Re: [Libguestfs] [libnbd PATCH v3 12/29] socket activation: clean up responsibilities of prep.sock.act.env.()

On Tue, Feb 21, 2023 at 01:24:17PM +0000, Richard W.M. Jones wrote: >>>> +++ b/generator/states-connect-socket-activation.c >>>> @@ -51,7 +51,7 @@ prepare_socket_activation_environment (string_vector *env) >>>> char *p; >>>> size_t i; >>>> >>>> - assert (env->len == 0); >>>> + *env = (string_vector)empty_vector; >>> >>> Do you actually need to cast this? >> >> This is not a cast, but a C99 compound literal. And yes, it is >> necessary, as empty_vector is just: >> >> #define empty_vector { .ptr = NULL, .len = 0, .cap = 0 } >> >> So this is *not* initialization, but assignment. We have a string_vector >> object (a structure) on the LHS, so we ned a structure on the RHS as >> well. The compound literal provides that (unnamed, automatic storage >> duration) structure. It looks like a cast (quite intentionally, I'd >> hazard), but it's not a cast. > > OK it's not a cast, but struct assignment is well defined so is the > change necessary? Struct assignment requires a source struct. Either a named one (which we do not have here without declaring one), or via a compound literal (which resembles a cast). It sounds like it would be nice to have a type-specific macro, as in 'empty_string_vector', 'empty_int_vector', and so on, per each use of DEFINE_VECTOR_TYPE() - but while our one macro can easily declare other type-specific functions, you can't really use the preprocessor to define further type-specific macros. So having to tell the user to supply their own type name when reusing the type-agnostic initializer portion of a compound literal is not too bad in the long run, although it might be worth documenting the practice in vector.h.