On 2015-02-18 17:29:27 -0500, Tom Lane wrote: > Michael Paquier <michael.paqu...@gmail.com> writes: > > On Wed, Feb 18, 2015 at 10:09 PM, Andres Freund <and...@2ndquadrant.com> > > wrote: > >> The compiler will complain if you use a FLEXIBLE_ARRAY_MEMBER in the > >> middle of a struct but not when when you embed a struct that uses it > >> into the middle another struct. At least gcc doesn't and I think it'd be > >> utterly broken if another compiler did that. If there's a compiler that > >> does so, we need to make it define FLEXIBLE_ARRAY_MEMBER to 1. > > > clang does complain on my OSX laptop regarding that ;) > > I'm a bit astonished that gcc doesn't consider this an error. Sure seems > like it should.
Why? The flexible arrary stuff tells the compiler that it doesn't have to worry about space for the array - it seems alright that it actually doesn't. There's pretty much no way you can do that sensibly if the variable length array itself is somewhere in the middle of a struct - but if you embed the whole struct somewhere you have to take care yourself. And e.g. the varlena cases Michael has shown do just that? > (Has anyone tried it on recent gcc?) Yes. > Moreover, if we have any code that is assuming such cases are okay, it > probably needs a second look. Isn't this situation effectively assuming > that a variable-length array is fixed-length? Not really. If you have struct varlena hdr; char data[TOAST_MAX_CHUNK_SIZE]; /* make struct big enough */ the variable length part is preallocated in the data? You're right that many of these structs could just be replaced with a union though. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers