LuaRocks Wiki: Creating a rock

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So, you wrote a Lua package (containing one or more modules) and want to make it available to users through LuaRocks. The process consists essentially of the following steps:

* Writing a rockspec file
* Publishing your code online
* Submitting a rockspec for inclusion in the rocks server

== Writing a rockspec ==

A rockspec file is the metadata file for your package, containing all the information LuaRocks needs in order to fetch, build and install your package. The [[Rockspec format]] supports various kind of build systems, but in this tutorial we'll use LuaRocks's own built-in build system -- that's why we're listing "Writing a rockspec" as the first step. We'll use the rockspec in place of a Makefile.

A rockspec is actually a Lua file, but it is loaded in an empty environment, so there are no Lua functions available. A skeleton for a basic rockspec looks like this:

package = "LuaFruits"
 version = "1.0-1"
 source = {
    url = "http://..." -- We don't have one yet
 }
 description = {
    summary = "An example for the LuaRocks tutorial.",
    detailed = [[
       This is an example for the LuaRocks tutorial.
       Here we would put a detailed, typically
       paragraph-long description.
    ]],
    homepage = "http://...", -- We don't have one yet
    license = "MIT/X11" -- or whatever you like
 }
 dependencies = {
    "lua >= 5.1"
    -- If you depend on other rocks, add them here
 }
 build = {
    -- We'll start here.
 }

This should be saved in a file called luafruits-1.0-1.rockspec. (The name must be a lowercase version of the "package" and "version" fields, or else LuaRocks will complain.)

There are some missing stuff in our rockspec which we will fill later, such as the source.url and source.homepage fields. We'll add those when we upload our sources to Github (or whatever server you choose to).

Right now our focus will be on the "build" section.

=== Building a module ===

LuaRocks supports a number of "build types": "make" for using Makefiles, "cmake" for using CMake, etc. In this tutorial, however, we'll use its built-in build system, called the "builtin" type.

build = {
    type = "builtin"
    -- Now we need to tell it what to build.
 }

In the "builtin" type, we add a subtable called "modules", in which keys are module names in Lua notation, and values indicate how to build them. This example shows the various possibilities supported:

build = {
    type = "builtin",
    modules = {
 
       -- A simple module written in Lua
       apricot = "src/apricot.lua",
 
       -- Note the required Lua syntax when listing submodules as keys
       ["apricot.seeds"] = "src/apricot/seeds.lua",
 
       -- A simple module written in C
       banana = "src/banana.c",
 
       -- C modules can span multiple files.
       cherry = {"src/cherry.c", "src/cherry_pie.c"},
 
       -- C modules also support an extended syntax, supporting
       -- cross-platform specifications of C defines, libraries and
       -- paths for external dependencies.
       date = {
          sources = {"src/date.c", "src/cali_date.c", "src/arab_date.c"},
          defines = {"MAX_DATES_PER_MEAL=50"}
          libraries = {"date"},
          incdirs = {"$(LIBDATE_INCDIR)"},
          libdirs = {"$(LIBDATE_LIBDIR)"}
       }
    }
 }

Since modules written in Lua do not need to be compiled, LuaRocks only needs to know where they are in your source directory tree  in order to copy them to the proper place on installation (see the "apricot" and "apricot.seeds" in the above example).

Similarly, for C code with no dependencies on external libraries, it suffices to say where the sources are and LuaRocks will invoke the appropriate compiler and linker for the platform. You can use a simple string value for single-file modules, such as the "banana" example above or an array value for multiple source files (as in the "cherry" example).

==== C modules linking to external libraries ====

:''If your code does not use third-party libraries, you may skip this subsection.''

For building C code that links to C libraries, you can use the long syntax given in the "date" example above, in which sources are listed in the "sources" subtable. You need to specify the libraries to be linked in the "libraries" subtable. The library name is specified in a platform-independent way: in the above example, libraries={"date"} will result in -ldate for GCC on Unix and DATE.LIB for MSVC on Windows. (Note that if this is not appropriate, the rockspec format allows [[Rockspec format|per-platform overrides]].) If you need to link code that uses libraries, you need to tell LuaRocks where to find them. You do this by adding a new section to the rockspec:

....
 dependencies = {
    "lua >= 5.1"
    -- If you depend on other rocks, add them here
 }
 external_dependencies = {
    LIBDATE = {
       header = "libdate.h"
    }
 }
 build = {
 ....

Adding the "external_dependencies" table will make LuaRocks search for the external dependencies in its lookup path (on Unix the defaults are /usr/local and /usr; on Windows, which doesn't have a standard for development trees, you'll probably have to specify  yourself through the [[Config file format|config file]] or the [[luarocks|command-line]] when invoking "luarocks"). We give a hint to LuaRocks, the libdate.h header, so it can test whether the development package is really there (on many Linux distros, one needs to install "-dev" packages in order to have all headers and libraries needed for compilation available, so header files are a good hint). In this case, for example, it would look for /usr/local/include/libdate.h and /usr/include/libdate.h on Unix. If you (or your users) have LibDate installed elsewhere, it's always possible to tell LuaRocks so through the command-line, passing for example LIBDATE_DIR=/opt/libdate as an argument.

