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construct/modules/README.md
Jason Volk 14040f4917 modules: Simplify comment.
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IRCd Module Tree

This directory contains dynamically loadable functionality to libircd. Many of these modules provide essential application functionality, but are not always required to be directly linked and loaded into libircd proper. Most application- specific functionality (i.e "business logic") is contained in modules within this tree.

Layout

The modules/ directory tree is primarily shaped the same as the HTTP resource tree in which most of its modules register themselves in.

Note that the installation layout is not the same as the development source layout (i.e in git). Upon installation, the module tree is collapsed into a single directory and installed into $prefix/lib/modules/construct/$directory_$module.so; where directories are replaced by underscores in the final SONAME. this may be subject to improvement.

Approach

Unlike most of the module systems found in traditional free software projects, our approach is oriented around global symbol availability to the address space rather than explicit imports from self-contained modules. This direction is made viable by C++ and advances in the compiler and linker toolchains. The result is significantly simpler and more convenient for developers to work with.

  • Modules are loaded with RTLD_GLOBAL on both posix and windows platforms. Use of C++ namespaces, visibility attributes, STB_GNU_UNIQUE, etc are adequate to make this modernization.

  • All project code is built to silently weaken undefined symbols. This means a complicated interface declared in a header, like a class interface with public and private and static members -- typical in C++ -- can be included by itself into any part of the project without knowing where the definitions of that interface are until they are first used at runtime. If said definitions are not available because they are in an unloaded module, a C++ exception is thrown directly from the callsite.

Getting started

The header mods/mapi.h is specific to modules and included for modules in addition to the core ircd.h. Both of these are included automatically via the compiler's command-line and the developer does not have to #include either in the module.

Every loadable module requires a static ircd::mapi::header with the explicit name of IRCD_MODULE. This is an object which will be sought by the module loader in libircd. Nothing else is required.