How to use SCNetworkReachability in Swift

Question

I'm trying to convert this code snippet to Swift. I'm struggling on getting off the ground due to some difficulties.

- (BOOL) connectedToNetwork
{
    // Create zero addy
    struct sockaddr_in zeroAddress;
    bzero(&zeroAddress, sizeof(zeroAddress));
    zeroAddress.sin_len = sizeof(zeroAddress);
    zeroAddress.sin_family = AF_INET;

    // Recover reachability flags
    SCNetworkReachabilityRef defaultRouteReachability = SCNetworkReachabilityCreateWithAddress(NULL, (struct sockaddr *)&zeroAddress);
    SCNetworkReachabilityFlags flags;

    BOOL didRetrieveFlags = SCNetworkReachabilityGetFlags(defaultRouteReachability, &flags);
    CFRelease(defaultRouteReachability);

    if (!didRetrieveFlags)
    {
        return NO;
    }

    BOOL isReachable = flags & kSCNetworkFlagsReachable;
    BOOL needsConnection = flags & kSCNetworkFlagsConnectionRequired;

    return (isReachable && !needsConnection) ? YES : NO;
}

The first and the main issue I'm having is on how to define and work with C structs. In the first line (struct sockaddr_in zeroAddress;) of the above code, I think they're defining a instance called zeroAddress from the struct sockaddr_in(?), I assume. I tried declaring a var like this.

var zeroAddress = sockaddr_in()

But I get the error Missing argument for parameter 'sin_len' in call which is understandable because that struct takes a number of arguments. So I tried again.

var zeroAddress = sockaddr_in(sin_len: sizeof(zeroAddress), sin_family: AF_INET, sin_port: nil, sin_addr: nil, sin_zero: nil)

As expected I get some other error Variable used within its own initial value. I understand the cause of that error too. In C, they declare the instance first and then fill up the parameters. Its not possible in Swift as far as I know. So I'm truly lost at this point on what to do.

I read Apple's official document on interacting with C APIs in Swift but it has no examples in working with structs.

Can anyone please help me out here? I'd really appreciate it.

Thank you.

---

UPDATE: Thanks to Martin I was able to get past the initial problem. But still Swift ain't making it easier for me. I'm getting multiple new errors.

func connectedToNetwork() -> Bool {

    var zeroAddress = sockaddr_in(sin_len: 0, sin_family: 0, sin_port: 0, sin_addr: in_addr(s_addr: 0), sin_zero: (0, 0, 0, 0, 0, 0, 0, 0))
    zeroAddress.sin_len = UInt8(sizeofValue(zeroAddress))
    zeroAddress.sin_family = sa_family_t(AF_INET)

    var defaultRouteReachability: SCNetworkReachabilityRef = SCNetworkReachabilityCreateWithAddress(UnsafePointer<Void>, UnsafePointer<zeroAddress>) // 'zeroAddress' is not a type
    var flags = SCNetworkReachabilityFlags()

    let didRetrieveFlags = SCNetworkReachabilityGetFlags(defaultRouteReachability, UnsafeMutablePointer<flags>) // 'flags' is not a type
    defaultRouteReachability.dealloc(1) // 'SCNetworkReachabilityRef' does not have a member named 'dealloc'

    if didRetrieveFlags == false {
        return false
    }

    let isReachable: Bool = flags & kSCNetworkFlagsReachable // Cannot invoke '&' with an argument list of type '(@lvalue UInt32, Int)'
    let needsConnection: Bool = flags & kSCNetworkFlagsConnectionRequired // Cannot invoke '&' with an argument list of type '(@lvalue UInt32, Int)'

    return (isReachable && !needsConnection) ? true : false
}

---

EDIT 1: Okay I changed this line to this,

var defaultRouteReachability: SCNetworkReachabilityRef = SCNetworkReachabilityCreateWithAddress(UnsafePointer<Void>(), &zeroAddress)

The new error I'm getting at this line is 'UnsafePointer' is not convertible to 'CFAllocator'. How to you pass NULL in Swift?

Also I changed this line and the error is gone now.

let didRetrieveFlags = SCNetworkReachabilityGetFlags(defaultRouteReachability, &flags)

---

EDIT 2: I passed nil in this line after seeing this question. But that answer contradicts with the answer here. It says there is no equivalent to NULL in Swift.

var defaultRouteReachability: SCNetworkReachabilityRef = SCNetworkReachabilityCreateWithAddress(nil, &zeroAddress)

Anyway I get a new error saying 'sockaddr_in' is not identical to 'sockaddr' at the above line.

