Re: Rationale behind 'extern' on protypes in .h files

From: Ravi Shekhar Jethani <rsjethani@gmail.com>
Date: Dec 23, 2013, 15:24 UTC
Message-ID: <CAKTJ_1z-pMePmh4phM2TXSMx0kOjGJ0afQ_JRESggi=k6+y-jA@mail.gmail.com>
In-Reply-To: <87eh54spw3.fsf@jedbrown.org>

2013/12/22 Jed Brown <jed@59a2.org>:

Show 11 quoted lines

> There is a difference for shared libraries if you would like to control
> which symbols are exported.  With gcc, for example, you might compile
> using -fvisibility=hidden.  Any functions explicitly declared with
> extern, bearing __attribute__((visibility("default")), or using
> visibility pragmas will be exported (similar to __declspec(dllexport) on
> Windows).  Other functions will be internal to the shared library so you
> don't have to worry about callers depending on those symbols and
> performance can be a bit better by skipping the PLT and avoiding symbol
> relocations at load time.  See Drepper's guide for more.
>
> http://www.akkadia.org/drepper/dsohowto.pdf

To check this I installed the libgit2-dev package which installed: /usr/include/git2/*.h , /usr/lib/libgit2.so Now, I exported all symbols using: $ readelf -s /usr/lib/libgit2.so and tried to match these with 'externed' prototypes in the Git source directory..no matches. I am confused!!!.

Also I checked this: $ ldd git There is no 'gitish' .so in the output; it seems everything is packed inside one executable. So your second point 'skipping the PLT...' also doesn't seem to apply here.

Regards, Ravi Shekhar Jethani