lftp 2.0 and later support loading modules (shared objects) at
runtime. Use command `module' to load a module.

It also supports loading certain modules (some of protocols and commands)
automatically on demand. To compile modular lftp use:

   configure --with-modules

You will need GCC and ELF platform (linux, freebsd-3.x, solaris, irix etc).


Below are the technical details.

Module is a shared object, after loading it with dlopen(3) lftp does
dlsym("module_init") and calls this function with parameters argc, argv:

   extern "C"
   void module_init(int argc, const char * const *argv);

The argv vector contains the arguments passed to `module' command after
module name. In case of loading module on demand it is empty.

Note: function _init of a module is called automatically by dlopen. It can
execute constructors if the module is properly compiled with `gcc -shared'.

To load modules on demand lftp uses protocol or command name to find
module file. For protocols it looks for proto-<PROTOCOL>.so and for
commands -- cmd-<COMMAND>.so. The modules register the protocols and
commands they provide with functions FileAccess::Register and
CmdExec::RegisterCommand.

lftp searches module for any protocol specified in URL in open command,
and only for certain compile time defined set of commands -- the commands
that have NULL instead of function pointer in command table in commands.cc.