Converting a variable to a single-element slice in Go via unsafe

July 3, 2019

I was recently reading Chris Wellons' Go Slices are Fat Pointers. At the end of the article, Wellons says:

Slices aren’t as universal as pointers, at least at the moment. You can take the address of any variable using &, but you can’t take a slice of any variable, even if it would be logically sound.

[...] However, if you really wanted to do this, the unsafe package can accomplish it. I believe the resulting slice would be perfectly safe to use:

// Convert to one-element array, then slice
fooslice = (*[1]int)(unsafe.Pointer(&foo))[:]

I had to read this carefully before I understood what it was doing, but then after I read the documentation for unsafe.Pointer() carefully, I believe that this is fully safe. So let's start with what it's doing. The important thing is this portion of the expression:


This is essentially reinterpreting foo from an integer to a one-element array of integers, by taking a pointer to it and then converting that to a pointer to a one-element array. I believe that this use of unsafe.Pointer() is probably valid, because it seems like it falls under the first valid use in the documentation:

(1) Conversion of a *T1 to Pointer to *T2.

Provided that T2 is no larger than T1 and that the two share an equivalent memory layout, this conversion allows reinterpreting data of one type as data of another type. [...]

In Go today, an integer and a one-element array of integers are the same size, making the first clause true and pretty much requiring that the second one is true as well. I don't think that Go requires this in the language specification, but in practice it's very likely to be the case in any implementation that wants to adhere to Go's ethos of efficiency and minimalism. Once we have a valid pointer to a (valid) one-element array of int, it's perfectly legal to create a slice from it, which is what the '[:]' does. So if this use of unsafe is valid, the resulting slice is fully safe and valid.

Now we get to the interesting question of why Go doesn't allow this without the use of unsafe.Pointer(). One possible answer is that this is not allowed simply because it would require extra work in the language specification and the compiler. This may well be the case (and it's certainly a very Go style reason), but another possibly reason is that Go doesn't want to require that all implementations make a one-element array have exactly the same memory layout and implementation as a single variable. By confining this to the limited assurances of unsafe and not making it part of the guaranteed language specification, Go keeps people's options open.

(Of course this is only theoretical, because in practice a new implementation will likely want to reuse as much of the standard library as possible and the current standard library uses unsafe in various places. If you don't match what works with unsafe today in mainline Go, you're going to have to rewrite some of that code. Also, see how unsafe type conversions are still garbage collection safe for some more discussion of this area.)

Comments on this page:

By lilydjwg at 2019-07-04 01:20:43:

I have one question: what use cases this conversion is suited for? You can always create a one-element array if you need.

If it's not very useful, then it's good reason Go doesn't specifically implement it.

Written on 03 July 2019.
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