src/unsafe-rust/unsafe-functions.md

---
minutes: 15
---

# Unsafe Functions

A function or method can be marked `unsafe` if it has extra preconditions you
must uphold to avoid undefined behaviour.

There are two main categories:

- Rust functions declared unsafe with `unsafe fn`.
- Foreign functions in `extern "C"` blocks.

<details>

We will look at the two kinds of unsafe functions next.

</details>
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00			`---`
Split large unsafe function slide (#2406) The old slice was doing several things at the same time: demonstrating both external functions as well as unsafe Rust functions. We now treat those two topics separately. In addition, the “Calling Unsafe Functions” heading has become its own slide with a non-crashing example that shows what can go wrong if an argument is misunderstood in a call to an unsafe function. The old example didn’t actually illustrate the danger clearly: it would produce mangled UTF-8 output, which the Playground server refuses to print. Part of #2445. --------- Co-authored-by: Dustin J. Mitchell <djmitche@google.com> Co-authored-by: Andrew Walbran <qwandor@google.com> 2025-02-05 15:00:14 +01:00			`minutes: 15`
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00			`---`

			`# Unsafe Functions`

			A function or method can be marked `unsafe` if it has extra preconditions you
Split large unsafe function slide (#2406) The old slice was doing several things at the same time: demonstrating both external functions as well as unsafe Rust functions. We now treat those two topics separately. In addition, the “Calling Unsafe Functions” heading has become its own slide with a non-crashing example that shows what can go wrong if an argument is misunderstood in a call to an unsafe function. The old example didn’t actually illustrate the danger clearly: it would produce mangled UTF-8 output, which the Playground server refuses to print. Part of #2445. --------- Co-authored-by: Dustin J. Mitchell <djmitche@google.com> Co-authored-by: Andrew Walbran <qwandor@google.com> 2025-02-05 15:00:14 +01:00			`must uphold to avoid undefined behaviour.`
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00
Split large unsafe function slide (#2406) The old slice was doing several things at the same time: demonstrating both external functions as well as unsafe Rust functions. We now treat those two topics separately. In addition, the “Calling Unsafe Functions” heading has become its own slide with a non-crashing example that shows what can go wrong if an argument is misunderstood in a call to an unsafe function. The old example didn’t actually illustrate the danger clearly: it would produce mangled UTF-8 output, which the Playground server refuses to print. Part of #2445. --------- Co-authored-by: Dustin J. Mitchell <djmitche@google.com> Co-authored-by: Andrew Walbran <qwandor@google.com> 2025-02-05 15:00:14 +01:00			`There are two main categories:`
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00
Split large unsafe function slide (#2406) The old slice was doing several things at the same time: demonstrating both external functions as well as unsafe Rust functions. We now treat those two topics separately. In addition, the “Calling Unsafe Functions” heading has become its own slide with a non-crashing example that shows what can go wrong if an argument is misunderstood in a call to an unsafe function. The old example didn’t actually illustrate the danger clearly: it would produce mangled UTF-8 output, which the Playground server refuses to print. Part of #2445. --------- Co-authored-by: Dustin J. Mitchell <djmitche@google.com> Co-authored-by: Andrew Walbran <qwandor@google.com> 2025-02-05 15:00:14 +01:00			- Rust functions declared unsafe with `unsafe fn`.
			- Foreign functions in `extern "C"` blocks.
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00
			`<details>`

Split large unsafe function slide (#2406) The old slice was doing several things at the same time: demonstrating both external functions as well as unsafe Rust functions. We now treat those two topics separately. In addition, the “Calling Unsafe Functions” heading has become its own slide with a non-crashing example that shows what can go wrong if an argument is misunderstood in a call to an unsafe function. The old example didn’t actually illustrate the danger clearly: it would produce mangled UTF-8 output, which the Playground server refuses to print. Part of #2445. --------- Co-authored-by: Dustin J. Mitchell <djmitche@google.com> Co-authored-by: Andrew Walbran <qwandor@google.com> 2025-02-05 15:00:14 +01:00			`We will look at the two kinds of unsafe functions next.`
Comprehensive Rust v2 (#1073) I've taken some work by @fw-immunant and others on the new organization of the course and condensed it into a form amenable to a text editor and some computational analysis. You can see the inputs in `course.py` but the interesting bits are the output: `outline.md` and `slides.md`. The idea is to break the course into more, smaller segments with exercises at the ends and breaks in between. So `outline.md` lists the segments, their duration, and sums those durations up per-day. It shows we're about an hour too long right now! There are more details of the segments in `slides.md`, or you can see mostly the same stuff in `course.py`. This now contains all of the content from the v1 course, ensuring both that we've covered everything and that we'll have somewhere to redirect every page. Fixes #1082. Fixes #1465. --------- Co-authored-by: Nicole LeGare <dlegare.1001@gmail.com> Co-authored-by: Martin Geisler <mgeisler@google.com> 2023-11-29 10:39:24 -05:00
			`</details>`