schedlua – refactor compactor

The subject of scheduling and async programming has been a long running theme in my blog.  From the very first entries related to LJIT2Win32, through the creation of TINN, and most recently (within the past year), the creation of schedlua, I have been exploring this subject.  It all kind of started innocently enough.  When node.js was born, and libuv was ultimately released, I thought to myself, ‘what prevents anyone from doing this in LuaJIT without the usage of any external libraries whatsovever?’

It’s been a long road.  There’s really no reason for this code to continue to evolve.  It’s not at the center of some massively distributed system.  These are merely bread crumbs left behind, mainly for myself, as I explore and evolve a system that has proven itself to be useful at least as a teaching aid.

In the most recent incarnation of schedlua kernel, I was able to clean up my act with the realization that you can implement all higher level semantics using a very basic ‘signal’ mechanism within the kernel.  That was pretty good as it allowed me to easily implement the predicate system (when, whenever, waitForTruth, signalOnPredicate).  In addition, it allowed me to reimplement the async io portion with the realization that a task waiting on IO to occur is no different than a task waiting on any other kind of signal, so I could simply build the async io atop the signaling.

schedlua has largely been a Linux based project, until now.  The crux of the difference between Linux and Windows comes down to two things in schedlua.  The first thing is timing operations.  Basically, how do you get a microsecond accurate clock on the system.  On Linux, I use the ‘clock_gettime()’ system call.  On Windows, I use ‘QueryPerformanceCounter, QueryPerformanceFrequency’.  In order to isolate these, I put them into their own platform specific timeticker.lua file, and they both just have to surface a ‘seconds()’ function.  The differences are abstracted away, and the common interface is that of a stopwatch class.

That was good for time, but what about alarms?

The functions in schedlua related to alarms, are: delay, periodic, runnintTime, and sleep.  Together, these allow you to run things based on time, as well as delay the current task as long as you like.  My first implementation of these routines, going all the way back to the TINN implementation, were to run a separate ‘watchdog’ task, which in turn maintained its list of tasks that were waiting, and scheduled them.  Recently, I thought, “why can’t I just use the ‘whenever’ semantics to implement this?”.

Now, the implementation of the alarm routines comes down to this:


local function taskReadyToRun()
	local currentTime = SWatch:seconds();

	-- traverse through the fibers that are waiting
	-- on time
	local nAwaiting = #SignalsWaitingForTime;

	for i=1,nAwaiting do
		local task = SignalsWaitingForTime[1];
		if not task then
			return false;

		if task.DueTime <= currentTime then
			return task
			return false

	return false;

local function runTask(task)
    table.remove(SignalsWaitingForTime, 1);

Alarm = whenever(taskReadyToRun, runTask)

The Alarm module still keeps a list of tasks that are waiting for their time to execute, but instead of using a separate watchdog task to keep track of things, I simply use the schedlua built-in ‘whenever’ function. This basically says, “whenever the function ‘taskReadyToRun()’ returns a non-false value, call the function ‘runTask()’ passing the parameter from taskReadyToRun()”. Convenient, end of story, simple logic using words that almost feel like an English sentence to me.

I like this kind of construct for a few reasons. First of all, it reuses code. I don’t have to code up that specialized watchdog task time and time again. Second, it wraps up the async semantics of the thing. I don’t really have to worry about explicitly calling spawn, or anything else related to multi-tasking. It’s just all wrapped up in that one word ‘whenever’. It’s relatively easy for me to explain this code, without mentioning semaphores, threads, conditions, or whatever. I can tell a child “whenever this is true, do that other thing”, and they will understand it.

So, that’s it. First I used signals as the basis to implement higher order functions, such as the predicate based flow control. Now I’m using the predicate based flow control to implement yet other functions such as alarms. Next, I’ll take that final step and do the same to the async IO, and I’ll be back to where I was a few months back, but with a much smaller codebase, and cross platform to boot.

SVG And Me – Don’t tell me, just another database!

A picture is worth 175Kb…


So, SVG right? Well, the original was, but this image was converted to a .png file for easy embedding in WordPress. The file size of the original grapes.svg is 75K. That savings in space is one of the reasons to use .svg files whenever you can.

But, I digress. The remotesvg project has been moving right along.

Last time around, I was able to use Lua syntax as a stand in for the raw .svg syntax.  That has some benefits because since your in a programming language, you can use programming constructs such as loops, references, functions and the like to enhance the development of your svg.  That’s great when you’re creating something from scratch programmatically, rather than just using a graphical editing tool such as inkscape to construct your .svg.  If you’re constructing a library of svg handling routines, you need a bit more though.

This time around, I’m adding in some parsing of svg files, as well as general manipulation of the same from within Lua.  Here’s a very simple example of how to read an svg file into a lua table:


local parser = require("remotesvg.parsesvg")

local doc = parser:parseFile("grapes.svg");

That’s it! You now have the file in a convenient lua table, ready to be manipulated. But wait, what do I have exactly? Let’s look at a section of that file and see what it gives us.

         id="stop4894" />
         id="stop4896" />
       y2="1093.8906" />

This is part of the definitions, which later get used on portions of representing the grapes. A couple of things to notice. As a straight ‘parsing’, you’ll get a bunch of text values. For example: y2 = “109.8906”, that will turn into a value in the lua table like this: {y2 = “109.8906”}, the ‘109.8906’ is still a string value. That’s useful, but a little less than perfect. Sometimes, depending on what I’m doing, retaining that value as a string might be just fine, but sometimes, I’ll want that value to be an actual lua number. So, there’s an additional step I can take to parse the actual attributes values and turn them into a more native form:

local parser = require("remotesvg.parsesvg")

local doc = parser:parseFile("grapes.svg");


That line with doc:parseAttributes(), tells the document to go through all its attributes and parse them, turning them into more useful values from the Lua perspective. In the case above, the representation of ‘y2’ would become: {y2 = 109.8906}, which is a string value.

This gets very interesting when you have values where the string representation and the useful lua representation are different.

<line x1="10", y1="20", width = "10cm", height= "12cm" />

This will be turning into:

  x1 = {value = 10},
  y1 = {value = 20},
  width = {value = 10, units = 'cm'},
  height = {value = 12, units = 'cm'}

Now, in my Lua code, I can access these values like so:

local doc = parser:parseFile("grapes.svg");

When I subsequently want to write this value out as valid svg, it will turn back into the string representation with no loss of fidelity.

Hidden in this example is a database query. How do I know that doc.svg[1] is going to give me the ” element that I’m looking for? In this particular case, it’s only because the svg is so simple that I know for a fact that the ” element is going to show up as the first child in the svg document. But, most of the time, that is not going to be the case.

In any svg that’s of substance, there is the usage of various ‘id’ fields, and that’s typically what is used to find an element. So, how to do that in remotesvg? If we look back at the example svg, we see this ‘id’ attribute on the first gradient: id=”linearGradient4892″.

