Many Lua rocks are bindings to C libraries: for example, luaossl is a binding library to the OpenSSL library. This means that, in our example, luaossl depends on OpenSSL. But since this is not a regular rock-to-rock dependency (if it were, LuaRocks could solve this by itself), we call this an external dependency.

When building a rock with external dependencies, LuaRocks needs to make sure the necessary C libraries and headers are installed, and know where those C libraries are.

When writing a rock with external dependencies, one needs to be careful to avoid the "works on my machine" situation: when you write code that hardcodes the location of external dependencies as they are in your system, but then fails to build in other people's systems.

The way to avoid this problem is to avoid any hardcoded paths. Instead, the build section of a rockspec should make use of path variables. If not using the builtin build type, paths set by the module's own build system must not be relied on and explicit paths should be passed to it by the rockspec instead.

Paths where LuaRocks should install files into are defined as the PREFIX, LUADIR, LIBDIR and BINDIR variables. (See the Config file format page for details.)

Paths for external dependencies (such as C libraries used in the compilation of modules) are generated from the external_dependencies section of the rockspec (see the Rockspec format page for details.) Lua itself is considered a special external dependency.

These path variables are set in the global variables table defined in the config file. Their values are populated automatically by LuaRocks, but can be overriden by the user (either by setting them directly in the config file, or by passing them through the "luarocks" command line).

The variables table always contains entries for LUA_BINDIR, LUA_INCDIR and LUA_LIBDIR. Like the other external dependency variables, these can be overriden in the LuaRocks config file or in the command line.