From: Jed Brown <jed@59a2.org>
Date: Sun, 22 Dec 2013 18:26:52 GMT
Subject: Re: Rationale behind 'extern' on protypes in .h files
Message-ID: <87eh54spw3.fsf@jedbrown.org>
In-Reply-To: <52B71D24.4000207@googlemail.com>

Stefan Beller <stefanbeller@googlemail.com> writes:
> From my understanding there is no
> difference for functions declarations being set to extern or not,
> because extern is the default on functions.

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