Embedding Lua in 5 Minutes

2008-04-03

So at the UKUUG Spring Conference I kind of decided that there weren’t enough different dynamic languages being talked about; in fact it was pretty much divided into Python land and Perl land (at least as far as dynamic languages were concerned). So I decided to give a 5 minute lightning talk at the end of the conference, on embedding Lua into an application in 5 minutes.

This was my first lightning talk and it was a bit scary and a lot of fun. I highly recommend the experience.

I decided that instead of talking about embedding Lua, I would actually do it, standing in front of the conference live. Including downloading the Lua sources and compiling them (yay for working conference wifi). I thought this was hilarious, I have no idea what anyone else thought. I surprised myself by being able to type code in vi and talk at the same time, though I’m not sure I made much sense.

Here’s an example using the new command line option I added to yes:

$ ./a.out -l 'x=x or 1; x=x*2; return x' | head
2
4
8
16
32
64
128
256
512
1024

For the sake of completeness here is the modified version of yes.c that I ended up with:

#include <sys/cdefs.h>

#include <stdio.h>
#include "lua.h"
#include "lauxlib.h"

int main __P((int, char **));

int
main(argc, argv)
        int argc;
        char **argv;
{
  if(argc >= 3 && strcmp(argv[2], "-l")) {
    lua_State *l = luaL_newstate();
    luaL_openlibs(l);

    while(1) {
      luaL_dostring(l, argv[2]);
      puts(lua_tostring(l, -1));
      lua_settop(l, 0);
    }
  }

        if (argc > 1)
                for(;;)
                        (void)puts(argv[1]);
        else for (;;)
                (void)puts("y");
}

/*
 * Copyright (c) 1987, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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 */
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4 Responses to “Embedding Lua in 5 Minutes”

  1. Thomas Guest Says:

    Well done on programming live, let alone talking at the same time! I’m always amazed and impressed when people can do this in front of an audience, even if it’s done using a canned command history. I think it’s important to show real live examples of how to do things.

    Congratulations too on connecting a computer to a projector. I’ve just returned from a couple of days at ACCU 2008 and it frightens me to see this still causes problems, even for very experienced presenters.

  2. mjb67 Says:

    Brilliant!

  3. daniel Says:

    i’m a bit amused at the twice-broken logic in line 14:

    - strcmp returns zero when the strings are equal, so your if statement only compares true when argv[2] is *not* “-l”

    but!

    - argv[2] is the lua code (the “-l” argument you’re trying to look for is in argv[1]), so by happy accident it does actually do what at a glance it would appear it’s meant to.

    but what you’re actually testing for is ‘there are at least two arguments, and the lua code in the second argument that i’m about to execute is not the string “-l”.’

    • drj11 Says:

      Yes, you’re right! Between then strcmp() and argv[] have two of the worst interfaces ever designed. That and coding live in front of an audience is not a good combination for making robust quality code. :)


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