How could I possibly find that gradient element based on the id field? Before that though, let’s look at how to enumerate elements in the document in the first place.

local function printElement(elem)
    if type(elem) == "string" then
        -- don't print content values
    print(string.format("==== %s ====", elem._kind))

    -- print the attributes
    for name, value in elem:attributes() do

local function test_selectAll()
    -- iterate through all the nodes in 
    -- document order, printing something interesting along
    -- the way

    for child in doc:selectAll() do

Here is a simple test case where you have a document already parsed, and you want to iterate through the elements, in document order, and just print them out. This is the first step in viewing the document as a database, rather than as an image. The working end of this example is the call to ‘doc:selectAll()’. This amounts to a call to an iterator that is on the BasicElem class, which looks like this:

	Traverse the elements in document order, returning
	the ones that match a given predicate.
	If no predicate is supplied, then return all the
function BasicElem.selectElementMatches(self, pred)
	local function yieldMatches(parent, predicate)
		for idx, value in ipairs(parent) do
			if predicate then
				if predicate(value) then

			if type(value) == "table" then
				yieldMatches(value, predicate)

  	return coroutine.wrap(function() yieldMatches(self, pred) end)	

-- A convenient shorthand for selecting all the elements
-- in the document.  No predicate is specified.
function BasicElem.selectAll(self)
	return self:selectElementMatches()

As you can see, ‘selectAll()’ just turns around and calls ‘selectElementMatches()’, passing in no parameters. The selectElementMatches() function then does the actual work. In Lua, there are a few ways to create iterators. In this particular case, where we want to recursive traverse down a hierarchy of nodes (document order), it’s easiest to use this coroutine method. You could instead keep a stack of nodes, pushing as you go down the hierarchy, popping as you return back up, but this coroutine method is much more compact to code, if a bit harder to understand if you’re not used to coroutines. The end result is an iterator that will traverse down a document hierarchy, in document order.

Notice also that the ‘selectElementMatches’ function takes a predicate. A predicate is simply a function that takes a single parameter, and will return ‘true’ or ‘false’ depending on what it sees there. This will become useful.

So, how to retrieve an element with a particular ID? Well, when we look at our elements, we know that the ‘id’ field is one of the attributes, so essentially, what we want to do is traverse the document looking for elements that have an id attribute that matches what we’re looking for.

function BasicElem.getElementById(self, id)
    local function filterById(entry)
        print("filterById: ",, id)
        if == id then
            return true;

    for child in self:selectMatches(filterById) do
        return child;

Here’s a convenient function to do just that. And to use it:

local elem = doc:getElementById("linearGradient10460")

That will retrieve the second linear gradient of the pair of gradients from our svg fragment. That’s great! And the syntax is looking very much like what I might write in javascript against the DOM. But, it’s just a database!

Given the selectMatches(), you’re not just limited to querying against attribute values. You can get at anything, and form as complex queries as you like. For example, you could find all the elements that are deep green, and turn them purple with a simple query loop.

Here’s an example of finding all the elements of a particular kind:

local function test_selectElementMatches()
    print("<==== selectElementMatches: entry._kind == 'g' ====>")
	for child in doc:selectElementMatches(function(entry) if entry._kind == "g" then return true end end) do

Or finding all the elements that have a ‘sodipodi’ attribute of some kind:

local function test_selectAttribute()
    -- select the elements that have an attribute
    -- with the name 'sodipodi' in them
    local function hasSodipodiAttribute(entry)
        if type(entry) ~= "table" then
            return false;

        for name, value in entry:attributes() do
            --print("hasSodipodi: ", entry._kind, name, value, type(name))
            if name:find("sodipodi") then
                return true;

        return false

    for child in doc:selectElementMatches(hasSodipodiAttribute) do
        if type(child) == "table" then

Of course, just finding these elements is one thing. Once found, you can use this to filter out those elements you don’t want. for example, eliminating the ones that are inkscape specific.

Well, there you have it. First, you can construct your svg programmatically using Lua syntax. Alternatively, you can simply parse a svg file into a lua structure. Last, you can query your document, no matter how it was constructed, for fun and profit.

Of course, the real benefit of being able to parse, and find elements and the like, is it makes manipulating the svg that much easier. Find the node that represents the graph of values, for example, and change those values over time for some form of animation…

SVG And Me


That’s a simple linear gradient, generated from an SVG document that looks like this:


<svg viewBox = '0 0 120 120' version = '1.1' xmlns = ''   width = '120' height = '120' xmlns:xlink = ''>
    	<linearGradient id = 'MyGradient'>
      <stop stop-color = 'green' offset = '5%' />
      <stop stop-color = 'gold' offset = '95%' />
  <rect x = '10' y = '10' height = '100' fill = 'url(#MyGradient)' width = '100' />


Fair enough. And of course there are a thousand and one ways to generate .svg files. For various reasons, I am interested in generating .svg files on the fly in a Lua context. So, the code I used to generate this SVG document looks like this:

local FileStream = require("remotesvg.filestream")
local SVGStream = require("remotesvg.SVGStream")

local ImageStream = SVGStream("test_lineargradient.svg"))

local doc = svg {
	width = "120",
	height = "120",
	viewBox = "0 0 120 120",
    ['xmlns:xlink'] ="",

        linearGradient {id="MyGradient",
            stop {offset="5%",  ['stop-color']="green"};
            stop {offset="95%", ['stop-color']="gold"};

    rect {
        x=10, y=10, width=100, height=100,


This comes from my remotesvg project. If you squint your eyes, these look fairly similar I think. In the second case, it’s definitely valid Lua script. Mostly it’s nested tables with some well known types. But, where are all the parenthesis, and how can you just put a name in front of ‘{‘ and have that do anything?

OK, so Lua has some nice syntactics tricks up its sleeve that make certain things a bit easier. For example, there’s this trick that if there’s only a single parameter to a function, you can leave off the ‘()’ combination. I’ve mentioned this before way long back when I was doing some Windows code, and supporting the “L” compiler thing for unicode literals.