Answer 1

(This answer was extended repeatedly due to changes in the Swift language, which made it a bit confusing. I have now rewritten it and removed everything which refers to Swift 1.x. The older code can be found in the edit history if somebody needs it.)

This is how you would do it in Swift 2.0 (Xcode 7):

import SystemConfiguration

func connectedToNetwork() -> Bool {

    var zeroAddress = sockaddr_in()
    zeroAddress.sin_len = UInt8(sizeofValue(zeroAddress))
    zeroAddress.sin_family = sa_family_t(AF_INET)

    guard let defaultRouteReachability = withUnsafePointer(&zeroAddress, {
        SCNetworkReachabilityCreateWithAddress(nil, UnsafePointer($0))
    }) else {
        return false
    }

    var flags : SCNetworkReachabilityFlags = []
    if !SCNetworkReachabilityGetFlags(defaultRouteReachability, &flags) {
        return false
    }

    let isReachable = flags.contains(.Reachable)
    let needsConnection = flags.contains(.ConnectionRequired)

    return (isReachable && !needsConnection)
}

Explanations:

• As of Swift 1.2 (Xcode 6.3), imported C structs have a default initializer in Swift, which initializes all of the struct's fields to zero, so the socket address structure can be initialized with

  var zeroAddress = sockaddr_in()
  

• sizeofValue() gives the size of this structure, this has to be converted to UInt8 for sin_len:

  zeroAddress.sin_len = UInt8(sizeofValue(zeroAddress))
  

• AF_INET is an Int32, this has to be converted to the correct type for sin_family:

  zeroAddress.sin_family = sa_family_t(AF_INET)
  

• withUnsafePointer(&zeroAddress) { ... } passes the address of the structure to the closure where it is used as argument for SCNetworkReachabilityCreateWithAddress(). The UnsafePointer($0) conversion is needed because that function expects a pointer to sockaddr, not sockaddr_in.

• The value returned from withUnsafePointer() is the return value from SCNetworkReachabilityCreateWithAddress() and that has the type SCNetworkReachability?, i.e. it is an optional. The guard let statement (a new feature in Swift 2.0) assigns the unwrapped value to the defaultRouteReachability variable if it is not nil. Otherwise the else block is executed and the function returns.

• As of Swift 2, SCNetworkReachabilityCreateWithAddress() returns a managed object. You don't have to release it explicitly.

• As of Swift 2, SCNetworkReachabilityFlags conforms to OptionSetType which has a set-like interface. You create an empty flags variable with

  var flags : SCNetworkReachabilityFlags = []
  

  and check for flags with

  let isReachable = flags.contains(.Reachable)
  let needsConnection = flags.contains(.ConnectionRequired)
  

• The second parameter of SCNetworkReachabilityGetFlags has the type UnsafeMutablePointer<SCNetworkReachabilityFlags>, which means that you have to pass the address of the flags variable.

Note also that registering a notifier callback is possible as of Swift 2, compare Working with C APIs from Swift and Swift 2 - UnsafeMutablePointer<Void> to object.

---

Update for Swift 3/4:

Unsafe pointers cannot be simply be converted to a pointer of a different type anymore (see - SE-0107 UnsafeRawPointer API). Here the updated code:

import SystemConfiguration

func connectedToNetwork() -> Bool {

    var zeroAddress = sockaddr_in()
    zeroAddress.sin_len = UInt8(MemoryLayout<sockaddr_in>.size)
    zeroAddress.sin_family = sa_family_t(AF_INET)

    guard let defaultRouteReachability = withUnsafePointer(to: &zeroAddress, {
        $0.withMemoryRebound(to: sockaddr.self, capacity: 1) {
            SCNetworkReachabilityCreateWithAddress(nil, $0)
        }
    }) else {
        return false
    }

    var flags: SCNetworkReachabilityFlags = []
    if !SCNetworkReachabilityGetFlags(defaultRouteReachability, &flags) {
        return false
    }

    let isReachable = flags.contains(.reachable)
    let needsConnection = flags.contains(.connectionRequired)

    return (isReachable && !needsConnection)
}