Cast to different C struct unsafe pointer in Swift

Question

I want to call the Posix socket functions socket and bind from Swift. socket is pretty easy—it takes Int32s, but bind is causing a problem, because I have a sockaddr_in pointer, but it wants a sockaddr pointer. In C, this would be a cast, like:

bind(sock, (struct sockaddr *)&sockAddress, sizeof(sockAddress))

Here's an attempt in Swift:

let sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)
var sockAddress = sockaddr_in()        
bind(sock, &sockAddress, UInt32(MemoryLayout<sockaddr_in>.size))

The bind line fails to compile with: cannot convert value of type 'sockaddr_in' to expected argument type 'sockaddr'

How do I cast the pointer?

score 8 · Accepted Answer · edited May 23 '17 at 11:45

8

You can write something like this:

withUnsafePointer(to: &sockAddress) {sockaddrInPtr in
    sockaddrInPtr.withMemoryRebound(to: sockaddr.self, capacity: 1) {sockaddrPtr in
        bind(sock, sockaddrPtr, UInt32(MemoryLayout<sockaddr_in>.stride))
    }
}

Or someone suggests this may be better:

withUnsafePointer(to: &sockAddress) {sockaddrInPtr in
    let sockaddrPtr = UnsafeRawPointer(sockaddrInPtr).assumingMemoryBound(to: sockaddr.self)
    bind(sock, sockaddrPtr, UInt32(MemoryLayout<sockaddr_in>.stride))
}

This article may be some help.

(UPDATE) As described in the link shown by Martin R, now MemoryLayout<T>.stride and MemoryLayout<T>.size return the same value which is consistent with C's sizeof, where T is an imported C-struct. I'll keep my stride version of answer here, but that is not something "required" in this case now.

edited May 23 '17 at 11:45

Community

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answered Aug 27 '16 at 18:57

OOPer

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OOPs, had not seen your answer :) – Martin R Aug 27 '16 at 19:00
[This article](https://forums.developer.apple.com/thread/60350#171911) suggests that `UnsafeRawPointer` works in some cases. But I'm not sure which is the better in this case. Any further info are welcome. – OOPer Aug 27 '16 at 19:06
According to https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20160815/002974.html, both are correct, with a slight favor of "withMemoryRebound", as I understand it. – Martin R Aug 27 '16 at 19:09
@MartinR, thanks. I could not be sure, with a slight glance to the article. I'll read it again taking more time. – OOPer Aug 27 '16 at 19:14
Here http://stackoverflow.com/a/33177600/1187415 is a case where "size" did not work, only "stride". It *is* low-level ... – Martin R Aug 27 '16 at 20:16
@MartinR, thanks. It seems we should always follow your third remark. I'll update my answer. – OOPer Aug 27 '16 at 20:21
I just noticed (http://stackoverflow.com/a/39302927/1187415) that in Swift 3, size == stride for structs imported from C. So that problem does not exist anymore. – Martin R Sep 03 '16 at 03:38
Started to write a comment regarding casting with double pointers, but I figured it would make more sense in a separate thread, if someone is interested in offering some help: https://stackoverflow.com/questions/51138411/cast-c-struct-double-pointer-to-a-swift-struct-unsafe-pointer – agirault Jul 02 '18 at 14:33

Martin R · Answer 2 · 2016-09-03T03:40:35.447

4

In Swift 3 you have to "rebind" the pointer (compare SE-0107 UnsafeRawPointer API):

let sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)
var sockAddress = sockaddr_in()

let result = withUnsafePointer(to: &sockAddress) {
    $0.withMemoryRebound(to: sockaddr.self, capacity: 1) {
        bind(sock, $0, socklen_t(MemoryLayout<sockaddr_in>.stride))
    }
}

Remarks:

The type annotations in let sock: Int32 and var sockAddress: sockaddr_in are not needed.
The memset() is not necessary because sockaddr_in() initializes all struct members to zero.
~~The Swift equivalent of the C sizeof is stride (which includes a possible struct padding), not size (which does not include the struct padding).~~ (This "problem" does not exist anymore. For structs imported from C, stride and size have the same value.)

edited Sep 03 '16 at 03:40

answered Aug 27 '16 at 18:57

Martin R

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If you try to use `sizeof`, Xcode recommends replacing it with `size`. – Rob N Aug 27 '16 at 20:07
@RobN: Then file a bug report for Xcode :) In C, sizeof is the size of the struct *including* a possible padding at the end. In Swift, stride includes the struct padding, and size doesn't. – Martin R Aug 27 '16 at 20:08
Ha. :) Well, I'll let someone else do that, since I don't know it's wrong. I did fix the question per your other remarks, to remove the noise code. Thanks! – Rob N Aug 27 '16 at 20:09
@RobN: It makes no difference if the struct has no padding at the end. With stride you are on the safe side. Here http://stackoverflow.com/a/33177600/1187415 is a (perhaps exotic) case where using size did not work. – Martin R Aug 27 '16 at 20:12

score 0 · Answer 3 · answered Jul 10 '19 at 10:16

Swift 5 obsolete the withUnsafeBytes(UnsafePointer<sockaddr>), so below is what I'm doing with Swift 5:

        var hostname = [CChar](repeating: 0, count: Int(NI_MAXHOST))
    return withUnsafeBytes { (p: UnsafeRawBufferPointer) -> String? in
        let addr = p.baseAddress?.assumingMemoryBound(to: sockaddr.self)
        guard getnameinfo(addr, socklen_t(self.count), &hostname, socklen_t(hostname.count), nil, 0, NI_NUMERICHOST) == 0 else {
            return nil
        }
        return String(cString: hostname)
    }

Cast to different C struct unsafe pointer in Swift

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