This is a follow-up on yesterday’s article about using a tuple to mimic a stack-allocated array. Please read that one first if you haven’t already.
Yesterday, we saw how you can obtain a pointer to a tuple’s underlying memory and treat it as a collection. Today, I’d like to discuss how we can use the same approach when interfacing with C APIs that work with fixed-size arrays.
Swift imports fixed-size C arrays as tuples
The Swift equivalent of the C type float[4] would be (Float, Float, Float, Float). This has the benefit of incurring no bridging overhead because the Swift compiler can lay out tuples in a C-compatible way.
A tuple often isn’t a convenient type for the user of the API, though — as we’ve seen, you can’t iterate over a tuple’s elements or access one via subscripting.
Example: char[256]
Let’s look at an example. The uname function returns some strings identifying the current platform. You call uname by passing it a pointer to a struct which uname then fills with values. The struct is defined like this in C:
struct utsname {
char sysname[256]; /* [XSI] Name of OS */
char nodename[256]; /* [XSI] Name of this network node */
char release[256]; /* [XSI] Release level */
char version[256]; /* [XSI] Version level */
char machine[256]; /* [XSI] Hardware type */
};
You know what’s coming next, right? Swift’s Clang importer sees the char[256] declarations and turns them into 256-tuples(!). This is how the same type appears in Swift (feel free to count the number of elements):
public struct utsname {
public var sysname: (Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8, Int8) /* [XSI] Name of OS */
public var nodename: (Int8, /* ... 254 more ... */, Int8) /* [XSI] Name of this network node */
public var release: (Int8, /* ... 254 more ... */, Int8) /* [XSI] Release level */
public var version: (Int8, /* ... 254 more ... */, Int8) /* [XSI] Version level */
public var machine: (Int8, /* ... 254 more ... */, Int8) /* [XSI] Hardware type */
}
Calling uname on macOS is as simple as this:
import Darwin
var utsInfo = utsname()
uname(&utsInfo)
But how do you then proceed to extract the information in the utsInfo struct? utsInfo.machine is a 256-tuple of integers, so it’s unusable in its current form:
print(utsInfo.machine)
// → (120, 56, 54, 95, 54, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
Using _FixedArray256
One approach is to generate a _FixedArray256 type as I showed yesterday. You can then pass one of the fields in utsInfo directly to the _FixedArray256 initializer. From then on, you have the full Collection API at your disposal to work with the data:
// Assuming we have generated a _FixedArray256 type
let machine = _FixedArray256(storage: utsInfo.machine)
This is probably the way to go if you’re dealing with smaller arrays of numbers etc., but in this particular case there are two significant downsides to this approach:
-
256 elements is a pretty big tuple, and the impact on code size and compile times is not negligible. I did a quick test: my version of
_FixedArray256(approximately 1,600 lines of generated Swift code) takes about 8 seconds to compile and adds nearly 750 KB to the compiled binary (with optimizations enabled). The convenience doesn’t come for free. -
The second problem only applies to APIs that return strings. The strings the
unamefunctions writes into the struct are actually null-terminated — the length of 256 bytes is just an upper limit. As a result, most of the elements in our “array” would contain garbage data and we’d have to process the data further (i.e. strip everything after the first null byte).
_FixedArray256 takes about 8 seconds to compile and adds nearly 750 KB to the compiled binary.
Using withUnsafeBytes(of:) directly
A better approach is to not use _FixedArray256 at all. Instead, let’s just take the idea of obtaining a pointer to the tuple’s storage and using that to initialize a string. Since String already has a matching initializer for a null-terminated C string, this can be done in very little code:
let machine = withUnsafeBytes(of: &utsInfo.machine) { (rawPtr) -> String in
let ptr = rawPtr.baseAddress!.assumingMemoryBound(to: CChar.self)
return String(cString: ptr)
}
print(machine) // → x86_64
This compiles much faster and doesn’t have a significant impact on code size.