In this case, it’s about tables, and later we’ll see about strings. The following two things are equivalent:

local function myFunc(tbl)
  for k,v in pairs(tbl) do

myFunc({x=1, y=1, id="MyID"})

-- Or this slightly shorter form

myFunc {x=1, y=1, id="MyID"}

OK. So that’s how we get rid of those pesky ‘()’ characters, which don’t add to the conversation. In lua, since tables are a basic type, I can easily include tables in tables, nesting as deeply as I please. So, what’s the other trick here then? The fact that all those things before the ‘{‘ are simply the names of tables. This is one area where a bit of trickery goes a long way. I created a ‘base type’ if you will, which knows how to construct these tables from a function, and do the nesting, and ultimately print out SVG. It looks like this:


	A base type for all other SVG Elements.
	This can do the basic writing
local BasicElem = {}
setmetatable(BasicElem, {
	__call = function(self, ...)
		return self:new(...);
local BasicElem_mt = {
	__index = BasicElem;

function, kind, params)
	local obj = params or {}
	obj._kind = kind;

	setmetatable(obj, BasicElem_mt);

	return obj;

-- Add an attribute to ourself
function BasicElem.attr(self, name, value)
	self[name] = value;
	return self;

-- Add a new child element
function BasicElem.append(self, name)
	-- based on the obj, find the right object
	-- to represent it.
	local child = nil;

	if type(name) == "table" then
		child = name;
	elseif type(name) == "string" then
		child = BasicElem(name);
		return nil;

	table.insert(self, child);

	return child;

function BasicElem.write(self, strm)

	local childcount = 0;

	for name, value in pairs(self) do
		if type(name) == "number" then
			childcount = childcount + 1;
			if name ~= "_kind" then
				strm:writeAttribute(name, tostring(value));

	-- if we have some number of child nodes
	-- then write them out 
	if childcount > 0 then
		-- first close the starting tag

		-- write out child nodes
		for idx, value in ipairs(self) do
			if type(value) == "table" then
				-- write out pure text nodes

And further on in the library, I have things like this:

defs = function(params) return BasicElem('defs', params) end;

So, ‘defs’ is a function, which takes a single parameter (typically a table), and it constructs an instance of the BasicElem ‘class’, handing in the name of the element, and the specified ‘params’. And that’s that…

BasicElem has a function ‘write(strm)’, which knows how to turn the various values and tables it contains into correct looking SVG elements and attributes. It’s all right there in the write() function. In addition, it adds a couple more tidbits, such as the attr() and append() functions.

Now that these basic constructs exist, what can be done? Well, first off all, every one of the SVG elements is covered with the simple construct we see with the ‘defs’ element. How might you used this:

	local doc = svg {
		width = "12cm", 
		height= "4cm", 
		viewBox="0 0 1200 400",

		:attr("x", 1)
		:attr("y", 2)
		:attr("width", 1198)
		:attr("height", 398)
		:attr("fill", "none")
		:attr("stroke", "blue")
		:attr("stroke-width", 2);

   local l1 = line({x1=100, y1=300, x2=300, y2=100, stroke = "green", ["stroke-width"]=5});
   local l2 = line({x1=300, y1=300, x2=500, y2=100, stroke = "green", ["stroke-width"]=20});
   local l3 = line({x1=500, y1=300, x2=700, y2=100, stroke = "green", ["stroke-width"]=25});
   local l4 = line({x1=700, y1=300, x2=900, y2=100, stroke = "green", ["stroke-width"]=20});
   local l5 = line({x1=900, y1=300, x2=1100, y2=100, stroke = "green", ["stroke-width"]=25});


In this case, instead of doing the ‘inlined table document’ style of the first example, I’m doing more of a ‘programmatic progressive document building’ style. I create the basic ‘svg’ element and save it in the doc variable. Then I use the ‘append()’ function, to create a ‘rect’ element. On that same element, I can use a short hand to add it’s attributes. Then, I can create separate ‘line’ elements, and append them onto the document as well. That’s pretty special if you need to construct the document based on some data you’re seeing, and you can’t use the embedded table style up front.

There are some special elements that get extra attention though. Aside from the basic table construction, and attribute setting, the ‘path’ element has a special retained mode graphics building capability.

	local p1 = path {
	p1:moveTo(100, 100);
	p1:lineTo(300, 100);
	p1:lineTo(200, 300);

	local doc = svg {
		viewBox="0 0 400 400",
		rect {
			x="1", y="1", 
			width="398", height="398",
        	fill="none", stroke="blue"};

In this case, I create my ‘path’ element, and then I use its various path construction functions such as ‘moveTo()’, and ‘lineTo()’. There’s the full set of arc, bezier curvs, and the like, so you have all the available path construction commands. Again, this works out fairly well when you are trying to build something on the fly based on some previously unknown data.

There’s one more important construct, and that’s string literals. There are cases where you might want to do something that this easy library just doesn’t make simple. In those cases, you might just want to embed some literal text into the output document. Well, luckily, Lua has a fairly easy ability to indicate single or multi-line text, and the BasicElem object knows what to do if it sees it.

    g {

      <text x="48" y="48">Test a motion path</text> 
      <text x="48" y="95" fill="red">'values' attribute.</text> 
      <path d="M90,258 L240,180 L390,180" fill="none" stroke="black" stroke-width="6" /> 
      <rect x="60" y="198" width="60" height="60" fill="#FFCCCC" stroke="black" stroke-width="6" /> 
      <text x="90" y="300" text-anchor="middle">0 sec.</text> 
      <rect x="210" y="120" width="60" height="60" fill="#FFCCCC" stroke="black" stroke-width="6" /> 
      <text x="240" y="222" text-anchor="middle">3+</text> 
      <rect x="360" y="120" width="60" height="60" fill="#FFCCCC" stroke="black" stroke-width="6" /> 
      <text x="390" y="222" text-anchor="middle">6+</text> 

      path {
        d="M-30,0 L0,-60 L30,0 z", 
        animateMotion {values="90,258;240,180;390,180", begin="0s", dur="6s", calcMode="linear", fill="freeze"} 

Notice the portion after the ‘font-size’ attribute is a Lua multi-line string literal. This section will be incuded in the form document verbatim. Another thing to notice here is that ‘path’ element. Although path is specialized, it still has the ability to have attributes, and even have child nodes of its own, such as for animation.

Another case where the literals may come in handy is for CSS style sheets.

	defs {
		style {type="text/css",
				fill: #CCCCCC;
				fill-opacity: 1;
				stroke-opacity: 1;

The ‘style’ element is well known, but the format of the actual content is a bit too specific to translate into a Lua form, so it can simply be included as a literal.

Well, that’s the beginning of this journey. Ultimately I want to view some live graphics generated from data, and send some commands back to the server to perform some functions. At this point, I can use Lua to generate the SVG on the fly, and there isn’t an SVG parser, or Javascript interpreter in sight.

Spelunking Linux – procfs or is that sysctl?

Last time around, I introduced some simple things with lj2procfs.  Being able to simply access the contents of the various files within procfs is a bit of convenience.  Really what lj2procfs is doing is just giving you a common interface to the data in those files.  Everything shows up as simple lua values, typically tables, with strings, and numbers.  That’s great for most of what you’d be doing with procfs, just taking a look at things.

But, on Linux, procfs has another capability.  The /proc/sys directory contains a few interesting directories of its own:



And if you look into these directories, you find some more interesting files. For example, in the ‘kernel/’ directory, we can see a little bit of this:


Now, these are looking kind of interesting. These files contain typically tunable portions of the kernel. On other unices, these values might be controlled through the sysctl() function call. On Linux, that function would just manipulate the contents of these files. So, why not just use lj2procfs to do the same.

