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20.1 A Module Loading Subsystem

As you saw in Using GNU libltdl, I need to put an invocation of the macro ‘AC_LIBTOOL_DLOPEN’ just before ‘AC_PROG_LIBTOOL’, in the file ‘configure.in’. But, as well as being able to use libtoolize --ltdl, which adds libltdl in a subdirectory with its own subconfigure, you can also manually copy just the ltdl source files into your project(45), and use AC_LIB_LTDL in your existing ‘configure.in’. At the time of writing, this is still a very new and (as yet) undocumented feature, with a few kinks that need to be ironed out. In any case you probably shouldn’t use this method to add ‘ltdl.lo’ to a C++ library, since ‘ltdl.c’ is written in C. If you do want to use libltdl with a C++ library, things will work much better if you build it in a subdirectory generated with libtoolize --ltdl.

For this project, lets:

 
$ cp /usr/share/libtool/libltdl/ltdl.[ch] sic/

The Sic module loader is probably as complicated as any you will ever need to write, since it must support two kinds of modules: modules which contain additional built-in commands for the interpreter; and modules which extend the Sic syntax table. A single module can also provide both syntax extensions and additional built-in commands.


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20.1.1 Initialising the Module Loader

Before using this code (or any other libltdl based module loader for that matter), a certain amount of initialisation is required:

Here is the start of the module loader, ‘sic/module.c’, including the initialisation code for libltdl:

 

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20.1.2 Managing Module Loader Errors

The error handling is a very simplistic wrapper for the libltdl error functions, with the addition of a few extra errors specific to this module loader code(46). Here are the error messages from ‘module.c’:

 

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20.1.3 Loading a Module

Individual modules are managed by finding specified entry points (prescribed exported symbols) in the module:

Variable: const Builtin * builtin_table

An array of names of built-in commands implemented by a module, with associated handler functions.

Function: void module_init (Sic *sic)

If present, this function will be called when the module is loaded.

Function: void module_finish (Sic *sic)

If supplied, this function will be called just before the module is unloaded.

Variable: const Syntax * syntax_table

An array of syntactically significant symbols, and associated handler functions.

Function: int syntax_init (Sic *sic)

If specified, this function will be called by Sic before the syntax of each input line is analysed.

Function: int syntax_finish (Sic *sic, BufferIn *in, BufferOut *out)

Similarly, this function will be call after the syntax analysis of each line has completed.

All of the hard work in locating and loading the module, and extracting addresses for the symbols described above is performed by libltdl. The module_load function below simply registers these symbols with the Sic interpreter so that they are called at the appropriate times – or diagnoses any errors if things don’t go according to plan:

 

Notice that the generalised List data type introduced earlier (see section A Small GNU Autotools Project) is reused to keep a list of accumulated module initialisation and finalisation functions.


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20.1.4 Unloading a Module

When unloading a module, several things must be done:

My first cut implementation of a module subsystem kept a list of the entry points associated with each module so that they could be looked up and removed when the module was subsequently unloaded. It also kept track of multiply loaded modules so that a module wasn’t unloaded prematurely. libltdl already does all of this though, and it is wasteful to duplicate all of that work. This system uses lt_dlforeach and lt_dlgetinfo to access libltdls records of loaded modules, and save on duplication. These two functions are described fully in(Libtool)Libltdl interface section ‘Libltdl interface’ in The Libtool Manual.

 

This function asks libltdl to call the function unload_ltmodule for each of the modules it has loaded, along with some details of the module it wants to unload. The tricky part of the callback function below is recalculating the entry point addresses for the module to be unloaded and then removing all matching addresses from the appropriate internal structures. Otherwise, the balance of this callback is involved in informing the calling lt_dlforeach loop of whether a matching module has been found and handled:

 

The userdata_address_compare helper function at the end is used to compare the address of recalculated entry points against the already registered functions and handlers to find which items need to be unregistered.

There is also a matching header file to export the module interface, so that the code for loadable modules can make use of it:

 

This header also includes some of the other Sic headers, so that in most cases, the source code for a module need only ‘#include <sic/module.h>’.

To make the module loading interface useful, I have added built-ins for ‘load’ and ‘unload’. Naturally, these must be compiled into the bare sic executable, so that it is able to load additional modules:

 

These new built-in commands are simply wrappers around the module loading code in ‘module.c’.

As with ‘dlopen’, you can use libltdl to ‘lt_dlopen’ the main executable, and then lookup its symbols. I have simplified the initialisation of Sic by replacing the sic_init function in ‘src/sic.c’ by ‘loading’ the executable itself as a module. This works because I was careful to use the same format in ‘sic_builtin.c’ and ‘sic_syntax.c’ as would be required for a genuine loadable module, like so:

 

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