A Guide to Porting C/C++ to Rust

This book is for people familiar with C or C++ who are thinking of using Rust.

Before we go into what Rust is or why it might be preferable to C/C++ in some cases, let’s think of software that is mission critical and must not or should not fail.

• Operating system services and daemons
• Internet of things devices
• Industrial control software
• Medical devices - MRI, ultrasound, X-ray, ventilators etc.
• High availability servers / databases / cloud storage etc.
• Avionics, telemetry, rocketry, drones etc.

All this code must run as efficiently and reliably as possible. It must run on devices for days, weeks, months or preferably years without failure. It cannot suffer intermittent freezes, erratic performance, memory leaks, crashes or other issues without impacting on its purpose.

Normally such software would be written in C or C++, but consider these every day programming issues that can afflict these languages:

• Dangling pointers. A program calls an invalid pointer causing a crash.
• Buffer overruns / underruns. Code writes beyond an allocated buffer causing memory corruption or a page exception.
• Memory leaks. Code that allocates memory or resources without calling the corresponding free action. C++ provides classes such as smart pointers and techniques like RAII to mitigate these issues but still occur.
• Data races. Multiple threads write to data at the same time causing corruption or other destabilizing behavior.

Rust stops these bad things happening by design. And it does so without impacting on runtime performance because all of these things are checked at compile time:

• Object lifetimes are tracked automatically to prevent memory leaks and dangling pointers.
• The length of arrays and collections is enforced.
• Data race conditions are prevented by strict enforcement of mutex / guards and object ownership.

Code that passes the compiler’s checks is transformed into machine code with similar performance and speed as the equivalent C or C++.

This is a “zero-cost” approach. The compiler enforces the rules so that there is zero runtime cost over the equivalent and correctly written program in C or C++. Safety does not compromise performance.

In addition Rust plays well C. You may invoke C from Rust or invoke Rust from C using foreign function interfaces. You can choose to rewrite a critical section of your codebase leave the remainder alone.

For example, the Firefox browser uses Rust to analyse video stream data - headers and such like where corrupt or malicious code could destabilize the browser or even be exploitable.

Why Rust?

Let’s start by saying if what you have works and is reliable, then the answer to the question is “there’s no reason” you should consider porting.

However if you have code that doesn’t work or isn’t reliable, or hasn’t been written yet or is due a major rewrite then perhaps you have answered your own question.

You could write the code or fixes in C/C++ in which case you have to deal with all the unsafe issues that the language does not protect you from. Or you might consider that choosing a safe-by-design language is a good way to protect you from suffering bugs in the field when the code is supposed to be ready for production.

Rust is not a magic wand

Despite the things the language can protect you against, it cannot protect you against the following:

• General race conditions such as deadlocks between threads
• Unbounded growth, e.g. a loop that pushes values onto a vector until memory is exhausted.
• Application logic errors, i.e. errors that have nothing to do with the underlying language, e.g. missing out the line that should say “if door_open { sound_alarm(); }”
• Explicit unsafe sections doing unsafe and erroneous things
• Errors in LLVM or something outside of Rust’s control.

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