rust/doc/tutorial-ffi.md

% Rust Foreign Function Interface Tutorial

# Introduction

This tutorial will use the [snappy](https://code.google.com/p/snappy/)
compression/decompression library as an introduction to writing bindings for
foreign code. Rust is currently unable to call directly into a C++ library, but
snappy includes a C interface (documented in
[`snappy-c.h`](https://code.google.com/p/snappy/source/browse/trunk/snappy-c.h)).

The following is a minimal example of calling a foreign function which will compile if snappy is
installed:

~~~~ {.xfail-test}
use core::libc::size_t;

#[link_args = "-lsnappy"]
extern {
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
}

fn main() {
    let x = unsafe { snappy_max_compressed_length(100) };
    println(fmt!("max compressed length of a 100 byte buffer: %?", x));
}
~~~~

The `extern` block is a list of function signatures in a foreign library, in this case with the
platform's C ABI. The `#[link_args]` attribute is used to instruct the linker to link against the
snappy library so the symbols are resolved.

Foreign functions are assumed to be unsafe so calls to them need to be wrapped with `unsafe {}` as a
promise to the compiler that everything contained within truly is safe. C libraries often expose
interfaces that aren't thread-safe, and almost any function that takes a pointer argument isn't
valid for all possible inputs since the pointer could be dangling, and raw pointers fall outside of
Rust's safe memory model.

When declaring the argument types to a foreign function, the Rust compiler will not check if the
declaration is correct, so specifying it correctly is part of keeping the binding correct at
runtime.

The `extern` block can be extended to cover the entire snappy API:

~~~~ {.xfail-test}
use core::libc::{c_int, size_t};

#[link_args = "-lsnappy"]
extern {
    fn snappy_compress(input: *u8,
                       input_length: size_t,
                       compressed: *mut u8,
                       compressed_length: *mut size_t) -> c_int;
    fn snappy_uncompress(compressed: *u8,
                         compressed_length: size_t,
                         uncompressed: *mut u8,
                         uncompressed_length: *mut size_t) -> c_int;
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
    fn snappy_uncompressed_length(compressed: *u8,
                                  compressed_length: size_t,
                                  result: *mut size_t) -> c_int;
    fn snappy_validate_compressed_buffer(compressed: *u8,
                                         compressed_length: size_t) -> c_int;
}
~~~~

# Creating a safe interface

The raw C API needs to be wrapped to provide memory safety and make use higher-level concepts like
vectors. A library can choose to expose only the safe, high-level interface and hide the unsafe
internal details.

Wrapping the functions which expect buffers involves using the `vec::raw` module to manipulate Rust
vectors as pointers to memory. Rust's vectors are guaranteed to be a contiguous block of memory. The
length is number of elements currently contained, and the capacity is the total size in elements of
the allocated memory. The length is less than or equal to the capacity.

~~~~ {.xfail-test}
pub fn validate_compressed_buffer(src: &[u8]) -> bool {
    unsafe {
        snappy_validate_compressed_buffer(vec::raw::to_ptr(src), src.len() as size_t) == 0
    }
}
~~~~

The `validate_compressed_buffer` wrapper above makes use of an `unsafe` block, but it makes the
guarantee that calling it is safe for all inputs by leaving off `unsafe` from the function
signature.

The `snappy_compress` and `snappy_uncompress` functions are more complex, since a buffer has to be
allocated to hold the output too.

The `snappy_max_compressed_length` function can be used to allocate a vector with the maximum
required capacity to hold the compressed output. The vector can then be passed to the
`snappy_compress` function as an output parameter. An output parameter is also passed to retrieve
the true length after compression for setting the length.

~~~~ {.xfail-test}
pub fn compress(src: &[u8]) -> ~[u8] {
    unsafe {
        let srclen = src.len() as size_t;
        let psrc = vec::raw::to_ptr(src);

        let mut dstlen = snappy_max_compressed_length(srclen);
        let mut dst = vec::with_capacity(dstlen as uint);
        let pdst = vec::raw::to_mut_ptr(dst);

        snappy_compress(psrc, srclen, pdst, &mut dstlen);
        vec::raw::set_len(&mut dst, dstlen as uint);
        dst
    }
}
~~~~

Decompression is similar, because snappy stores the uncompressed size as part of the compression
format and `snappy_uncompressed_length` will retrieve the exact buffer size required.

~~~~ {.xfail-test}
pub fn uncompress(src: &[u8]) -> Option<~[u8]> {
    unsafe {
        let srclen = src.len() as size_t;
        let psrc = vec::raw::to_ptr(src);

        let mut dstlen: size_t = 0;
        snappy_uncompressed_length(psrc, srclen, &mut dstlen);

        let mut dst = vec::with_capacity(dstlen as uint);
        let pdst = vec::raw::to_mut_ptr(dst);

        if snappy_uncompress(psrc, srclen, pdst, &mut dstlen) == 0 {
            vec::raw::set_len(&mut dst, dstlen as uint);
            Some(dst)
        } else {
            None // SNAPPY_INVALID_INPUT
        }
    }
}
~~~~

For reference, the examples used here are also available as an [library on
GitHub](https://github.com/thestinger/rust-snappy).

# Linking

In addition to the `#[link_args]` attribute for explicitly passing arguments to the linker, an
`extern mod` block will pass `-lmodname` to the linker by default unless it has a `#[nolink]`
attribute applied.

# Unsafe blocks

Some operations, like dereferencing unsafe pointers or calling functions that have been marked
unsafe are only allowed inside unsafe blocks. Unsafe blocks isolate unsafety and are a promise to
the compiler that the unsafety does not leak out of the block.

Unsafe functions, on the other hand, advertise it to the world. An unsafe function is written like
this:

~~~~
unsafe fn kaboom(ptr: *int) -> int { *ptr }
~~~~

This function can only be called from an `unsafe` block or another `unsafe` function.

# Foreign calling conventions

Most foreign code exposes a C ABI, and Rust uses the platform's C calling convention by default when
calling foreign functions. Some foreign functions, most notably the Windows API, use other calling
conventions. Rust provides the `abi` attribute as a way to hint to the compiler which calling
convention to use:

~~~~
#[cfg(target_os = "win32")]
#[abi = "stdcall"]
extern mod kernel32 {
    fn SetEnvironmentVariableA(n: *u8, v: *u8) -> int;
}
~~~~

The `abi` attribute applies to a foreign module (it cannot be applied to a single function within a
module), and must be either `"cdecl"` or `"stdcall"`. The compiler may eventually support other
calling conventions.

# Interoperability with foreign code

Rust guarantees that the layout of a `struct` is compatible with the platform's representation in C.
A `#[packed]` attribute is available, which will lay out the struct members without padding.
However, there are currently no guarantees about the layout of an `enum`.

Rust's owned and managed boxes use non-nullable pointers as handles which point to the contained
object. However, they should not be manually because they are managed by internal allocators.
Borrowed pointers can safely be assumed to be non-nullable pointers directly to the type. However,
breaking the borrow checking or mutability rules is not guaranteed to be safe, so prefer using raw
pointers (`*`) if that's needed because the compiler can't make as many assumptions about them.

Vectors and strings share the same basic memory layout, and utilities are available in the `vec` and
`str` modules for working with C APIs. Strings are terminated with `\0` for interoperability with C,
but it should not be assumed because a slice will not always be nul-terminated. Instead, the
`str::as_c_str` function should be used.

The standard library includes type aliases and function definitions for the C standard library in
the `libc` module, and Rust links against `libc` and `libm` by default.