When LuaRocks succeeds finding an external dependency, it creates special variables for it which can be used in incdirs and libdirs fields. The example above shows two such variables, LIBDATE_INCDIR and LIBDATE_LIBDIR being use. It's important to always pass those variables: if LibDate happened to be in the system lookup path of your compiler, compilation would succeed without those variables, but they would fail in a user's system where they are somewhere else, such as in the LIBDATE_DIR=/opt/libdate example given earlier.

=== Using LuaRocks as a build system ===

Now that your build section is written, you can use LuaRocks as a build system and have it compile and install your code as a rock. Just type:

luarocks make

and it will do its thing. Like "make", it will look for the appropriate rules file (in our case, the rockspec) in the current directory and then will proceed to build and install the package in your rocks tree, assuming the sources are in the current directory as well.

== Publishing your code online ==

Now, to complete the rockspec for public consumption we need to fill the sources.url field. For that, we need the code to be available online. By default, LuaRocks expects the code to be packed in a zip or tarball containing a directory with the lowercase name of the package and its version number (without the LuaRocks revision number). In our example, we should make a tarball with the following contents:

luafruits-1.0/
 luafruits-1.0/src/apricot.lua
 luafruits-1.0/src/apricot/seeds.lua
 luafruits-1.0/src/banana.c
 luafruits-1.0/src/cherry.c
 luafruits-1.0/src/cherry_pie.c
 luafruits-1.0/src/date.c
 luafruits-1.0/src/cali_date.c
 luafruits-1.0/src/arab_date.c

Note that the rockspec itself doesn't need to be stored in the tarball (actually, storing the final rockspec inside the package is a chicken-egg problem because rockspecs can contain the source.md5 field with a checksum of the tarball).

It's important to note that LuaRocks does not enforce the internal structure. Our example only has a src/ subdirectory because the build rules listed above used it (we could have done without it if we wanted to). The tarball could contain other files as well, such as licenses and documentation. The only convention used here is the top level directory name, and if desired this can be overridden in the rockspec using the sources.dir field. So, now we pack our code. On Unix, for example, that would be:

tar czvpf luafruits-1.0.tar.gz luafruits-1.0/

And now we're ready to publish it online. A good option is to create a project in [http://github.com/ Github]. Once your tarball is uploaded there, we're ready to complete our rockspec. By now it will look like this:

package = "LuaFruits"
 version = "1.0-1"
 source = {
    url = "http://github.com/downloads/me/luafruits/luafruits-1.0.tar.gz"
 }
 description = {
    summary = "An example for the LuaRocks tutorial.",
    detailed = [[
       This is an example for the LuaRocks tutorial.
       Here we would put a detailed, typically
       paragraph-long description.
    ]],
    homepage = "http://me.github.com/luafruits",
    license = "MIT/X11"
 }
 dependencies = {
    "lua >= 5.1"
 }
 external_dependencies = {
    LIBDATE = {
       header = "libdate.h"
    }
 }
 build = {
    type = "builtin",
    modules = {
       apricot = "src/apricot.lua",
       ["apricot.seeds"] = "src/apricot/seeds.lua",
       banana = "src/banana.c",
       cherry = {"src/cherry.c", "src/cherry_pie.c"},
       date = {
          sources = {"src/date.c", "src/cali_date.c", "src/arab_date.c"},
          defines = {"MAX_DATES_PER_MEAL=50"}
          libraries = {"date"},
          incdirs = {"$(LIBDATE_INCDIR)"},
          libdirs = {"$(LIBDATE_LIBDIR)"}
       }
    }
 }

== Submitting a rockspec for inclusion in the rocks server ==

This is the simplest step: just send the rockspec to the LuaRocks [[Mailing list|mailing list]] and we'll include it in the public LuaRocks repository. We will usually publish both the .rockspec file and the appropriate .rock file in the server.

You can also create .rock files yourself. If you run the "pack" command on your rockspec, like this:

luarocks pack luafruits-1.0-1.rockspec

you will get a file called luafruits-1.0-1.src.rock, which contains the rockspec itself and your sources. If you have the rock installed in your local tree, you can also pack the binaries you just built:

luarocks pack luafruits

(The version is optional, latest is assumed.) This will create a file with a name such as luafruits-1.0-1.linux-x86.rock.

== Conclusion ==

And we're done -- by writing a simple rules file which simply describes which sources compose the project and which libraries they use, we actually achieved a lot behind the scenes: LuaRocks takes care of using the right compiler for your platform (GCC? MSVC? Not mingw though), passing the right flags (-fpic? -shared? -bundle -undefined dynamic_lookup?) and checking external dependencies (/usr/lib? /usr/local/lib? Is it really there?). Getting all these little portability details right is not always easy to do on hand-written Makefiles, so that's why we recommend using LuaRocks's own build system whenever possible. There are many other features of LuaRocks we haven't covered in this tutorial (per-platform overrides, support for command-line scripts, etc.), but this should get you started -- browse around the wiki for more.

And of course, we're eager to receive all kinds of feedback in the LuaRocks [[Mailing list|mailing list]].

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