Access violation while copying a string of wchar from network packet

Question

I'm programming game server and it uses unicode since I'm in Korea. The problem is that I can't copy wchar string from packet.

I checked wcsncpy_s(chat, 100, inChat.c_str(), inChat.length()); this works fine but something like this doesn't work.

wchar_t strId[10];  // I'm trying to copy L"player11" here.    
wcsncpy_s(
            strId,
            10,
            (WCHAR*)(buffer[2]),  // buffer[0] : packet size, buffer[1] : packet type
            9                     
        );

Memory view

I checked so many times to figure out what I've done wrong but I have no idea what is wrong about it.

It throws the same exception every time.

Exception thrown at 0x00007FFF20DFE5A0 (ucrtbased.dll) in SimplestMMORPG-Server.exe: 0xC0000005: Access violation reading location 0x0000000000000070.

Please help me...

Answer 1

You need just to specify offset from the buffer's beginning, instead of trying to interpret buffer[2] as memory address:

wcsncpy_s(
            strId,
            10,
            (WCHAR*)(buffer + 2),
            9                     
        );

Because: given that buffer is declared as char* buffer (or std::byte* buffer) that is buffer contains a pointer to some memory (shown on screenshot), buffer[2] takes 3-rd element from the buffer. buffer[2] has type char, and type char is incompatible with 3rd parameter of wcsncpy_s. (From the memory view 3-rd byte is 0x70. Then 0x70 was passed as third parameter to wcsncpy_s, which expects 3rd parameter to be a memory address, and it tries to read from 0x70 memory address, and fails, that's why error message was Access violation reading location 0x0000000000000070)

Instead you need to calculate an address of that string inside buffer, as buffer + 2, buffer + 2 has type char*. wcsncpy_s expects exactly char* as a third parameter.