lua_ex.cpp

// lua_ex.c - How to embed LUA library into C/C++ - example

// EXIT_* constants
#include <stdlib.h>
// printf(3)....
#include <stdio.h>
// variable errno
#include <errno.h>
// strerror(3)
#include <string.h>
// uname(2)
#include <sys/utsname.h>
// sysinfo(2) - uptime
#include <sys/sysinfo.h>

// NOTE: C++ must use "lua.hpp", however C must use "lua.h"
#include <lua.hpp>


#define LOCAL_VALIDATE_LUA_STACK \
	if ( lua_gettop(ls) ){ \
		fprintf(stderr, "LUA has unbalanced stack before lua_close() call: %d <> %d", \
				lua_gettop(ls), 0); \
		return EXIT_FAILURE; \
	}


static int l_uname_machine(lua_State *ls){
    struct utsname un;

    if (uname(&un)){
    	char msg[256];
    	snprintf(msg, sizeof(msg), "Error calling uname(): %s",
    			strerror(errno));
    	// reporting error: insert NIL and Error message
    	lua_pushnil(ls);
    	lua_pushstring(ls, msg);
	    return 2;  // number of results
    } else {
    	lua_pushstring(ls, un.machine);
    	return 1; // number of results
    }

}

static int l_uptime_seconds(lua_State *ls){
	struct sysinfo in;

	if (sysinfo(&in)){
    	char msg[256];
    	snprintf(msg, sizeof(msg), "Error calling sysinfo(): %s",
    			strerror(errno));
    	// reporting error: insert NIL and Error message
    	lua_pushnil(ls);
    	lua_pushstring(ls, msg);
	    return 2;  // number of results
	} else {
		// according to https://stackoverflow.com/a/4079267
		// sizeof() can't be used in C++ preprocessor
		if ( sizeof(in.uptime) > sizeof(lua_Integer) ){
			// lua_Number is typically double
			lua_pushnumber(ls, (lua_Number) in.uptime);
		} else {
			lua_pushinteger(ls, (lua_Integer) in.uptime);
		}
		return 1; // number of results
	}

}

static int Ex_ExtendLUA(lua_State *ls){

	lua_pushcfunction(ls, l_uptime_seconds);
	lua_setglobal(ls, "uptime_seconds");

	LOCAL_VALIDATE_LUA_STACK;

	lua_pushcfunction(ls, l_uname_machine);
	lua_setglobal(ls, "uname_machine");

	LOCAL_VALIDATE_LUA_STACK;

	return EXIT_SUCCESS;
}

// reports error on LUA call and show error message from stack.
// also pops-out error message from stack.
static int Ex_ReportLuaError(lua_State *ls, const char *fn){
	fprintf(stderr, "Error calling %s(): %s", fn, lua_tostring(ls, -1));
    lua_pop(ls, 1);  /* pops LUA error message from the stack */
	return EXIT_FAILURE;
}

// runs LUA commands in luaCommands, returns EXIT_SUCESS or EXIT_FAILURE
// this call always uses fresh LUA instance
static int Ex_RunLua(const char *luaCommands){
	int rc  = EXIT_FAILURE; // our return value
	int err = 0;            // for errors from LUA calls

	lua_State *ls = luaL_newstate();
	if ( ls == NULL ){
		fprintf(stderr,"luaL_newstate() returned NULL\n");
		goto exit0;
	}

	luaL_openlibs(ls);

	if (Ex_ExtendLUA(ls)){
		goto exit1;
	}

	err = luaL_loadstring(ls, luaCommands);
	if (err){
		Ex_ReportLuaError(ls, "luaL_loadstring");
		goto exit1;
	}

	// now there is "luaCommmands" string on LUA stack (!)
	// Verify that it is true.
	if (lua_gettop(ls)!=1){
		fprintf(stderr, "Invalid LUA stack - got %d, expected %d\n", 1, 0);
		goto exit1;
	}

	// this executes our commands
	err = lua_pcall(ls, 0, 0, 0);
	if (err){
		fprintf(stderr, "This LUA command failed:\n%s\n", luaCommands);
		Ex_ReportLuaError(ls, "lua_pcall");
		goto exit1;
	}

	rc = EXIT_SUCCESS;

	exit1:
	// additionally report unbalanced stack
	if ( lua_gettop(ls) ){
		fprintf(stderr, "LUA has unbalanced stack before lua_close() call: %d <> %d",
				lua_gettop(ls), 0);
		rc = EXIT_FAILURE;
	}
	lua_close(ls);

	exit0:
	return rc;
}


int main(int argc, char **argv)
{
	int rc = EXIT_SUCCESS;

	// NOTE: unlike TCL there is no global Init function

	rc = Ex_RunLua("print('Hello, world on ' .. uname_machine() .. '!')");

	if (rc == EXIT_SUCCESS){
		rc = Ex_RunLua("print('System uptime is ', uptime_seconds() ,' seconds.')");
	}

	// NOTE: unlike TCL there is no global Finalize function
	return rc;
}