https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112761
Bug ID: 112761 Summary: Using incomplete array types in function prototypes doesn't work Product: gcc Version: 13.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: ville.syrjala at linux dot intel.com Target Milestone: --- struct foo; void func(struct foo bar[]); int main(void) { } error: array type has incomplete element type ‘struct foo’ 3 | void func(struct foo bar[]); | ^~~ This prevents using the array syntax in function prototypes with opaque types. So I have to give up either: - the array syntax, which means there is no longer any hint to the reader whether the function takes an array or a pointer to a single object - or the opaque type which is even worse since the clean abstraction is ruined (not to mention the resulting stable ABI issues). I couldn't immediately find anything in the spec that says this is illegal: - "6.7.6.2 Array declarators ... If the size is not present, the array type is an incomplete type." - "6.7.6.3 Function declarators ... If the function declarator is not part of a definition of that function, parameters may have incomplete type and may use the [*] notation in their sequences of declarator specifiers to specify variable length array types." But maybe I just don't know how to read the spec correctly? Side note: This also affects the (arguably hideous) "ptr[static 1]" syntax which would be beneficial to indicate that you can't pass in a null pointer. But I suppose that one is explicitly forbidden by the spec since we do specify a size for the "array" and thus it is not allowed to be an incomplete type.