Let’s take a look at a few relatively simple tasks. First, I want to get the version of the OS running on my machine. This can be obtained through the file /proc/sys/kernel/version

local procfs = require("lj2procfs.procfs")

$ #15-Ubuntu SMP Thu Apr 16 23:32:37 UTC 2015

This is the same string returned from the call ‘uname -v’

And, to get the hostname of the machine:

$ ubuntu

Which is what the ‘hostname’ command returns on my machine.

And what about setting the hostname? First of all, you’ll want to do this as root, but it’s equally simple:

procfs.sys.kernel.hostname = 'alfredo'

Keep in mind that setting the hostname in this way is transient, and it will seriously mess up things, like your about to sudo after this. But, there you have it.

Any value under /proc/sys can be retrieved or set using the fairly simple mechanism. I find this to be very valuable for two reasons. First of all, spelunking these values makes for great discovery. More importantly, being able to capture and set the values makes for a fairly easily tunable system.

An example of how this can be used for system diagnostics and tuning, you can capture the kernel values, using a simple command that just dumps what you want into a table. Send that table to anyone else for analysis. Similarly, if someone has come up with a system configuration that is great for a particular task, tuning the VM allocations, networking values, and the like, they can send you the configuration (just a string value that is a lua table) and you can apply it to your system.

This is a tad better than simply trying to look at system logs to determine after the fact what might be going on with a system. Perhaps the combination of these live values, as well as correlation with system logs, makes it easier to automate the process of diagnosing and tuning a system.

Well, there you have it. The lj2procfs thing is getting more concise, as well as becoming more usable at the same time.

Spelunking Linux – procfs, and a bag of chips

Recently, as a way to further understand all things Linux, I’ve been delving into procfs.  This is one of those virtual file systems on linux, meaning, the ‘files’ and ‘directories’ are not located on some real media anywhere, they are conjured up in realtime from within the kernel.  If you take a look at the ‘/proc’ directory on any linux machine, you’ll find a couple of things.  First, there are a bunch of directories with numeric values as their names.


1	     10			100	      1003	     1004
10075	     101		10276	      10683	     10695
10699	     1071		10746	      10756	     10757
1081	     11			11927	      12	     1236
12527	     12549		12563	      1296	     13

Yes, true to the unix philosophy, all the things are but files/directories. Each one of these numbers represents a process that is running on the system at the moment. Each one of these directories contains additional directories, and files. The files contain interesting data, and the directories lead into even more places where you can find more interesting things about the process.

Here are the contents of the directory ‘1’, which is the first process running on the system:

attr/	    autogroup	     auxv	 cgroup     clear_refs	cmdline
comm	    coredump_filter  cpuset	 cwd@	    environ	exe@
fd/	    fdinfo/	     gid_map	 io	    limits	loginuid
map_files/  maps	     mem	 mountinfo  mounts	mountstats
net/	    ns/		     numa_maps	 oom_adj    oom_score	oom_score_adj
pagemap     personality      projid_map  root@	    sched	schedstat
sessionid   setgroups	     smaps	 stack	    stat	statm
status	    syscall	     task/	 timers     uid_map	wchan

Some actual files, some more directories, some symbolic links. To find out the details of what each of these contains, and their general meaning, you need to consult the procfs man page, as well as the source code of the linux kernel, or various utilities that use them.

Backing up a bit, the /proc directory itself contains some very interesting files as well:

acpi/	      asound/		 buddyinfo     bus/	      cgroups
cmdline       consoles		 cpuinfo       crypto	      devices
diskstats     dma		 driver/       execdomains    fb
filesystems   fs/		 interrupts    iomem	      ioports
irq/	      kallsyms		 kcore	       keys	      key-users
kmsg	      kpagecount	 kpageflags    loadavg	      locks
mdstat	      meminfo		 misc	       modules	      mounts@
mtrr	      net@		 pagetypeinfo  partitions     sched_debug
schedstat     scsi/		 self@	       slabinfo       softirqs
stat	      swaps		 sys/	       sysrq-trigger  sysvipc/
thread-self@  timer_list	 timer_stats   tty/	      uptime
version       version_signature  vmallocinfo   vmstat	      zoneinfo

Again, the meanings of each of these is buried in the various documentation and source code files that surround them, but let’s take a look at a couple of examples. How about that uptime file?

8099.41 31698.74

OK. Two numbers. What do they mean? The first one is how many seconds the system has been running. The second one is the number of seconds all cpus on the system have been idle since the system came up. Yes, on a multi-proc system, the second number can be greater than the first. And thus begins the actual journey into spelunking procfs. If you’re like me, you occasionally need to know this information. Of course, if you want to know it from the command line, you just run the ‘uptime’ command, and you get…

 06:38:22 up  2:18,  2 users,  load average: 0.17, 0.25, 0.17

Well, hmmm, I get the ‘up’ part, but what’s all that other stuff, and what happened to the idle time thing? As it turns out, the uptime command does show the up time, but it also shows the logged in users, and the load average numbers, which actually come from different files.

It’s like this. Whatever you want to know about the system is probably available, but you have to know where to look for it, and how to interpret the data from the files. Often times there’s either a libc function you can call, or a command line utility, if you can discover and remember them.

What about a different way? Since I’m spelunking, I want to discover things in a more random fashion, and of course I want easy lua programmatic access to what I find. In steps the lj2procfs project.

In lj2procfs, I try to provide a more manageable interface to the files in /proc.  Most often, the information is presented as lua tables.  If the information is too simple (like /proc/version), then it is presented as a simple string.  Here is a look at that uptime example, done using lj2procfs:


return {
    ['uptime'] = {
        seconds = 19129.39,
        idle = 74786.86,

You can see that the simple two numbers in the uptime file are converted to meaningful fields within the table. In this case, I use a simple utility program to turn any of the files into simple lua value output, suitable for reparsing, or transmitting. First, what does the ‘parsing’ look like?

	seconds idle 

	The first value is the number of seconds the system has been up.
	The second number is the accumulated number of seconds all processors
	have spent idle.  The second number can be greater than the first
	in a multi-processor system.
local function decoder(path)
	path = path or "/proc/uptime"
	local f =
	local str = f:read("*a")

	local seconds, idle = str:match("(%d*%.?%d+)%s+(%d*%.?%d+)")
	return {
		seconds = tonumber(seconds);
		idle = tonumber(idle);

return {
	decoder = decoder;

In most cases, the output of the /proc files are meant to be human readable. At least with Linux. Other platforms might prefer these files to be more easily machine readable (binary). As such, they are readily parseable mostly by simple string patterns.

