Re: [ANN] Lua 5.4.0 (rc3) now available

[Lorenzo Donati <lorenzodonatibz@...>]

On 09/05/2020 20:03, Hugo Musso Gualandi wrote:
> Hi,
>
> GCC 10 just came out and its -Wstringop-overflow warning is pickier
> than it used to be. Attached to this message is the warning that I get
> when compiling 5.4-rc3 using GCC 10.0.1.
>
> The culprit seems to be the getstr macro in lobject.h. It appears that
> GCC doesn't like that the string bytes are after the struct and would
> prefer if the byte array were a member of the struct:
>
>   typedef struct TString {
>     CommonHeader;
>     lu_byte extra;
>     lu_byte shrlen;
>     unsigned int hash;
>     union {
>       size_t lnglen;
>       struct TString *hnext;
>     } u;
>     char bytes[1];
>   } TString;
>
>
>   #define getstr(ts) check_exp(sizeof((ts)->extra), ts->bytes)
>
>   #define sizelstring(l)  (sizeof(TString) + (l) * sizeof(char))
>
> -- Hugo
I'm not a C great expert, but isn't that a common C idiom, I think it is 
called "flexible array member". I read somewhere that C standard made 
special provisions for that idiom because it is expected that the bytes 
making up the array (except the first element, which must be in the 
struct) will be allocated dynamically "outside" the boundary of the 
struct object itself.

Funny that a standard-compliant compiler complains about that, unless it 
is a "belt and braces" approach that signals any spot in code that 
usually goes wrong unless "you know what you are doing".

-- Lorenzo

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