variant length array?

[bjorn@mork.no (Bjørn Mork)]

"Robert P. J. Day" <rpjday@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.


Bj?rn