So, this decoder is one of many. There is one for each of the file types in the /proc directory, or at least the list is growing.

They are in turn accessed using the Decoders class.

local Decoders = {}
local function findDecoder(self, key)
	local path = "lj2procfs.codecs."..key;

	-- try to load the intended codec file
	local success, codec = pcall(function() return require(path) end)
	if success and codec.decoder then
		return codec.decoder;

	-- if we didn't find a decoder, use the generic raw file loading
	-- decoder.
	-- Caution: some of those files can be very large!
	return getRawFile;
setmetatable(Decoders, {
	__index = findDecoder;


This is a fairly simple forwarding mechanism. You could use this in your code by doing the following:

procfs = require("Decoders")
local uptime = procfs.uptime


When you try to access the procfs.uptime field of the Decoders class, it will go; “Hmmm, I don’t have a field in my table with that name, I’ll defer to whatever was set as my __index value, which so happens to be a function, so I’m going to call that function and see what it comes up with”. The findDecoder function will in turn look in the codecs directory for something with that name. It will find the code in uptime.lua, and execute it, handing it the path specified. The uptime function will read the file, parse the values, and return a table.

And thus magic is practiced!

It’s actually pretty nice because having things as lua tables and lua values such as numbers and strings, makes it really easy to do programmatic things from there.

Here’s meminfo.lua

local function meminfo(path)
	path = path or "/proc/meminfo"
	local tbl = {}
	local pattern = "(%g+):%s+(%d+)%s+(%g+)"

	for str in io.lines(path) do
		local name, size, units = str:match(pattern)
		if name then
			tbl[name] = {
				size = tonumber(size), 
				units = units;

	return tbl

return {
	decoder = meminfo;

The raw ‘/proc/meminfo’ file output looks something like this:

MemTotal:        2045244 kB
MemFree:          273464 kB
MemAvailable:     862664 kB
Buffers:           72188 kB
Cached:           629268 kB
SwapCached:            0 kB

And the parsed output might be something like this:

    ['meminfo'] = {
        ['Active'] = {
            size = 1432756,
            units = [[kB]],
        ['DirectMap2M'] = {
            size = 1992704,
            units = [[kB]],
        ['MemFree'] = {
            size = 284604,
            units = [[kB]],
        ['MemTotal'] = {
            size = 2045244,
            units = [[kB]],

Very handy.

In some cases, the output can be a bit tricky, since it’s trying to be human readable, there might be some trickery, like header lines, and variable number of columns. This can get tricky, but you have the full power of lua to do the parsing, including using something like lpeg if you so choose. Here’s the parser for the ‘/proc/interrupts’ file, for example:

local strutil = require("lj2procfs.string-util")

local namepatt = "(%g+):(.*)"
local numberspatt = "[%s*(%d+)]+"
local descpatt = "[%s*(%d+)]+(.*)"

local function numbers(value)
	num, other = string.match(value, numpatt)
	return num;

local function interrupts(path)
	path = path or "/proc/interrupts"

	local tbl = {}
	for str in io.lines(path) do
		local name, remainder = str:match(namepatt)
		if name then
			local numbers = remainder:match(numberspatt)
			local desc = remainder:match(descpatt)

			local cpu = 0;
			local valueTbl = {}
			for number in numbers:gmatch("%s*(%d+)") do
				--print("NUMBER: ", number)
				valueTbl["cpu"..cpu] = tonumber(number);
				cpu = cpu + 1;
			valueTbl.description = desc
			tbl[name] = valueTbl


	return tbl

return {
	decoder = interrupts;

And it deals with raw file data that looks like this:

          CPU0       CPU1       CPU2       CPU3       
  0:         38          0          0          0   IO-APIC-edge      timer
  1:      25395          0          0          0   IO-APIC-edge      i8042
  8:          1          0          0          0   IO-APIC-edge      rtc0
  9:      70202          0          0          0   IO-APIC-fasteoi   acpi

In this case, I’m running on a VM which was configured with 4 cpus. I had run previously with a VM with only 3 CPUs, and there were only 3 CPU columns. So, in this case, the patterns first isolate the interrupt number from the remainder of the line, then the numeric columns are isolated from the interrupt description field, then the numbers themselves are matched off using an iterator (gmatch). The table generated looks something like this:

    ['interrupts'] = {
        ['SPU'] = {
            cpu2 = 0,
            cpu3 = 0,
            cpu1 = 0,
            cpu0 = 0,
            description = [[Spurious interrupts]],
        ['22'] = {
            cpu2 = 0,
            cpu3 = 0,
            cpu1 = 0,
            cpu0 = 0,
            description = [[IO-APIC  22-fasteoi   virtio1]],
        ['NMI'] = {
            cpu2 = 0,
            cpu3 = 0,
            cpu1 = 0,
            cpu0 = 0,
            description = [[Non-maskable interrupts]],


To make spelunking easier, I’ve created a simple script which just calls the procfs thing, given a command line argument of the name of the file you’re interested in looking at.

#!/usr/bin/env luajit


	This is a general purpose /proc/<file> interpreter.
		$ sudo ./procfile filename

	Here, 'filname' is any one of the files listed in the 
	/proc directory.

	In the cases where a decoder is implemented in Decoders.lua
	the file will be parsed, and an appropriate value will be
	returned and printed in a lua form appropriate for reparsing.

	When there is no decoder implemented, the value returned is

		$ sudo ./procfile cpuinfo
		$ sudo ./procfile partitions


package.path = "../?.lua;"..package.path;

local procfs = require("lj2procfs.procfs")
local putil = require("lj2procfs.print-util")

if not arg[1] then
	print ([[

	$ sudo ./procfile <filename>

where <filename> is the name of a file in the /proc

	$ sudo ./pocfile cpuinfo


local filename = arg[1]

print("return {")
putil.printValue(procfs[filename], "    ", filename)

Once you have these basic tools in hand, you can begin to look at the various utilities that are used within linux, and try to emulate them. For example, the ‘free’ command will show you roughly how memory currently sits on your system, in terms of how much is physically available, how much is used, and the like. It’s typical output, without any parameters, might look like:

             total       used       free     shared    buffers     cached
Mem:       2045244    1760472     284772      28376      73276     635064
-/+ buffers/cache:    1052132     993112
Swap:      1046524          0    1046524

Here’s the code to generate something similar, using lj2procfs

#!/usr/bin/env luajit

	This lua script acts similar to the 'free' command, which will
	display some interesting information about how much memory is being
	used in the system.
package.path = "../?.lua;"..package.path;

local procfs = require("lj2procfs.procfs")

local meminfo = procfs.meminfo;

local memtotal = meminfo.MemTotal.size
local memfree = meminfo.MemFree.size
local memused = memtotal - memfree
local memshared = meminfo.Shmem.size
local membuffers = meminfo.Buffers.size
local memcached = meminfo.Cached.size

local swaptotal = meminfo.SwapTotal.size
local swapfree = meminfo.SwapFree.size
local swapused = swaptotal - swapfree

print(string.format("%18s %10s %10s %10s %10s %10s",'total', 'used', 'free', 'shared', 'buffers', 'cached'))
print(string.format("Mem: %13d %10d %10d %10d %10d %10d", memtotal, memused, memfree, memshared, membuffers, memcached))
--print(string.format("-/+ buffers/cache: %10d %10d", 1, 2))
print(string.format("Swap: %12d %10d %10d", swaptotal, swapused, swapfree))

The working end of this is simply ‘local meminfo = procfs.meminfo’

The code generates the following output.

             total       used       free     shared    buffers     cached
Mem:       2045244    1768692     276552      28376      73304     635100
Swap:      1046524          0    1046524

I couldn’t quite figure out where the -/+ buffers/cache: values come from yet. I’ll have to look at the actual code for the ‘free’ program to figure it out. But, the results are otherwise the same.

