Programming Rust, 2nd Edition

Chapter 23. Foreign Functions

Cyberspace. Unthinkable complexity. Lines of light ranged in the non-space of the mind, clusters and constellations of data. Like city lights, receding . . .

William Gibson, Neuromancer

Tragically, not every program in the world is written in Rust. There are many critical libraries and interfaces implemented in other languages that we would like to be able to use in our Rust programs. Rust’s foreign function interface (FFI) lets Rust code call functions written in C, and in some cases C++. Since most operating systems offer C interfaces, Rust’s foreign function interface allows immediate access to all sorts of low-level facilities.

In this chapter, we’ll write a program that links with libgit2, a C library for working with the Git version control system. First, we’ll show what it’s like to use C functions directly from Rust, using the unsafe features demonstrated in the previous chapter. Then, we’ll show how to construct a safe interface to libgit2, taking inspiration from the open source git2-rs crate, which does exactly that.

We’ll assume that you’re familiar with C and the mechanics of compiling and linking C programs. Working with C++ is similar. We’ll also assume that you’re somewhat familiar with the Git version control system.

There do exist Rust crates for communicating with many other languages, including Python, JavaScript, Lua, and Java. We don’t have room to cover them here, but ultimately, all these interfaces are built using the C ...