add README for vortex-ffi

Author: a10y

vortex-ffi/Cargo.toml

@@ -1,6 +1,9 @@
 [package]
 name = "vortex-ffi"
 description = "Native C FFI bindings for Vortex"
+readme = "README.md"
+# This crate is not meant to be consumed by other Rust code but rather produces a static binary
+# that can be linked to by other languages.
 publish = false
 version.workspace = true
 homepage.workspace = true

vortex-ffi/README.md

@@ -0,0 +1,48 @@
+# Foreign Function Interface
+
+Vortex is a file format that can be used by any execution engine. Nearly every programming language supports
+the C ABI (Application Binary Interface), so by providing an FFI interface to work with Vortex objects we can
+make it easy to support a variety of languages.
+
+## Design
+
+The FFI is designed to be very simple and follows a very object-oriented approach:
+
+- **Constructors** are simple C functions that return opaque pointers
+- **Methods** are functions that receive an opaque pointer as the first argument, followed by subsequent arguments.
+  Methods may return a value or void.
+- **Destructors** free native resources (allocations, file handles, network sockets) and must be explicitly called by
+  the foreign language to avoid leaking resources.
+
+Constructors will generally allocate rust memory, and destructors free that memory.
+
+For example:
+
+```c
+// DType is an opaque pointer to the Rust DType enum.
+// typedef void* DType;
+
+
+// Pointer to memory allocated by Rust (via `Box::new()`)
+DType d_i32 = DType_new(DTYPE_PRIMITIVE_I32, false);
+
+printf("nullable = %d\n", DType_nullable(d_i32));
+// "nullable = 1"
+
+// Rust memory is freed, d_i32 is now a dangling pointer.
+DType_free(d_i32);
+```
+
+## C Strings
+
+C strings are null-terminated, while Rust's are not. This means that unfortunately, we cannot simply return a pointer
+to a `&str` or `&String` but instead need to copy the data into a new allocation. Instead, methods that return a string
+should instead receive two arguments:
+a `*mut c_void` which is a pointer to the start of a buffer that is large enough to hold the largest string, and a
+`*mut c_int` to store the length of the buffer after writing.
+
+This means that we can actually request a pointer instead
+
+Because C and Rust have different string representations, functions that return Strings must instead receive
+a pointer to a buffer, and a pointer to an integer. Any `str` or `String` from Rust will be copied into the output
+buffer,