Some of these files can be quite large, like kallsyms, which might argue for an iterator interface instead of a table interface. But, some of the files have meta information, as well as a list of fields. Since the number of large files is fairly small, it made more sense to cover the broader cases instead, and tables do that fine. kallsyms being fairly large, it will still nicely fit into a table.

So, what can you do with that?

package.path = "../?.lua;"..package.path;

local sym = arg[1]
assert(sym, "must specify a symbol")

local putil = require("lj2procfs.print-util")
local sutil = require("lj2procfs.string-util")
local fun = require("")
local procfs = require("lj2procfs.procfs")

local kallsyms = procfs.kallsyms

local function isDesiredSymbol(name, tbl)
    return sutil.startswith(name, sym)

local function printSymbol(name, value)

fun.each(printSymbol, fun.filter(isDesiredSymbol, kallsyms))

In this case, a little utility which will traverse through the symbols, looking for something that starts with whatever the user specified on the command line. So, I can use it like this:

luajit findsym.lua mmap

And get the following output:

    location = [[0000000000000000]],
    name = [[mmap_init]],
    kind = [[T]],
    location = [[0000000000000000]],
    name = [[mmap_rnd]],
    kind = [[t]],
    location = [[0000000000000000]],
    name = [[mmap_zero]],
    kind = [[t]],
    location = [[0000000000000000]],
    name = [[mmap_min_addr_handler]],
    kind = [[T]],
    location = [[0000000000000000]],
    name = [[mmap_vmcore_fault]],
    kind = [[t]],
    location = [[0000000000000000]],
    name = [[mmap_region]],
    kind = [[T]],
    location = [[0000000000000000]],
    name = [[mmap_vmcore]],
    kind = [[t]],
    location = [[0000000000000000]],
    name = [[mmap_kset.19656]],
    kind = [[b]],
    location = [[0000000000000000]],
    name = [[mmap_min_addr]],
    kind = [[B]],
    location = [[0000000000000000]],
    name = [[mmap_mem_ops]],
    kind = [[r]],
    location = [[0000000000000000]],
    name = [[mmap_mem]],
    kind = [[t]],

Of course, you’re not limited to simply printing to stdout. In fact, that’s the least valuable thing you could be doing. What you really have is programmatic access to all these values. If you had run this command as root, you would get the actual addresses of these routines.

And so it goes. lj2procfs gives you easy programmatic access to all the great information that is hidden in the procfs file system on linux machines. These routines make it relatively easy to gain access to the information, and utilize tools such as luafun to manage it. Once again, the linux system is nothing more than a very large database. Using a tool such a lua makes it relatively easy to access all the information in that database.

So far, lj2procfs just covers reading the values. In this article I did not cover the fact that you can also get information on individual processes as well. Aside from this, procfs actually allows you to set some values as well. This is why I structured the code as ‘codecs’. You can encode, and decode. So, in future, setting a value will be as simple as ‘procfs.something.somevalue = newvalue’. This will eliminate the guess work out of doing command line ‘echo …’ commands for esoteric values, which are seldom used. It also makes easy to achieve great things programmatically through script, without even relying on various libraries that are meant to do the same.

And there you have it. procfs wrapped up in lua goodness.

Spelunking Linux – what is this auxv thing anyway

While spelunking Linux, trying to find an easier way to do this or that, I ran across this vDSO thing (virtual ELF Dynamic Shared Object). What?

Ok, it’s like this. I was implementing direct calling of syscalls on Linux, and reading up on how the C libraries do things. On Linux, when you wan to talk to the kernel, you typically go through syscalls, or ioctl calls, or netlinks. With syscall, it’s actually a fairly expensive process, switching from userspace to kernel space, issuing and responding to interrupts, etc. In some situations this could be a critical performance hit. So, to make things easier/faster, some of the system calls are implemented in this little ELF package (vDSO). This little elf package (a dynamic link library) is loaded into every application on Linux. Then, the C library can decide to make calls into that library, instead of syscalls, thus saving a lot of overhead and speeding things up. Not all systems have this capability, but many do.

Alright, so how does the C runtime know whether the capability is there or now, and where this little library is, and how to get at functions therewith? In steps our friend auxv. In the GNU C library, there is a lone call:

unsigned long getauxval(unsigned long);

What values can you get out of this thing? Well, the constants can be found in the elf.h file, and look like:

#define AT_PLATFORM 15
#define AT_PAGESZ 6

And about 30 others. How you use each of these depends on the type of the value that you are looking up. For instance, the AT_PLATFORM returns a pointer to a null terminated string. The AT_PAGESZ returns an integer which represents the memory page size of the machine you’re running on.

OK, so what’s the lua version?

static const int AT_NULL = 0;
static const int AT_IGNORE = 1;
static const int AT_EXECFD = 2;
static const int AT_PHDR = 3;
static const int AT_PHENT = 4;
static const int AT_PHNUM = 5;
static const int AT_PAGESZ = 6;
static const int AT_BASE = 7;
static const int AT_FLAGS = 8;
static const int AT_ENTRY = 9;
static const int AT_NOTELF = 10;
static const int AT_UID = 11;
static const int AT_EUID = 12;
static const int AT_GID = 13;
static const int AT_EGID = 14;
static const int AT_CLKTCK = 17;
static const int AT_PLATFORM = 15;
static const int AT_HWCAP = 16;
static const int AT_FPUCW = 18;
static const int AT_DCACHEBSIZE = 19;
static const int AT_ICACHEBSIZE = 20;
static const int AT_UCACHEBSIZE = 21;
static const int AT_IGNOREPPC = 22;
static const int AT_SECURE = 23;
static const int AT_BASE_PLATFORM = 24;
static const int AT_RANDOM = 25;
static const int AT_HWCAP2 = 26;
static const int AT_EXECFN = 31;
static const int AT_SYSINFO = 32;
static const int AT_SYSINFO_EHDR = 33;
static const int AT_L1I_CACHESHAPE = 34;
static const int AT_L1D_CACHESHAPE = 35;
static const int AT_L2_CACHESHAPE = 36;

unsigned long getauxval(unsigned long);

