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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: ", entry.id, id)
        if entry.id == 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 = 'http://www.w3.org/2000/svg'   width = '120' height = '120' xmlns:xlink = 'http://www.w3.org/1999/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(FileStream.open("test_lineargradient.svg"))

local doc = svg {
	width = "120",
	height = "120",
	viewBox = "0 0 120 120",
    ['xmlns:xlink'] ="http://www.w3.org/1999/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 BasicElem.new(self, 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.

New Year, new tools – OpenScad Tetrahedron

Screen Shot 2016-01-02 at 8.16.57 AM

I fired up OpenScad, and created a new tetrahedron model using it.  I’ve done tons with platonic solids in OpenScad in the past.  This time around though, I figured I’d create the simplest tetrahedron I know.  Here’s the OpenScad code for the same:




    translate([0, 100, 100])
    translate([100, 0, 100])

The trick here is in the fact of the special relationship between a tetrahedron and a cube. The cube has 8 vertices, and you can create a tetrahedron by using 4 of those vertices. You just have to pick the right ones. You can easily choose the right ones by picking a set at opposite corners of any face (cutting a diagonal across that face). Then, choose the face exactly opposite, and select the two corners that are different from the ones you picked on the first face. Now you have these 4 vertices.

OpenScad has had the ‘hull()’ module since forever, but it didn’t always work so well. In this modern OpenScad, and for this particular case, it works extremely well. Just take the 4 vertices, throw them into a hull(), and out comes a nice tetrahedron!

This is the tetrahedron for the masses. No math required, except to know the vertices of a unit cube. And of course you can scale it up to any size you please.

If you want to do anything tricky with it though, like subtract some holes and the like, you’ll have to know the math again. But, there you have it, tetrahedron solid in a few lines of OpenScad.

End of 2015 Already!

Over the past few years, I figured out that in the US, if you spend on your business, you’ll recoup more money than you’ll ever make by simply saving that money in a bank account.  So, here we are, it’s the end of the year, what to do?

Well, first up was a new laser cutter:

G. Weike LG 900N

Then came another large format powder printer

3D Systems Pro Jet 660Pro

A replacement for my original Up! automated glue gun

Afinia H800

And finally, I took a flyer on the GlowForge laser cutter

GlowForge (link to save $100: http://glowforge.com/referred/?kid=Y1kbpm)

Now, what business does any sane person have putting all this stuff into their garage?  Well, first of all, the Pro Jet isn’t actually in my garage, it’s actually being put into production in California.  That does leave me with the Weike and Afinia (ignore the GlowForge).  The Weike is an interesting tool in that it’s big enough to cut large enough pieces to incorporate into real furniture.  The way I see it is, I can cut 1/8″ or 1/16″ slices and create my own “plywood”.  That can result in a lot of possibilities.  And of course there’s always the paper/cardboard cutting for kids crafts, ‘living hinge’ jewelry boxes, and the like.  The Afinia, whenever it shows up, can serve for in house ‘print/cut’, rather than being the heavy duty garage bound tool.  It will go in my office.

The Afinia is just an evolution of a fairly good ‘just print’ printer.  I purchased one of the earliest Up! printers a few years back, after first spending quite a few frustrating months with the original MakerBot Cupcake.  I’ve loved my Up! ever since, but it’s getting long in the tooth, so it’s time for an upgrade.  The H800 has a larger print area, and probably more importantly it has an enclosed volume with a HEPA filter.  This makes it a candidate for living inside the house if I make space for it.  The larger print volume will make it feasible to print fun parts for construction, and not just little trinkets.  I’m looking forward to having an enclosed build volume, it will likely make for better temperature control, and thus less warping of ABS.

I recently had one of my ancient designs printed on the powder printer, just to see what it would look like.  I have plans to have it made using injection molding process, meanwhile, using the Afinia will help me make some small runs of the same, in true rapid prototyping fashion.

Speaking of designs, I recently installed the 2015 version of OpenScad.  It renders my old designs nicely enough, and it has some new features which will make some work I had done previously even better.  So, what better time to dust off some of those old designs and update them with the modern OpenScad.  We’ll see how hard it is to get back into the mindset of that programming environment.

And how can the year end purchases ignore electronics!

Raspberry Pi Zero (from Adafruit)

Raspberry Pi Microsoft iOT package

At $5 (for basic board), how could any self respecting tinkerer pass this up.  Yes, if you really add things up, it’s much more than this, but I already have enough power supplies, keyboards and wifi dongles in stock that these are not additional costs, and the incremental cost to me is truly $5.  At the very least, I can see using these as a learning tool to teach Linux.  A Raspberry Pi 0, with LuaJIT installed, and tons of “Lua Tools For Linux”, and you’re all set.

So, finishing the year with a wallet busting blowout, taking back some tax dollars in the process, and generally setting up to have some great fun in the new year.

Happy New Year!

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 = io.open(path)
	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("lj2procfs.fun")
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.


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