From: Mikael Magnusson <mikachu@gmail.com>
To: gustavo@embeddedor.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 16:00:53 +0100 [thread overview]
Message-ID: <20200121150053.31457-1-mikachu@gmail.com> (raw)
In-Reply-To: <957960eb-118f-21c7-8901-50f54d65d7cb@embeddedor.com>
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.
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.
--
Mikael Magnusson
next prev parent reply other threads:[~2020-01-21 15:01 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 [this message]
2020-01-21 15:14 ` Gustavo A. R. Silva
2020-01-21 15:24 ` Gustavo A. R. Silva
2020-01-21 16:03 ` Gustavo A. R. Silva
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