[cfe-dev] question about function with linkage type AvailableExternallyLinkage

Akira Hatanaka ahatanak at gmail.com
Thu Feb 3 11:42:55 PST 2011

Okay, that explains the difference of linkage type assignment between clang
and llvm-gcc.

Thank you for the clarification.

On Wed, Feb 2, 2011 at 8:00 PM, John McCall <rjmccall at apple.com> wrote:

> On Feb 2, 2011, at 2:28 PM, Akira Hatanaka wrote:
> > I am getting linkage errors when I try to compile foo1.c:
> >
> > $: clang foo1.c -o foo1 -O3
> > /tmp/cc-VXVfGf.o:(.data+0x0): undefined reference to `f1'
> > /tmp/cc-VXVfGf.o:(.data+0x4): undefined reference to `f2'
> > clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
> >
> > I inspected the bitcode foo1.ll, and it seems that definitions of f1 and
> f2 are not emitted because their linkage types are "available_externally".
> Clang compiles C files in C99 mode by default.  In C99, 'inline' is a
> promise that a non-inline declaration exists somewhere else in your program.
>  Therefore, if the compiler doesn't actually inline the functions, it's
> supposed to emit an external reference instead of emitting them in this
> file.
> You have five options, which I've listed in descending order of
> recommendation.
>  - Make the functions 'static inline'.  This will cause them to be emitted
> in this translation unit if they're used but not inlined.  This is the best
> option if you don't need to call these functions from a different file.
>  - Remove 'inline'.  It isn't required in order to get the compiler to
> inline a function, and while we do honor it as an optimization hint,
> functions this small will always get inlined anyway.  (Here it's impossible
> to inline because we can't statically prove what farray[i] will call, in
> part because farray is mutable.)
>  - Make sure there's a different file in your program which provides
> non-inline definitions of these functions.
>  - Compile the file with -std=gnu89, which will cause the functions to be
> emitted with strong linkage despite the 'inline'.  gcc uses gnu89 by
> default.
>  - Write this in C++ instead of C.  In C++, you can provide definitions of
> inline functions in multiple translation units and they'll get automatically
> merged together by the linker.
> John.
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