATCH] tty: n_hdlc: Use flexible-array member Gustavo A. R. Silva
2020-01-21 5:54 ` Jiri Slaby
2020-01-21 14:27 ` Gustavo A. R. Silva
2020-01-21 15:00 ` Mikael Magnusson
2020-01-21 15:14 ` Gustavo A. R. Silva [this message]
2020-01-21 15:24 ` Gustavo A. R. Silva
2020-01-21 16:03 ` Gustavo A. R. Silva
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