With this, I can then write code that looks like the following:

local function getStringAuxVal(atype)
	local res = libc.getauxval(atype)

	if res == 0 then 
		return false, "type not found"

	local str = ffi.string(ffi.cast("char *", res));
	return str

local function getIntAuxValue(atype)
	local res = libc.getauxval(atype)

	if res == 0 then 
		return false, "type not found"

	return tonumber(res);

local function getPtrAuxValue(atype)
	local res = libc.getauxval(atype)

	if res == 0 then 
		return false, "type not found"

	return ffi.cast("intptr_t", res);

-- convenience functions
local function getExecPath()
	return getStringAuxVal(libc.AT_EXECFN);

local function getPlatform()
	return getStringAuxVal(libc.AT_PLATFORM);

local function getPageSize()
	return getIntAuxValue(libc.AT_PAGESZ);

local function getRandom()
	return getPtrAuxValue(libc.AT_RANDOM);

	Some test cases
print(" Platform: ", getPlatform());
print("Exec Path: ", getExecPath());
print("Page Size: ", getPageSize());
print("   Random: ", getRandom());

And so on and so forth, assuming you have the proper ‘libc’ luajit ffi binding, which gives you access to constants through the ‘libc.’ mechanism.

OK, fine, if I’m a C programmer, and I just want to port some code that’s already doing this sort of thing. By the way, the one value that we’re interested in is: AT_SYSINFO_EHDR. That contains a pointer to the beginning of our vDSO. Then you can call functions directly from there (there’s an API for that).

But, if I’m a lua programmer, I’ve come to expect more out of my environment, largely because I’m lazy and don’t like so much typing. Upon further examination, you can get this information yourself. If you’re hard core, you can look at the top of memory in your program, and map that location to a pointer you can fiddle with directly. Otherwise, you can actually get this information from a ‘file’ on Linux.

Turns out that you can get this info from ‘/proc/self/auxv’, if you’re running this command about your current process (which you most likely are). So, now what can I do with that? Well, the lua way would be the following:

-- auxv_iter.lua
local ffi = require("ffi")
local libc = require("libc")

local E = {}

-- This table maps the constant values for the various
-- AT_* types to their symbolic names.  This table is used
-- to both generate cdefs, as well and hand back symbolic names
-- for the keys.
local auxtbl = {
	[0] =  "AT_NULL";
	[1] =  "AT_IGNORE";
	[2] = "AT_EXECFD";
	[3] = "AT_PHDR";
	[4] = "AT_PHENT";
	[5] = "AT_PHNUM";
	[6] = "AT_PAGESZ";
	[7] = "AT_BASE";
	[8] = "AT_FLAGS";
	[9] = "AT_ENTRY";
	[10] = "AT_NOTELF";
	[11] = "AT_UID";
	[12] = "AT_EUID";
	[13] = "AT_GID";
	[14] = "AT_EGID";
	[17] = "AT_CLKTCK";
	[15] = "AT_PLATFORM";
	[16] = "AT_HWCAP";
	[18] = "AT_FPUCW";
	[22] = "AT_IGNOREPPC";
	[23] = "AT_SECURE";
	[25] = "AT_RANDOM";
	[26] = "AT_HWCAP2";
	[31] = "AT_EXECFN";
	[32] = "AT_SYSINFO";
	[33] = "AT_SYSINFO_EHDR";
	[34] = "AT_L1I_CACHESHAPE";
	[35] = "AT_L1D_CACHESHAPE";
	[36] = "AT_L2_CACHESHAPE";

-- Given a auxv key(type), and the value returned from reading
-- the file, turn the value into a lua specific type.
-- string pointers --> string
-- int values -> number
-- pointer values -> intptr_t

local function auxvaluefortype(atype, value)
	if atype == libc.AT_EXECFN or atype == libc.AT_PLATFORM then
		return ffi.string(ffi.cast("char *", value))

	if atype == libc.AT_UID or atype == libc.AT_EUID or
		atype == libc.AT_GID or atype == libc.AT_EGID or 
		atype == libc.AT_FLAGS or atype == libc.AT_PAGESZ or
		atype == libc.AT_HWCAP or atype == libc.AT_CLKTCK or 
		atype == libc.AT_PHENT or atype == libc.AT_PHNUM then

		return tonumber(value)

	if atype == libc.AT_SECURE then
		if value == 0 then 
			return false
			return true;

	return ffi.cast("intptr_t", value);

-- iterate over the auxv values at the specified path
-- if no path is specified, use '/proc/self/auxv' to get
-- the values for the currently running program
local function auxviterator(path)
	path = path or "/proc/self/auxv"
	local fd =, libc.O_RDONLY);

	local params = {
		fd = fd;
		keybuff ="intptr_t[1]");
		valuebuff ="intptr_t[1]");
		buffsize = ffi.sizeof(ffi.typeof("intptr_t"));

	local function gen_value(param, state)
		local res1 =, param.keybuff, param.buffsize)
		local res2 =, param.valuebuff, param.buffsize)
		if param.keybuff[0] == 0 then
			return nil;

		local atype = tonumber(param.keybuff[0])
		return state, atype, auxvaluefortype(atype, param.valuebuff[0])

	return gen_value, params, 0


-- generate ffi.cdef calls to turn the symbolic type names
-- into constant integer values
local cdefsGenerated = false;

local function gencdefs()
	for k,v in pairs(auxtbl) do		
		-- since we don't know if this is already defined, we wrap
		-- it in a pcall to catch the error
		pcall(function() ffi.cdef(string.format("static const int %s = %d;", v,k)) end)
	cdefsGenerated = true;

-- get a single value for specified key.  A path can be specified
-- as well (default it '/proc/self/auxv')
-- this is most like the gnuC getauxval() function
local function getOne(key, path)
	-- iterate over the values, looking for the one we want
	for _, atype, value in auxviterator(path) do
		if atype == key then
			return value;

	return nil;

E.gencdefs = gencdefs;
E.keyvaluepairs = auxviterator;	
E.keynames = auxtbl;
E.getOne = getOne;

setmetatable(E, {
	-- we allow the user to specify one of the symbolic constants
	-- when doing a 'getOne()'.  This indexing allows for the creation
	-- and use of those constants if they haven't already been specified
	__index = function(self, key)
		if not cdefsGenerated then

		local success, value = pcall(function() return ffi.C[key] end)
		if success then
			rawset(self, key, value);
			return value;

		return nil;


return E

In a nutshell, this is all you need for all the lua based auxv goodness in your life. Here are a couple of examples of usage in action:

local init = require("test_setup")()
local auxv_util = require("auxv_iter")
local apairs = auxv_util.keyvaluepairs;
local keynames = auxv_util.keynames;
local auxvGetOne = auxv_util.getOne;

print("==== Iterate All ====")
local function printAll()
	for _, key, value in apairs(path) do
		io.write(string.format("%20s[%2d] : ", keynames[key], key))

