lkml.org 
[lkml]   [2020]   [Jan]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] tty: n_hdlc: Use flexible-array member
Date


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.
>

\
 
 \ /
  Last update: 2020-01-21 16:35    [W:0.835 / U:0.192 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site