Add a type checking macro __is_type_defined(type)

[yonghong-song]

Hi,

We would like to propose to implement a type checking macro like
__is_type_defined(type)
to check whether a type has been defined or not so far during preprocessor.

The possible usage of this macro can be illustrated by below.
…
#if !__is_type_defined(struct abc)
struct abc {
…
};
#endif

If ‘struct abc’ is defined before the ‘#if’ the __is_type_defined(struct abc) should be evaluated as true or 1. Otherwise, __is_type_defined(struct abc) should be evaluated as false or 0.

Previously we proposed attribute((bpf_dominating_decl)) and later__attribute__((bpf_accept_identical_del)). The previous discussion is at

BPF: adding new clang extension bpf_dominating_decl attribute

Hi, Aaron, For bpf_dominating_decl attribute, our previous use cases target both user header files (like /usr/include/...) and linux kernel internal header files. The reason bpf_dominating_decl attribute is needed because some popular linux kernel internal headers changed quite often so those types in vmlinux.h are often not the same as the ones in host kernel internal files. To address your above concern where we really need to diagnose the potential mis-matched types, we are proposi…

The attribute((bpf_dominating_decl)) is not favored by upstream, and attribute((bpf_accept_identical_del)). has its own limitation for our use case.

So we propose a macro __is_type_defined(type) so clang doesn’t need to make non-default decision how to handle duplicated types. User code can handle them in their particular way with the new macro.

Is this something clang could implement?