Messages in this thread | | | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH] tty: n_hdlc: Use flexible-array member | Date | Tue, 21 Jan 2020 09:14:15 -0600 |
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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. >
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