From: Jed Brown <jed@59a2•org>
To: Ravi Shekhar Jethani <rsjethani@gmail•com>
Cc: Stefan Beller <stefanbeller@googlemail•com>, git@vger•kernel.org
Subject: Re: Rationale behind 'extern' on protypes in .h files
Date: Mon, 23 Dec 2013 09:59:25 -0700 [thread overview]
Message-ID: <87fvpjqz9u.fsf@jedbrown.org> (raw)
In-Reply-To: <CAKTJ_1z-pMePmh4phM2TXSMx0kOjGJ0afQ_JRESggi=k6+y-jA@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 2202 bytes --]
Ravi Shekhar Jethani <rsjethani@gmail•com> writes:
> 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!!!.
libgit2 is an entirely different package from Git. If you look at the
libgit2 sources (https://github.com/libgit2/libgit2), look in
include/git2/common.h:
/** Declare a public function exported for application use. */
#if __GNUC__ >= 4
# define GIT_EXTERN(type) extern \
__attribute__((visibility("default"))) \
type
#elif defined(_MSC_VER)
# define GIT_EXTERN(type) __declspec(dllexport) type
#else
# define GIT_EXTERN(type) extern type
#endif
I have always used __attribute__((visibility("default"))), but the gcc
man page says
extern declarations are not affected by -fvisibility, so a lot of
code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to "extern" functions
with no explicit visibility use the PLT, so it is more effective to
use "__attribute ((visibility))" and/or "#pragma GCC visibility" to
tell the compiler which "extern" declarations should be treated as
hidden.
However, I don't understand what the first statement means
(documentation bug?) since -fvisibility=hidden causes functions declared
with 'extern' to be hidden.
symbols.c:
EXTERN int foo(void);
int foo(void) {return 1;}
$ gcc -fvisibility=hidden -DEXTERN=extern -shared -o libsymbols.so symbols.c
$ nm -D libsymbols.so | grep foo
$
meanwhile,
$ gcc -fvisibility=hidden -DEXTERN='__attribute((visibility("default")))' -shared -o libsymbols.so symbols.c
$ nm -D libsymbols.so | grep foo
000000000000055c T foo
> 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.
My comment applied to shared libraries in general, not specifically to
Git (which isn't a shared library).
[-- Attachment #2: Type: application/pgp-signature, Size: 835 bytes --]
prev parent reply other threads:[~2013-12-23 16:59 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-22 15:51 Rationale behind 'extern' on protypes in .h files Ravi Shekhar Jethani
2013-12-22 17:11 ` Stefan Beller
2013-12-22 18:26 ` Jed Brown
2013-12-23 15:24 ` Ravi Shekhar Jethani
2013-12-23 16:59 ` Jed Brown [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87fvpjqz9u.fsf@jedbrown.org \
--to=jed@59a2$(echo .)org \
--cc=git@vger$(echo .)kernel.org \
--cc=rsjethani@gmail$(echo .)com \
--cc=stefanbeller@googlemail$(echo .)com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox