On Tue, Apr 30, 2024 at 05:51:20PM -0400, Jason Merrill wrote: > On 4/30/24 14:45, Qing Zhao wrote: > > > > > > > On Apr 30, 2024, at 15:27, Jason Merrill <ja...@redhat.com> wrote: > > > > > > On 4/30/24 07:58, Qing Zhao wrote: > > > > The request for GCC to accept that the C99 flexible array member can be > > > > in a union or alone in a structure has been made a long time ago > > > > around 2012 > > > > for supporting several practical cases including glibc. > > > > A GCC PR has been opened for such request at that time: > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548 > > > > However, this PR was closed as WONTFIX around 2015 due to the > > > > following reason: > > > > "there is an existing extension that makes the requested > > > > functionality possible" > > > > i.e GCC fully supported that the zero-length array can be in a > > > > union or alone > > > > in a structure for a long time. (though I didn't see any > > > > official documentation > > > > on such extension) > > > > It's reasonable to close PR53548 at that time since zero-length > > > > array extension > > > > can be used for such purpose. > > > > However, since GCC13, in order to improve the C/C++ security, we > > > > introduced > > > > -fstrict-flex-arrays=n to gradually eliminate the "fake flexible array" > > > > usages from C/C++ source code. As a result, zero-length arrays > > > > eventually > > > > will be replaced by C99 flexiable array member completely. > > > > Therefore, GCC needs to explicitly allow such extensions directly for > > > > C99 > > > > flexible arrays, since flexable array member in unions or alone > > > > in structs > > > > are common code patterns in active use by the Linux kernel (and > > > > other projects). > > > > For example, these do not error by default with GCC: > > > > union one { > > > > int a; > > > > int b[0]; > > > > }; > > > > union two { > > > > int a; > > > > struct { > > > > struct { } __empty; > > > > int b[]; > > > > }; > > > > }; > > > > But these do: > > > > union three { > > > > int a; > > > > int b[]; > > > > }; > > > > struct four { > > > > int b[]; > > > > } > > > > Clang has supported such extensions since March, 2024 > > > > https://github.com/llvm/llvm-project/pull/84428 > > > > GCC should also support such extensions. This will allow for > > > > a seamless transition for code bases away from zero-length arrays > > > > without > > > > losing existing code patterns. > > > > gcc/ChangeLog: > > > > * doc/extend.texi: Add documentation for Flexible Array Members in > > > > Unions and Flexible Array Members alone in Structures. > > > > --- > > > > gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++ > > > > 1 file changed, 34 insertions(+) > > > > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi > > > > index 7b54a241a7bf..cba98c8aadd7 100644 > > > > --- a/gcc/doc/extend.texi > > > > +++ b/gcc/doc/extend.texi > > > > @@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++. > > > > * Named Address Spaces::Named address spaces. > > > > * Zero Length:: Zero-length arrays. > > > > * Empty Structures:: Structures with no members. > > > > +* Flexible Array Members in Unions:: Unions with Flexible > > > > Array Members. > > > > +* Flexible Array Members alone in Structures:: Structures with > > > > only Flexible Array Members. > > > > * Variable Length:: Arrays whose length is computed at run time. > > > > * Variadic Macros:: Macros with a variable number of arguments. > > > > * Escaped Newlines:: Slightly looser rules for escaped newlines. > > > > @@ -1873,6 +1875,38 @@ The structure has size zero. In C++, > > > > empty structures are part > > > > of the language. G++ treats empty structures as if they had a single > > > > member of type @code{char}. > > > > +@node Flexible Array Members in Unions > > > > +@section Unions with Flexible Array Members > > > > +@cindex unions with flexible array members > > > > +@cindex unions with FAMs > > > > + > > > > +GCC permits a C99 flexible array member (FAM) to be in a union: > > > > + > > > > +@smallexample > > > > +union with_fam @{ > > > > + int a; > > > > + int b[]; > > > > +@}; > > > > +@end smallexample > > > > + > > > > +If all the members of a union are flexible array member, the size of > > > > It’s for the following case: > > > > union with_fam_3 { > > char a[]; > > int b[]; > > } > > > > And also include: (the only member of a union is a flexible array > > member as you mentioned below) > > > > union with_fam_1 { > > char a[]; > > } > > > > So, I think the original sentence: > > > > “If all the members of a union are flexible array member, the size of” > > > > Should be better than the below: > > > > > > "If the only member of a union is a flexible array member” > > Makes sense, but then it should be "members" both times rather than > "members" and then "member".
"If every member of a union is a flexible array, the size ..." ? -- Kees Cook
