Question

How do I get the anonymous struct/union behaviour activated by -fplan9-extensions in GCC to work in Clang?

I’m getting errors assigning to members of anonymous when using designated initializers, and I’m not getting the free casting to the type of an anonymous member. Both these work under GCC with the aforementioned extension activated.

typedef struct {int hi;} Embedded;
typedef struct {Embedded;} Encapsulating;

Encapsulating poo = {.hi = 3;};
error: field designator 'hi' does not refer to any field in type 'Encapsulating'

void takes_embedded(Embedded *m);
takes_embedded(&poo);
warning: incompatible pointer types passing 'Encapsuating *' to parameter of type 'Embedded *'

Answer 1

Here’s how to get the -fplan9-extensions functionality in Clang:

• Ensure that the functionality meets the seven criteria for Clang extensions.
• Mail the cfe-dev mailing list and ask whether the Clang community would accept a patch implementing this extension.
• Familiarize yourself with the LLVM coding standards.
• Check out and build LLVM and clang from SVN.
• Make a patch implementing the functionality and mail it to the cfe-commits mailing list.
• Respond to comments on that mailing list. For a first patch to Clang, you should expect to need to revise it several times before it is accepted by the community. Be patient and persevere.
• If all goes well, your patch will be checked into Clang.

Some of the -fplan9-extensions functionality (the struct { Embedded; } part) is already available under the -fms-extensions argument, but designated initializers for such anonymous members are not supported. The other part is similar in spirit to GCC’s __attribute__((transparent_union)) functionality, which Clang already supports.