lua require function does not find my required file on iOS

Question 1

Most likely, "." is not the folder containing main.lua, but a working directory (such as where xcode runs from). The app that runs your script probably runs it via a path, like

lua /full/path/to/your/main.lua

So having ./?.lua in LUA_PATH does not help here. Instead, you should have the script run a command to determine where it is running from, and append that to package.path. This should be the path part of arg[0]. So you could try (not tested):

local scriptPath = arg[0]
local dir = string.match(scriptPath, '^.*/')
package.path = package.path .. ';' .. dir .. '?.lua'

The arg is automatically populated by the interpreter, see Section 6 of Lua ref man.

You definitely don't need C to do what you want.

Question 2

Okay, I finally find a good answer and the job is done, thank to this answer.

So, the solution is to modify the path within c++ code, add this function

#include <string>

int setLuaPath(lua_State* L, const char* path) {
    lua_getglobal(L, "package");
    lua_getfield(L, -1, "path"); // get field "path" from table at top of stack (-1)
    std::string cur_path = lua_tostring(L, -1); // grab path string from top of stack
    cur_path.append(";"); // do your path magic here
    cur_path.append(path);
    lua_pop(L, 1); // get rid of the string on the stack we just pushed on line 5
    lua_pushstring(L, cur_path.c_str()); // push the new one
    lua_setfield(L, -2, "path"); // set the field "path" in table at -2 with value at top of stack
    lua_pop(L, 1); // get rid of package table from top of stack
    return 0; // all done!
}

and then, call this function when init lua_State:

// yourfile.cpp
void runLua() {
    lua_State *L;
    L = luaL_newstate();
    luaL_openlibs(L);

    OCCaller oc_caller;
    std::string bundlePath = oc_caller.get_ios_bundle_path();
    bundlePath.append("/?.lua");
    setLuaPath(L, bundlePath.c_str());

    ....
}

your oc_caller class might look like this:

// OCCaller.h
#ifndef __LuaThirdTry__OCCaller__
#define __LuaThirdTry__OCCaller__

#include <iostream>

class OCCaller {
public:
    OCCaller();
    ~OCCaller();
    std::string get_ios_bundle_path();

};

#endif

impl file:

// OCCaller.mm
#include "OCCaller.h"
#import <Foundation/Foundation.h>

OCCaller::OCCaller() { }
OCCaller::~OCCaller() { }

std::string OCCaller::get_ios_bundle_path() {
    NSString *bundlePath = [[NSBundle mainBundle]resourcePath];
    return std::string([bundlePath UTF8String]);
}

Question 3

This is a revision on Bryophyte's answer. To me, this was far too complex. I'm also not sure why he did all these extra classes and used C++ strings instead of NSStrings. Here is my revised and hopefully easier to understand iOS code based on this solution:

-(void)addBundlePathToLuaState:(lua_State*)L
{ 
    lua_getglobal(L, "package");
    lua_getfield(L, -1, "path"); // get field "path" from table at top of stack (-1)

    const char* current_path_const = lua_tostring(L, -1); // grab path string from top of stack
    NSString* current_path = [NSString stringWithFormat:@"%s;%@/?.lua", current_path_const, [[NSBundle mainBundle]resourcePath]];

    lua_pop(L, 1); // get rid of the string on the stack we just pushed on line 5
    lua_pushstring(L, [current_path UTF8String]); // push the new one
    lua_setfield(L, -2, "path"); // set the field "path" in table at -2 with value at top of stack
    lua_pop(L, 1); // get rid of package table from top of stack
}