Example on how to implement Lua functions that run C Code

compile.sh

#!/bin/sh
gcc -Wall -shared -fPIC -o power.so -I/usr/include/lua5.1 -llua5.1 hellofunc.c

# Notes:

# gcc
# The compiler

# -Wall
# Warn on almost anything

# -shared
# Compile to .so format, and don't error out just because there's no main()

# -fPIC
# Goes with -shared to tell the executable format. Subtle errors can sometimes creep in if you don't do this.

# -o power.so
# Name the shared library power.so

# -I/usr/include/lua5.1
# Look for lua.h etc in /usr/include/lua5.1. Note this will be different on every system.

# -llua5.1
# Compile with the Lua library. Note this will be different on every system. See here for how you deduce the -l argument.

# hellofunc.c
# The source file to the power library. It doesn't have to be named "power".

# How it worked on my Ubuntu with Lua 5.4:
# $ gcc -Wall -shared -fPIC -o power.so -I/usr/local/include/lua5.4 -llua hellofunc.c

hellofunc.c

/* hellofunc.c (C) 2011 by Steve Litt
 * gcc -Wall -shared -fPIC -o power.so -I/usr/include/lua5.1 -llua5.1 hellofunc.c
 * Note the word "power" matches the string after the underscore in
 * function luaopen_power(). This is a must.
 * The -shared arg lets it compile to .so format.
 * The -fPIC is for certain situations and harmless in others.
 * On your computer, the -I and -l args will probably be different.
 */

#include <lua.h>                               /* Always include this */
#include <lauxlib.h>                           /* Always include this */
#include <lualib.h>                            /* Always include this */

static int isquare(lua_State *L){              /* Internal name of func */
	float rtrn = lua_tonumber(L, -1);      /* Get the single number arg */
	printf("Top of square(), nbr=%f\n",rtrn);
	lua_pushnumber(L,rtrn*rtrn);           /* Push the return */
	return 1;                              /* One return value */
}
static int icube(lua_State *L){                /* Internal name of func */
	float rtrn = lua_tonumber(L, -1);      /* Get the single number arg */
	printf("Top of cube(), number=%f\n",rtrn);
	lua_pushnumber(L,rtrn*rtrn*rtrn);      /* Push the return */
	return 1;                              /* One return value */
}


/* Register this file's functions with the
 * luaopen_libraryname() function, where libraryname
 * is the name of the compiled .so output. In other words
 * it's the filename (but not extension) after the -o
 * in the cc command.
 *
 * So for instance, if your cc command has -o power.so then
 * this function would be called luaopen_power().
 *
 * This function should contain lua_register() commands for
 * each function you want available from Lua.
 *
*/
int luaopen_power(lua_State *L){
	lua_register(
			L,               /* Lua state variable */
			"square",        /* func name as known in Lua */
			isquare          /* func name in this file */
			);  
	lua_register(L,"cube",icube);
	return 0;
}

test.lua

require("power")

-- Test the 2 functions
print(square(1.414213598))
print(cube(5))