On Sun, Jun 11, 2017 at 7:56 PM, andrey mirtchovski
<mirtchov...@gmail.com> wrote:
>
> I have a piece of cgo code which doesn't do much, but links a lot of
> libraries in. As far as I can tell all my includes are there and they
> shouldn't stomp on each other (they don't when compiled in C), however
> with cgo I get the following weirdness:
>
> $ go build
> # mypkg
> Undefined symbols for architecture x86_64:
> "___DARWIN_NULL", referenced from:
> __cgohack___DARWIN_NULL in _cgo_main.o
> (maybe you meant: __cgohack___DARWIN_NULL)
> ld: symbol(s) not found for architecture x86_64
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
>
> If I decide to be bold and #define __DARWIN_NULL in my Cgo preamble
> (before "import C"), I get the following:
>
> $ go build
> # mypkg
> ./main.go:53:9: warning: '__DARWIN_NULL' macro redefined [-Wmacro-redefined]
> /usr/include/sys/_types.h:52:9: note: previous definition is here
>
> I'm a little bit stumped. How to debug the chain on #includes to get
> to the bottom of this? Why is this not a problem elsewhere?
Presumably your Go code is referring to C.__DARWIN_NULL. In some
cases that will cause cgo to generate C code that looks like
extern char __DARWIN_NULL[];
void *_cgohack___DARWIN_NULL = __DARWIN_NULL;
This kind of thing works for a variable or function, but not for a
macro. This may be a version of https://golang.org/issue/18720. The
simplest fix is to write, in your C preamble,
void *my_DARWIN_NULL = __DARWIN_NULL; // use the appropriate type here
and then refer to C.my_DARWIN_NULL in your Go code.
Ian
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