File: libtool.info,  Node: Modules for libltdl,  Next: Thread Safety in libltdl,  Prev: Libltdl interface,  Up: Using libltdl

11.2 Creating modules that can be ‘dlopen’ed
============================================

Libtool modules are created like normal libtool libraries with a few
exceptions:

   You have to link the module with libtool's ‘-module’ switch, and you
should link any program that is intended to dlopen the module with
‘-dlopen MODULENAME.LA’ where possible, so that libtool can dlpreopen
the module on platforms that do not support dlopening.  If the module
depends on any other libraries, make sure you specify them either when
you link the module or when you link programs that dlopen it.  If you
want to disable versioning (*note Versioning::) for a specific module
you should link it with the ‘-avoid-version’ switch.  Note that libtool
modules don't need to have a "lib" prefix.  However, Automake 1.4 or
higher is required to build such modules.

   Usually a set of modules provide the same interface, i.e. exports the
same symbols, so that a program can dlopen them without having to know
more about their internals: In order to avoid symbol conflicts all
exported symbols must be prefixed with "modulename_LTX_" (MODULENAME is
the name of the module).  Internal symbols must be named in such a way
that they won't conflict with other modules, for example, by prefixing
them with "_modulename_".  Although some platforms support having the
same symbols defined more than once it is generally not portable and it
makes it impossible to dlpreopen such modules.

   libltdl will automatically cut the prefix off to get the real name of
the symbol.  Additionally, it supports modules that do not use a prefix
so that you can also dlopen non-libtool modules.

   ‘foo1.c’ gives an example of a portable libtool module.  Exported
symbols are prefixed with "foo1_LTX_", internal symbols with "_foo1_".
Aliases are defined at the beginning so that the code is more readable.

     /* aliases for the exported symbols */
     #define foo  foo1_LTX_foo
     #define bar  foo1_LTX_bar

     /* a global variable definition */
     int bar = 1;

     /* a private function */
     int _foo1_helper() {
       return bar;
     }

     /* an exported function */
     int foo() {
       return _foo1_helper();
     }

The ‘Makefile.am’ contains the necessary rules to build the module
‘foo1.la’:

     ...
     lib_LTLIBRARIES = foo1.la

     foo1_la_SOURCES = foo1.c
     foo1_la_LDFLAGS = -module
     ...