Re: use of modules, static vs shared, and dependencies

[Howard Chu]

> > We have this situation in OpenLDAP slapd, since a number of modules can 
> > be built either statically (linked into the slapd executable) or 
> > dynamically (loaded by libltdl), and several of the modules depend on 
> > each other in addition to the main slapd executable and the 
> > libldap/liblber libraries (which may be static or dynamic). In this 
> > case, each module's compilation is controlled by a three way --enable 
> > switch at configure time: no (don't build), yes (static), mod (dynamic). 
> > The list of static and dynamic modules go into two separate Makefile 
> > macros. The slapd Makefile compiles all static modules, the slapd 
> > executable, and then all dynamic modules, three separate steps. There 
> > are appropriate dummies in case either of those two macros is empty, e.g.:
>
> OK, you have one single Makefile from which to build stuff.

Actually no, each module is in its own subdirectory with its own 
Makefile. We also set a BUILD_FOO=yes|mod macro for each of the modules 
so that its own Makefile knows what to do. There's also a subdirectory 
(overlays) that contains a bunch of modules that can be either static or 
dynamic, we just descend into it twice - once with "make static" and 
once with "make dynamic". (Yes, we should probably streamine this so 
that there aren't two different approaches to building modules...)

> You don't use dlpreopening though, right?

Right.

> One thing I haven't grasped is this: suppose one would like to use
> dlpreopening for the static case.  Will this help in any way with the
> dependency mess?  Is there anything we can do to automate this or keep
> the user from shooting himself in the foot with wrong dependencies?

I remember when we first added dynamic module support, we also tried to 
support dlpreopening, but that was just confusing. I.e., why should we 
go through the module-handling work for something that is statically 
linked into the main executable... So in our case, there didn't seem to 
be anything to be gained from that.

There is of course some other overhead for static modules; for example 
our configure script generates a backends.c file that contains an array 
of references to the entry points of all the static backends.

> Cheers, and thanks for your explanations and the pointer!
> Ralf

--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/