Simple Lua Api Example

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First, create the below Lua script file and save it as "script.lua". This script will dump to the screen the contents of a table named "foo" (that will be created from the C program) and return the sum of this table's components.

-- script.lua
-- Receives a table, returns the sum of its components.
io.write("The table the script received has:\n");
x = 0
for i = 1, #foo do
  print(i, foo[i])
  x = x + foo[i]
end
io.write("Returning data back to C\n");
return x

In order to access it from C, we could hack a little program that would construct the table, pass it to the script, and get the returned value. Create a C program called "test.c" as follows:

/*
 * test.c
 * Example of a C program that interfaces with Lua.
 * Based on Lua 5.0 code by Pedro Martelletto in November, 2003.
 * Updated to Lua 5.1. David Manura, January 2007.
 */

#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <stdio.h>


int
main(void)
{
    int status, result, i;
    double sum;
    lua_State *L;

    /*
     * All Lua contexts are held in this structure. We work with it almost
     * all the time.
     */
    L = luaL_newstate();

    luaL_openlibs(L); /* Load Lua libraries */

    /* Load the file containing the script we are going to run */
    status = luaL_loadfile(L, "script.lua");
    if (status) {
        /* If something went wrong, error message is at the top of */
        /* the stack */
        fprintf(stderr, "Couldn't load file: %s\n", lua_tostring(L, -1));
        exit(1);
    }

    /*
     * Ok, now here we go: We pass data to the lua script on the stack.
     * That is, we first have to prepare Lua's virtual stack the way we
     * want the script to receive it, then ask Lua to run it.
     */
    lua_newtable(L);    /* We will pass a table */

    /*
     * To put values into the table, we first push the index, then the
     * value, and then call lua_rawset() with the index of the table in the
     * stack. Let's see why it's -3: In Lua, the value -1 always refers to
     * the top of the stack. When you create the table with lua_newtable(),
     * the table gets pushed into the top of the stack. When you push the
     * index and then the cell value, the stack looks like:
     *
     * <- [stack bottom] -- table, index, value [top]
     *
     * So the -1 will refer to the cell value, thus -3 is used to refer to
     * the table itself. Note that lua_rawset() pops the two last elements
     * of the stack, so that after it has been called, the table is at the
     * top of the stack.
     */
    for (i = 1; i <= 5; i++) {
        lua_pushnumber(L, i);   /* Push the table index */
        lua_pushnumber(L, i*2); /* Push the cell value */
        lua_rawset(L, -3);      /* Stores the pair in the table */
    }

    /* By what name is the script going to reference our table? */
    lua_setglobal(L, "foo");

    /* Ask Lua to run our little script */
    result = lua_pcall(L, 0, LUA_MULTRET, 0);
    if (result) {
        fprintf(stderr, "Failed to run script: %s\n", lua_tostring(L, -1));
        exit(1);
    }

    /* Get the returned value at the top of the stack (index -1) */
    sum = lua_tonumber(L, -1);

    printf("Script returned: %.0f\n", sum);

    lua_pop(L, 1);  /* Take the returned value out of the stack */
    lua_close(L);   /* Cya, Lua */

    return 0;
}

It's now time to compile it. Keep in mind you'll need to link against -llua (Lua) possibly -lm (the math library). In the system I wrote this simple test, "cc -o test test.c -I/usr/local/include -L/usr/local/lib -llua -lm" generated the binary as expected. On another system (OpenSuse? 11.4, gcc4.7) -ldl was also needed to resolve undefined references to "dlopen" etc.

Finally, running the test you should get something like:

$ ./test                                           
The table the script received has:
1       2
2       4
3       6
4       8
5       10
Returning data back to C
Script returned: 30

That should demonstrate how the basic API works and how values are passed between C and Lua.

For more, see [Lua API Demo]. It is a module designed for the Lua interpreter that allows you to set up a fake Lua state with a stack, call Lua API functions, and observe their effect on the stack.