output

lua test.lua 
lua: test.lua:5: attempt to index a nil value (global 'zstring')
stack traceback:
	test.lua:5: in main chunk
	[C]: in ?

zstring.c

#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
#include <string.h>

/* gcc zstring.c -Wall -shared -o zstring.so */

static int zst_strlen(lua_State *L)
{
    size_t len;
    len = strlen(lua_tostring(L, 1));
    lua_pushnumber(L, len);
    return 1;
}

int luaopen_zstring(lua_State *L)
{
    lua_register(L,"zst_strlen", zst_strlen);
    return 0;
}