How to convert a luajit pointer to a string and back? - lua

Question

I need some help converting a luajit pointer to a string and back.
First I define the ctype:
ffi.cdef[[
typedef struct {
unsigned char Bytes[16];
} EncryptionKeys[100000000];
void* malloc(size_t);
void free(void*);
]]
Then use malloc to allocate some memory and then create the 'EncryptionKeys' variable.
local EncryptionKeyMemoryAddress = ffi.C.malloc(ffi.sizeof("EncryptionKeys"))
local EncryptionKeys = ffi.cast("EncryptionKeys(&)", EncryptionKeyMemoryAddress)
I first convert the variable into a lua string using:
ffi.string(EncryptionKeyMemoryAddress)
But I can't figure out how to convert it back!
Can someone please help me?
FYI: I am passing the 'EncryptionKeyMemoryAddress' variable to one of the function parameters for a lua lane (https://lualanes.github.io/lanes/).
Edit:
Here is the section of code that I am working on:
This is for the client managers module of my server that manages a list of lua states that all have access to any clients connected to the server. They all use a shared section of memory that I want them to have access to using a pointer.
local ClientFFIString = [[
typedef struct {
unsigned char Bytes[16];
} EncryptionKeys[100000000];
void* malloc(size_t);
void free(void*);
]]
ffi.cdef(Matchpools.FFIString)
local EncryptionKeyMemoryAddress = ffi.C.malloc(ffi.sizeof("EncryptionKeys"))
--------------------------------------------
function ClientManagers.CreateNewClientManager()
local EncryptionKeys = ffi.cast("EncryptionKeys(&)", EncryptionKeyMemoryAddress)
EncryptionKeys[0].Bytes[0] = 24
print("___a", EncryptionKeys[0].Bytes[0])
local NewIndex = #ClientManagers.List+1
ClientManagers.List[NewIndex] = ClientManagerFunc(
ClientFFIString,
ffi.string(EncryptionKeysMemoryAddress)
)
end
--------------------------------------------
local ClientManagerFunc = Lanes.gen("*", function(ClientFFIString, EncryptionKeysMemoryAddress)
ffi = require("ffi")
ffi.cdef(ClientFFIString)
local EncryptionKeys = ffi.cast("EncryptionKeys(&)", EncryptionKeyMemoryAddress)
print("___a", EncryptionKeys[0].Bytes[0])
-- I want this to be 24 just like it is in the function that created this lua state
local ClientManagerRunning = true
while ClientManagerRunning do
--local dt = GetDt()
--UpdateClientData(dt)
--UpdateMatchmaking(dt)
end
end)

Answer 1

You can convert Lua string to your structure pointer (and later use it as an array):
ffi.cdef"typedef struct {unsigned char Bytes[16];} Key;"
ptr=ffi.cast("Key *", your_string)
print("First Byte of the First Key in your Array:", ptr[0].Bytes[0])
UPDATE:
Let's test how it works for an array containing three keys:
local ffi = require'ffi'
ffi.cdef"typedef struct {unsigned char Bytes[16];} Key;"
local your_string = string.char(11):rep(16)..string.char(22):rep(16)..string.char(33):rep(16)
local ptr=ffi.cast("Key *", your_string)
print("First Byte of the First Key in your Array:", ptr[0].Bytes[0])
print("First Byte of the Second Key in your Array:", ptr[1].Bytes[0])
print("First Byte of the Third Key in your Array:", ptr[2].Bytes[0])
It prints 11, 22, 33
UPDATE 2:
Pass the address of buffer instead of the content of buffer
In the main thread
-- allocate the buffer
local Keys = ffi.cast("EncryptionKeys&", ffi.C.malloc(ffi.sizeof("EncryptionKeys")))
-- write to the buffer
Keys[2].Bytes[5] = 42
-- create string containing 64-bit address
local string_to_send = tostring(ffi.cast("uint64_t", Keys))
-- Send string_to_send to a lane
Inside the lane
-- receive the string
local received_string = .....
-- restore the buffer pointer
local Keys = ffi.cast("EncryptionKeys&", loadstring("return "..received_string)())
-- read the data from the buffer
print(Keys[2].Bytes[5])