LuaJIT FFI pass char** to C function

Question

Been taking a stab at LuaJIT FFI and have run into a curious problem. I have the following code:

local ffi = require("ffi")
ffi.cdef [[
    int decode(char* encodedToken, char** payload);
]]

local lib = ffi.load("lib.so")

local function decode(encoded_token)
    local c_encoded_token = ffi.new("char[?]", #encoded_token, encoded_token)
    local c_token = ffi.new("char*[1]")
    lib.decode(c_encoded_token, c_token)

    local resonse = ffi.string(c_token[0])
    return response
end

Basically, the shared object has a method to decode an incoming token, and writes the output to char** payload. The weird part is, this code works when I run a single iteration of the decode function, but when run in a loop, causes Segmentation fault.

This leads me to believe there is a memory leak with my ffi.new("char*[1]") allocation. I even tried wrapping it in a ffi.gc call, which didn't help either:

local c_token = ffi.gc(ffi.new("char*[1]"), ffi.free)
-- do stuff
c_token = nil

This is the first time I'm trying out FFI, so any pointers appreciated!

Checksum Checksum · Accepted Answer · 2019-05-03T09:39:29

Figured it out myself. Apparently it was due to null terminated C string taking up one more character.

This worked:

local c_encoded_token = ffi.new("char[?]", #encoded_token + 1, encoded_token)

LuaJIT FFI pass char** to C function

1 Answers