package.searchers

package.searchers

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) plus an extra value that will be passed to that 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 searcher 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. The search is done as described in function package.searchpath.

The third searcher looks for a loader as a C library, using the path given by the variable package.cpath. Again, the search is done as described in function package.searchpath. 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 suffix after (and including) the first hyphen is removed. For instance, if the module name is a.b.c-v2.1, the function name will be luaopen_a_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.

All searchers except the first one (preload) return as the extra value the file name where the module was found, as returned by package.searchpath. The first searcher returns no extra value.