-- print all the entries

-- try to get a specific one
print("==== Get Singles ====")
print(" Platform: ", auxvGetOne(auxv_util.AT_PLATFORM))
print("Page Size: ", auxvGetOne(auxv_util.AT_PAGESZ))

The output from printAll() might look like this:

==== Iterate All ====
     AT_SYSINFO_EHDR[33] : 140721446887424LL
            AT_HWCAP[16] : 3219913727
           AT_PAGESZ[ 6] : 4096
           AT_CLKTCK[17] : 100
             AT_PHDR[ 3] : 4194368LL
            AT_PHENT[ 4] : 56
            AT_PHNUM[ 5] : 10
             AT_BASE[ 7] : 140081410764800LL
            AT_FLAGS[ 8] : 0
            AT_ENTRY[ 9] : 4208720LL
              AT_UID[11] : 1000
             AT_EUID[12] : 1000
              AT_GID[13] : 1000
             AT_EGID[14] : 1000
           AT_SECURE[23] : false
           AT_RANDOM[25] : 140721446787113LL
           AT_EXECFN[31] : /usr/local/bin/luajit
         AT_PLATFORM[15] : x86_64

The printAll() function uses the auxv iteration function, which in turns reads the key/value pairs directly from the /proc/self/auxv file. No need for the GNU C lib function at all. It goes further and turns the raw ‘unsigned long’ values into the appropriate data type based on what kind of data the key specified represents. So, you get lua string, and not just a pointer to a C string.

In the second example, getting singles, the output is simply this:

 Platform: 	x86_64
Page Size: 	4096

The code for this goes into a bit of trickery that’s possible with luajit. first of all, notice the use of ‘auxv_util.AT_PAGESZ’. There is nothing in the auxv_iter.lua file that supports this value directly. There is the table of names, and then there’s that ‘setmetatable’ at the end of things. Here’s where the trickery happens. Basically, this function is called whenever you put a ‘.’ after a table to try and access something, and that something isn’t in the table. You get a chance to make something up and return it. Ini this case, we first call ‘gencdefs()’ if that hasn’t already been called. This will generate ‘static const int XXX’ for all the values in the table of names, so that we can then do a lookup of the values in the ‘ffi.C.’ namespace, using the name. If we find a name, then we add it to the table, so next time the lookup will succeed, and we won’t end up calling the __index function.

At any rate, we now have the requisite value to lookup. Then we just roll through the iterator, and return when we’ve got the value we were looking for. The conversion to the appropriate lua type is automatic.

And there you have it! From relative obscurity, to complete usability, in one iterator. Being able to actually get function pointers in the vDSO is the next step. That will require another API wrapper, or worst case, and all encompassing ELF parser…

cUrling up to the net – a LuaJIT binding

I have need to connect to some Azure services from Linux, using C++. There are a few C/C++ libraries around that will make life relatively easy in this regard, but I thought I’d go with an option that has me learn about something I don’t use that often, but will be fairly powerful and complete.  I chose to use cURL, or to be more precise.  Why?  cURL has been around for ages, has continued to evolve, and makes it fairly easy to do anything from ftp down/upload to https connections.  It has tons of options and knobs, including dealing with authentication, SSL, or just acting like an ordinary socket of you prefer that.

First I created a luajit binding to libcurl.  This just follows my usual learning pattern.  In order to conquer an API, you must first render it useful from Lua.  I did my typical two part binding, first a fairly faithful low level binding, then added some luajit idioms atop.  In this particular case, there’s not a big database to query, although there are quite a few options that get listed in a table:



This is an excerpt from the original curl.h header file. There is a listing of some 200 options which can be set on a curl connection (depending on the context of the connection). This CINIT is a macro that sets the value of an enum appropriately. Well, those macros don’t work in the luajit ffi.cdef call, so I needed a way to convert these. I could have just run the lot through the gcc pre-processor, and that would give me the values I needed, but I thought I’d take a different approach.

I wrote a bit of script to scan the curl.h file looking for the CINIT lines, and turn them into something interesting.

function startswith(s, prefix)
    return string.find(s, prefix, 1, true) == 1

local function writeClean(filename)
	for line in io.lines(filename) do
		if startswith(line, "CINIT") then
			name, tp, num = line:match("CINIT%((%g+),%s*(%g+),%s*(%d+)")
			print(string.format("\t%-25s = {'%s', %s},", name, tp, num))


	COOKIE                    = {'OBJECTPOINT', 22},
	HTTPHEADER                = {'OBJECTPOINT', 23},
	HTTPPOST                  = {'OBJECTPOINT', 24},
	SSLCERT                   = {'OBJECTPOINT', 25},
	KEYPASSWD                 = {'OBJECTPOINT', 26},
	CRLF                      = {'LONG', 27},

Well, that’s nice. Now I have it as a useful table. I can write another script to turn that inti enums, or anything else.

local ffi = require("ffi")

local filename = arg[1] or "CurlOptions"

local options = require(filename)

typedef enum {
	CURLOPTTYPE_LONG          = 0,
	CURLOPTTYPE_OFF_T         = 30000

local function CINIT(na,t,nu) 
	return string.format("\tCURLOPT_%s = CURLOPTTYPE_%s+%d,", na, t, nu)

local tbl = {}
local function addenum(name, type, number)
	table.insert(tbl, CINIT(name, type, number));

table.insert(tbl, "local ffi = require('ffi')");
table.insert(tbl, "ffi.cdef[[\ntypedef enum {")

for k,v in pairs(options) do
	addenum(k, v[1], v[2]);

table.insert(tbl, "} CURLoption;]]");

local tblstr = table.concat(tbl,'\n')
-- now get the definitions as a giant string
-- and execute it
local defs = loadstring(tblstr);

It’s actually easier than this. I threw in the loadstring just to ensure the output was valid. Yah, ok basics.

libcurl is an extremely convenient library. As such, it can be a challenge to use. Fortunately, it has an ‘easy’ interface as well. Here, I chose to wrap the easy interface in an object like wrapper to make it even easier. Here’s how you use it:


That will retrieve the entirety of any given url, and dump the output to stdout. Well, that’s somewhat useful, and its only one line of code. This simple interface will become more sophisticated over time, including being the basis for REST calls, but for now, it suffices.

So, there you have it. libcurl seems to be a viable choice for web access from within lua. You could of corse just do it all with pure lua code, but, you probably wont be wrong to leverage libcurl.