Re: dynamically linking one of mysq, postgresql or sqlite3

[Bob Friesenhahn]

On Fri, 11 Sep 2009, santilin wrote:

> Hi, I'm quite new to libtool though I have read the tree GNU manuals:
> automake, autoconf and libtool.
>
> I want to solve the typical problem of loading dinamically one of the
> sql servers drivers from my own shared library.
>
> I know I have to create a common interface for the drivers.
>
> What I am not sure is wheter I must use dlopen or there is any other way
> of loading the library. If I use dlopen, lets say, to open mysql.so, I
> have to read all the symbols and use pointers to them, which is a lot of
> work. I wonder if there is a way of loading the libraries the same way
> they are loaded when they are linked within the main program.

I believe that you can use libltdl from libtool 2.X to do this by 
using lt_dlopenadvise() with the lt_dladvise_global option.  This 
invokes dlopen() with the RTLD_GLOBAL flag.

I don't recommend that you do this since it introduces ordering 
problems, is NOT PORTABLE, and does not solve the static linkage 
issue.  If your code accidentally references a symbol before it is 
defined, then your program goes "poof".

If you design a common interface for the drivers, then you can use 
libltdl to locate just one symbol in the driver which is a 
registration function which registers all of the interface functions 
that the driver supports.  You could pass a structure containing all 
of the possible function vectors and the driver module updates it, or 
you could have the module call back into your library (which it can 
do) and add itself as a driver.  This is more efficient than using 
libltdl to search for all of the possible symbols (some of which may 
not exist).  This allows all of the implementation functions (except 
for the registration function) to be 'static' since they don't require 
external visibility.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/