Some c-ares symbols in Node headers not exported by Node executable

[murgatroid99]

• Version: 6.3.1
• Platform:Linux <hostname redacted> 3.13.0-87-generic #133-Ubuntu SMP Tue May 24 18:32:09 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

The ares.h file in the Node header distribution (the one that node-gyp downloads) contains symbols that are not dynamically exported by the Node binary. For example, ares_gethostbyname appears in that header file, but objdump -T $(which node) shows that that symbol appears nowhere in the dynamic symbol table. Because of this, I can build a native addon using that header without errors, but have it fail to find the symbol when I load it.

[bnoordhuis]

I don't think it's intentional that c-ares symbols are visible, we should probably tighten up our link step so people don't run into issues like yours. If you want to use c-ares, your best bet is to vendor it with your add-on.

[murgatroid99]

If symbols are not supposed to be visible, then those headers shouldn't be there. node-gyp automatically includes that headers directory in the include path, so any headers there override any vendored headers.

I've had similar issues in the past with OpenSSL. On Windows, for example, the OpenSSL headers are provided but the symbols are not available (#4932), so awkward workarounds are needed to compile against any vendored OpenSSL (or drop in replacement) whose headers aren't identical to those provided by Node.

[richardlau]

c-ares headers have been removed from the release tarballs by #10283