Lua coding guide

5. Standard libraries

package.cpath

The path used by require to search for a C loader.

Lua initializes the C path package.cpath in the same way it initializes the Lua path package.path, using the environment variable LUA_CPATH or a default path defined in luaconf.h.

--------------------------------------------------------------------------------

package.loaded

A table used by require to control which modules are already loaded. When you require a module modname and package.loaded[modname] is not false, require simply returns the value stored there.

--------------------------------------------------------------------------------

package.loaders

A table used by require to control how to load modules.

Each entry in this table is a searcher function. When looking for a module, require calls each of these searchers in ascending order, with the module name (the argument given to require) as its sole parameter. The function can return another function (the module loader) or a string explaining why it did not find that module (or nil if it has nothing to say). Lua initializes this table with four functions.

The first searcher simply looks for a loader in the package.preload table.

The second searcher looks for a loader as a Lua library, using the path stored at package.path. A path is a sequence of templates separated by semicolons. For each template, the searcher will change each interrogation mark in the template by filename, which is the module name with each dot replaced by a "directory separator" (such as "/" in Unix); then it will try to open the resulting file name. So, for instance, if the Lua path is the string

"./?.lua;./?.lc;/usr/local/?/init.lua"

the search for a Lua file for module foo will try to open the files ./foo.lua, ./foo.lc, and /usr/local/foo/init.lua, in that order.

The third searcher looks for a loader as a C library, using the path given by the variable package.cpath. For instance, if the C path is the string

"./?.so;./?.dll;/usr/local/?/init.so"

the searcher for module foo will try to open the files ./foo.so, ./foo.dll, and /usr/local/foo/init.so, in that order. Once it finds a C library, this searcher first uses a dynamic link facility to link the application with the library. Then it tries to find a C function inside the library to be used as the loader. The name of this C function is the string "luaopen_" concatenated with a copy of the module name where each dot is replaced by an underscore. Moreover, if the module name has a hyphen, its prefix up to (and including) the first hyphen is removed. For instance, if the module name is a.v1-b.c, the function name will be luaopen_b_c.

The fourth searcher tries an all-in-one loader. It searches the C path for a library for the root name of the given module. For instance, when requiring a.b.c, it will search for a C library for a. If found, it looks into it for an open function for the submodule; in our example, that would be luaopen_a_b_c. With this facility, a package can pack several C submodules into one single library, with each submodule keeping its original open function.

--------------------------------------------------------------------------------

package.loadlib (libname, funcname)

Dynamically links the host program with the C library libname. Inside this library, looks for a function funcname and returns this function as a C function. (So, funcname must follow the protocol (see lua_CFunction)).