On Tue, 7 Feb 2023, Qing Zhao via Gcc-patches wrote: > Then, this routine (flexible_array_type_p) is mainly for diagnostic purpose. > It cannot be used to determine whether the structure/union type recursively > include a flexible array member at the end. > > Is my understanding correct?
My comments were about basic principles of what gets diagnosed, and the need for different predicates in different contexts; I wasn't trying to assert anything about how that maps onto what functions should be used in what contexts. > >> 2. Only C99 standard flexible array member be included, [0] and [1] are > >> not included, for example: > > > > Obviously we can't diagnose use of structures with [1] trailing members, > > because it's perfectly valid to embed those structures at any position > > inside other structures. And the same is the case for the [0] extension > > when it's used to mean "empty array" rather than "flexible array". > > With the -fstrict-flex-arrays available, we should be able to diagnose > the flexible array member per gnu extension (i.e [0] or [1]) the same as []. There are different sorts of diagnostic that might be involved. * Simply having [0] or [1] at the end of a structure embedded in another structure isn't appropriate to diagnose, because [0] and [1] have perfectly good meanings in such a context that aren't trying to be flexible array members at all. [0] might be an empty type (possibly one that wouldn't be empty when built with a different configuration). [1] might be the use of arrays in C to produce a passed-by-reference type. * Trying to use such an embedded [0] or [1] array as if it were a flexible array member - i.e. accessing any member of the [0] array, or any member other than the [0] member of the [1] array - *is* a sign of the problematic use as a flexible array member, that might be appropriate to diagnose. (Actually I'd guess the array index tends to be non-constant in accesses, and it would be odd to use a non-constant index when you mean that constant always to be 0, which it would need to be in the non-flexible case.) -- Joseph S. Myers jos...@codesourcery.com
