Skip to main content

Go: A Surprising Edge Case Concerning append and Slice Aliasing

In Go, the append function reallocates the underlying array if there's no more room. Hence, if you have two slices that point to the same array, after you append to one of the slices, the other slice may or may not point to the same array. This can lead to some unexpected behavior:

package main

import "fmt"

func main() {
 // Create a slice of length 1 and capacity 2.
 a := make([]int, 1, 2)

 // b "aliases" a
 b := a

 // If I set something in a, you see it in b.
 a[0] = 1
 fmt.Println(a[0], b[0]) // Prints 1 1; they're equal.

 // append returns a new slice, but with the same array.
 // Hence, if I set something in a, you still see it in b.
 a = append(a, 2)
 a[0] = 2
 fmt.Println(a[0], b[0]) // Prints 2 2; they're equal.

 // I'm doing the same thing as last time. However, this time,
 // append has to allocate a new array because there's not enough
 // space. Hence, if I set something in a, you don't see it in b.
 a = append(a, 3)
 a[0] = 3
 fmt.Println(a[0], b[0]) // Prints 3 2; they're *not* equal.
}

Having multiple aliases to the same mutable data can lead to some subtle behavior and lots of bugs if you're not careful. It's perhaps even more surprising to the naive reader since a and b don't look like pointers. In fact, they're not. They're slices which are a value type that have a pointer within them.

Immutable data structures (like lists in Lisp) don't have these issues. However, you lose some of the performance that a raw array can provide.


Comments

Nick said…
This is called, as I'm sure you know, a dangling pointer. One of the many endemic problems with this little language we all tried so hard to love.
Actually doubt very much `b` is dangling to deallocated memory in this case.
Anonymous said…
Prints 2. That's not a dangling pointer. One of the many endemic problems with this little developer that the language tried so hard to help.
Brian Olson said…
Nothing to see here. The contract for append() has always been that it might return the original pointer or it might copy-and-extend into a new pointer. That you can keep a reference of the previous version is programmer error for not having clear ownership of that pointer. There should be one canonical context for it, being a member of a struct or a local variable on the stack. Then you don't have to worry about having two copies of it. If you need multiple threads accessing the pointer, then you should have put a mutex on it.
Anonymous said…
> Immutable data structures (like lists in Lisp) don't have these issues. However, you lose some of the performance that a raw array can provide.

Um, what?

Lisp lists are very highly mutable. Aaaand they're not arrays.
jjinux said…
What I mean is that adding to the head of a list in Lisp does not impact the original list.
Unknown said…
It's not an alias. a := b copies the value of b into a.
jjinux said…
> It's not an alias. a := b copies the value of b into a.

Yes, I know. However, a and b are both references to the same underlying array. "alias" was the best term I could come up with.
Akshay said…
It is very clearly defined in Effective Go
Hence it is not a edge case

"Slices hold references to an underlying array, and if you assign one slice to another, both refer to the same array."

https://golang.org/doc/effective_go.html#slices
jjinux said…
This is known as the "Stale" Slice problem: http://devs.cloudimmunity.com/gotchas-and-common-mistakes-in-go-golang/index.html#stale_slices

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

Creating Windows 10 Boot Media for a Lenovo Thinkpad T410 Using Only a Mac and a Linux Machine

TL;DR: Giovanni and I struggled trying to get Windows 10 installed on the Lenovo Thinkpad T410. We struggled a lot trying to create the installation media because we only had a Mac and a Linux machine to work with. Everytime we tried to boot the USB thumb drive, it just showed us a blinking cursor. At the end, we finally realized that Windows 10 wasn't supported on this laptop :-/ I've heard that it took Thomas Edison 100 tries to figure out the right material to use as a lightbulb filament. Well, I'm no Thomas Edison, but I thought it might be noteworthy to document our attempts at getting it to boot off a USB thumb drive: Download the ISO. Attempt 1: Use Etcher. Etcher says it doesn't work for Windows. Attempt 2: Use Boot Camp Assistant. It doesn't have that feature anymore. Attempt 3: Use Disk Utility on a Mac. Erase a USB thumb drive: Format: ExFAT Scheme: GUID Partition Map Mount the ISO. Copy everything from