Re: server-side extension in c++

Tom Lane wrote:
> Craig Ringer <craig@postnewspapers.com.au> writes:
> > On 01/06/10 11:05, Tom Lane wrote:
> >> I'd be interested to see a section like this written by someone who'd
> >> actually done a nontrivial C++ extension and lived to tell the tale.
>
> > I can't speak up there - my own C++/Pg backend stuff has been fairly
> > trivial, and has been where I can maintain a fairly clean separation of
> > the C++-exposed and the Pg-backend-exposed parts. I was able to keep
> > things separate enough that my C++ compilation units didn't include the
> > Pg backend headers; they just exposed a pure C public interface. The Pg
> > backend-using compilation units were written in C, and talked to the C++
> > part over its exposed pure C interfaces.
>
> Yeah, if you can design your code so that C++ never has to call back
> into the core backend, that eliminates a large chunk of the pain.
> Should we be documenting design ideas like this one?

I have incorporated the new ideas into the C++ documentation section,
and removed the comment block in the attached patch.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + None of us is going to be here forever. +
Index: doc/src/sgml/extend.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v
retrieving revision 1.42
diff -c -c -r1.42 extend.sgml
*** doc/src/sgml/extend.sgml    1 Jun 2010 03:19:36 -0000    1.42
--- doc/src/sgml/extend.sgml    2 Jun 2010 01:20:23 -0000
***************
*** 273,280 ****
    &xoper;
    &xindex;

- <!-- Use this someday when C++ is easier to use. bjm 2010-05-31
-
    <sect1 id="extend-Cpp">
     <title>Using C++ for Extensibility</title>

--- 273,278 ----
***************
*** 284,313 ****

     <para>
      It is possible to use a compiler in C++ mode to build
!     <productname>PostgreSQL</productname> extensions;  you must simply
!     follow the standard methods for dynamically linking to C executables:

      <itemizedlist>
       <listitem>
        <para>
!         Use <literal>extern C</> linkage for all functions that must
!         be accessible by <function>dlopen()</>.  This is also necessary
!         for any functions that might be passed as pointers between
!         the backend and C++ code.
!       </para>
!      </listitem>
!      <listitem>
!       <para>
!        Use <function>palloc()</> to allocate any memory that might be
!        freed by the backend C code (don't pass <function>new()</>-allocated
!        memory).
        </para>
       </listitem>
       <listitem>
        <para>
!        Use <function>pfree()</> to free memory allocated by the backend
!        C code (do not use <function>delete()</> for such cases).
!       </para>
       </listitem>
       <listitem>
        <para>
--- 282,307 ----

     <para>
      It is possible to use a compiler in C++ mode to build
!     <productname>PostgreSQL</productname> extensions by following these
!     guidelines:

      <itemizedlist>
       <listitem>
        <para>
!         All functions accessed by the backend must present a C interface
!         to the backend;  these C functions can then call C++ functions.
!         For example, <literal>extern C</> linkage is required for
!         backend-accessed functions.  This is also necessary for any
!         functions that are passed as pointers between the backend and
!         C++ code.
        </para>
       </listitem>
       <listitem>
        <para>
!        Free memory using the appropriate deallocation method.  For example,
!        most backend memory is allocated using <function>palloc()</>, so use
!        <function>pfree()</> to free it, i.e. using C++
!        <function>delete()</> in such cases will fail.
       </listitem>
       <listitem>
        <para>
***************
*** 320,325 ****
     </para>

    </sect1>
- -->

   </chapter>
--- 314,318 ----