---
title: ffi.cdef
summary: Declares C types and signatures for FFI; it does not load a library or implement a function.
section: reference
kind: api
status: reference
updated: 2026-10-26
tags: ["API","X4"]
sources: ["https://wiki.egosoft.com/X4%20Foundations%20Wiki/Modding%20Support/","https://www.lua.org/manual/5.1/manual.html","https://luajit.org/ext_ffi_api.html"]
---
## Return value

| Type | Meaning |
| --- | --- |
| [`none`](wiki:type-results) | Types describe the declared interface, not proof of a successful game effect. |

[Types and methods](wiki:types)

## Parameters

| Name | Type |
| --- | --- |
| `declarations` | [`string`](wiki:type-string) |

? marks an optional argument; ... means a variable number of arguments. Invalid input can raise a Lua error rather than return nil.

## Example and context

```lua
local ffi = require("ffi")
ffi.cdef("typedef struct { int count; } WikiExampleCounter;")
```

**What happens:** FFI learns the WikiExampleCounter type with its count field; no instance has been allocated.

This fragment does not print or write a log message. The values shown are the expected results of the example.

See [language basics](wiki:x4-code-basics), [types and methods](wiki:types) and [Lua / MD events](wiki:md-lua-bridge). Examples there are independent learning examples. No project source files or private implementation fragments are distributed with this reference.