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Re: dynamically linking one of mysq, postgresql or sqlite3

From: Bob Friesenhahn
Subject: Re: dynamically linking one of mysq, postgresql or sqlite3
Date: Fri, 11 Sep 2009 10:10:56 -0500 (CDT)

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, 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 Friesenhahn
GraphicsMagick Maintainer,

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