code review 853042: Add //export to cgo.

Add //export to cgo.

The new //export comment marks a Go function as callable from
C.  The syntax is "//export NAME" where NAME is the name of
the function as seen from C.  If such a comment is seen, cgo
will generate two new files: _cgo_export.h and _cgo_export.c.
The _cgo_export.h file provides declarations which C code may
use to call Go functions.  The _cgo_export.c file contains
wrappers, and is to be compiled with gcc.

The changes to Make.pkg support using this from a Go Makefile,
though it could probably be more convenient.

[iant, 2010-04-02 04:06:15]

Hello rsc (cc: golang-dev@googlegroups.com),

I'd like you to review this change.

[rsc1, 2010-04-02 06:35:13]

LGTM

I wonder if instead of //export (which is fine for now)
we should think about using the name itself.  For example,
C_foo could, by convention, be exported to C as foo.

[iant2, 2010-04-03 02:17:39]

rsc@google.com writes:

> I wonder if instead of //export (which is fine for now)
> we should think about using the name itself.  For example,
> C_foo could, by convention, be exported to C as foo.

I was originally thinking about methods, for which we might want to
use a different name for the method and the C callable function.  But
the type issues are such that I think people will always be writing
little wrappers, so it's true that permitting the function to be named
in the export comment is not too helpful.

Ian

[rsc, 2010-04-06 05:18:22]

LGTM

Either way.  It's easy to change later.

[iant, 2010-04-09 20:31:09]

*** Submitted as http://code.google.com/p/go/source/detail?r=09271c841e33 ***

Add //export to cgo.

The new //export comment marks a Go function as callable from
C.  The syntax is "//export NAME" where NAME is the name of
the function as seen from C.  If such a comment is seen, cgo
will generate two new files: _cgo_export.h and _cgo_export.c.
The _cgo_export.h file provides declarations which C code may
use to call Go functions.  The _cgo_export.c file contains
wrappers, and is to be compiled with gcc.

The changes to Make.pkg support using this from a Go Makefile,
though it could probably be more convenient.

R=rsc
CC=golang-dev
http://codereview.appspot.com/853042