ffi

I want to do Rust bindings to a C library which requires a callback, and this callback must return a C-style char* pointer to the C library which will then free it. The callback must be in some sense exposed to the user of my library (probably using closures), and I want to provide a Rust interface as convenient as possible (meaning accepting a String output if possible). However, the C library complains when trying to free() a pointer coming from memory allocated by Rust, probably because Rust uses jemalloc and the C library uses malloc. So currently I can see two workarounds using libc:

I want to return a vector in a pub extern "C" fn . Since a vector has an arbitrary length, I guess I need to return a struct with the pointer to the vector, and the number of elements in the vector My current code is: extern crate libc; use self::libc::{size_t, int32_t, int64_t}; // struct to represent an array and its size #[repr(C)] pub struct array_and_size { values: int64_t, // this is probably not how you denote a pointer, right? size: int32_t, } // The vector I want to return the address of is already in a Boxed struct, // which I have a pointer to, so I guess the vector is on the heap

I have a scenario where Rust will call C to malloc a buffer and stash the resulting pointer into a struct. Later on, the struct will be moved to a thread and passed to a C function which mutates it. The naive approach to my problem looks like this ( playground ): extern crate libc; use libc::{c_void, malloc, size_t}; use std::thread; const INITIAL_CAPACITY: size_t = 8; extern "C" { fn mutate(s: *mut Storage); } #[repr(C)] struct Storage { #[allow(dead_code)] buf: *mut c_void, capacity: usize, } fn main() { let buf = unsafe { malloc(INITIAL_CAPACITY) }; let mut s = Storage { buf: buf, capacity:

I want to pass in a String to a Rust lib, but it always throws a segmentation fault. Here's the code: // lib.rs #[no_mangle] pub extern fn process(foo: String) -> String { foo } And the Ruby file: # embed.rb require 'ffi' module Hello extend FFI::Library ffi_lib 'target/release/libembed.dylib' attach_function :process, [ :string ], :string end puts Hello.process("foo") DK. Disclaimer : I've never used Ruby-FFI before; I'm going on what I can find in the documentation. According to the Ruby-FFI wiki page on types , :string is equivalent to a NUL-terminated C string. This is not the same as a