variant length array?

[Bjørn Mork]

"Robert P. J. Day" <rpjday at crashcourse.ca> writes:
> On Tue, 5 Apr 2016, Wenda Ni wrote:
>
>> Hi all,
>>
>> I come across the following code in a kernel module code. It defines
>> an array whose length is variant at runtime, depending on the actual
>> inputs. It seems that kernel compiler supports this, which is
>> obvious an error in the standard ANSI C. Do I have the correct
>> understanding on it?
>>
>> Thank you.
>>
>>
>> u32 rxe_icrc_hdr(struct rxe_pkt_info *pkt, struct sk_buff *skb)
>> {
>>          ...
>>          int hdr_size = sizeof(struct udphdr) +
>>                  (skb->protocol == htons(ETH_P_IP) ?
>>                  sizeof(struct iphdr) : sizeof(struct ipv6hdr));
>>          u8 tmp[hdr_size + RXE_BTH_BYTES];
>>          ...
>> }
>
>   pretty sure "sizeof" can be calculated at compile time so i don't
> see a problem here.

Yes, but skb->protocol is variable and sizeof(struct iphdr) !=
sizeof(struct ipv6hdr)).  Is the compiler smart enough to just use the
largest possible value?  The logic here is pretty similar to a union,
which it of course wouldn't have any problems calculating the size of.


Bjørn