clang error: unsupported: anonymous type given name for linkage purposes by typedef declaration after its linkage was computed

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Someone knows what does this error mean? My piece of code is shown below：

// test.c
inline void fun() {
    typedef struct {
        int i;
    } S;
}

GCC could compile without an error while clang (clang 12.0.0) claims an error:

root:~/test # clang -c test.c
test.c:2:19: error: unsupported: anonymous type given name for linkage purposes by typedef declaration after its linkage was computed; add a tag name here to establish linkage prior to definition
    typedef struct {
                  ^
                  S
test.c:4:7: note: type is given name 'S' for linkage purposes by this typedef declaration
    } S;
      ^
1 error generated.

According to the error text from clang, it looks like I need to add a tag name for the anonymous typedef. After adding a tag name, it won’t show an error. However, this piece of code from a team work program in my department so I need a strong reason to modify it. Someone knows what’s this error mean?

UPDATE: In the comments below someone mentioned, The code could compile but fail to link with GCC. Actually that because the GCC optimization is not turned on. Use -O2 to compile and link will pass.

compiler-errors clang c
1 Answers

I mentioned this in a comment, but I think I might be able to expand it to a proper answer. The problem here is in how the inline keyword works in C. Quite frankly, I don't know what the C standards committee was smoking when they came up with this ridiculous concept, and it's probably best not to know. Don't ask why the rules are this way: I don't know.

There is an important difference between extern inline and static inline functions, as well as ones declared simply inline.

inline functions may be defined in a header and included in multiple compilation units. This is because they aren't considered "real" definitions. No symbol for them is emitted, thus there is no multiple-definition error. However, the compiler is not required to actually inline the function call. It may instead try to call it. Since no symbol exists, there will be an error at link time. Never use inline functions in C. They make no sense at all.

extern inline functions provide the definition for an inline function. If the above case happens, and the compiler tries to call the function, so long as some file includes an extern inline definition, there won't be an error because it emits a symbol. This making sense? No? Good.

static inline functions are the only one that make any sense whatsoever. Because they're static and therefore local, no symbol need exist. If the compiler decides to inline it, it does. If not, it calls the function. No problems. Declare your function static inline and it will compile.

But that wasn't your question. Why is clang complaining about the struct? Why doesn't gcc complain? The answer is "idunnoman". Best guess is that clang is taking the rules very seriously, in which case because there is no "real" definition for fun, there is also no real definition for your anonymous struct, and therefore can't be typedef'd.

But that would also be the case if it were a named struct. And why does the linker care whether the structure has a name? Once again, I can only guess it has to do with the fact that there can be numerous definitions for an inline function, and clang wants the structure to have a constant name between them. But different compilation units can have different structures with the same name, so that doesn't make sense either.

Ugh. I thought I had an answer for you but now I'm even more confused than when I started. I'll still post this but it probably ought to be downvoted. Oh well.

user_55111370
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