Working with C APIs from Swift

Working with C APIs from Swift

Your first problem can be solved with

let reachability = host.withCString {
    SCNetworkReachabilityCreateWithName(nil, $0).takeRetainedValue()
}

Inside the closure, $0 is a pointer to the NUL-terminated UTF-8 representation of the
String.

Update: As Nate Cook said in
a now deleted answer and also here,
you can actually pass a Swift string to a function taking a UnsafePointer
directly:

let reachability = SCNetworkReachabilityCreateWithName(nil, host).takeRetainedValue()

Unfortunately there is (as far as I know) currently no solution to your second problem.
SCNetworkReachabilitySetCallback expects a pointer to a C function as second
parameter, and there is currently no method to pass a Swift function or closure.
Note that the documentation for SCNetworkReachabilityCallBack
shows only Objective-C but no Swift.

See also Does Swift not work with function pointers?.

Update for Swift 2: It is now possible to pass a Swift closure
to a C function taking a function pointer parameter. Also
SCNetworkReachabilityCreateWithName()
does not return an unmanaged object anymore:

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
}, &context) 

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes)

Update for Swift 3 (Xcode 8):

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
    }, &context)

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(),
                                         CFRunLoopMode.commonModes.rawValue)

Is it possible to mix C and Swift?

It is very much possible to mix Swift and C. In fact it's pretty easy. For example, if you type import Darwin into a playground, you will find all sorts of C functions:

import Darwin

let i = atoi("42")
let path = String.fromCString(getenv("PATH"))

let voidptr = malloc(10 * UInt(sizeof(Int)))
let ptr = UnsafeMutablePointer(voidptr)
for i in 0..<10 {
    ptr[i] = i
}
let buffer = UnsafeBufferPointer(start: ptr, count: 10)
reduce(buffer, 0, +) // sum(1 to 10) = 45
free(ptr)

There are various convenience conversions, for example in the above code, "42" was automatically converted to a C-style string to be passed into atoi. Similarly, if you declare var i: Int = 0, you can pass that into a C API that takes a C pointer-to-int as f(&i), and arrays are similarly auto-converted to pointers when you pass them in.

See the docs on the Apple website for more info on how you can interact with C from Swift.

Working with C APIs from Swift

Your first problem can be solved with

let reachability = host.withCString {
    SCNetworkReachabilityCreateWithName(nil, $0).takeRetainedValue()
}

Inside the closure, $0 is a pointer to the NUL-terminated UTF-8 representation of the
String.

Update: As Nate Cook said in
a now deleted answer and also here,
you can actually pass a Swift string to a function taking a UnsafePointer
directly:

let reachability = SCNetworkReachabilityCreateWithName(nil, host).takeRetainedValue()

Unfortunately there is (as far as I know) currently no solution to your second problem.
SCNetworkReachabilitySetCallback expects a pointer to a C function as second
parameter, and there is currently no method to pass a Swift function or closure.
Note that the documentation for SCNetworkReachabilityCallBack
shows only Objective-C but no Swift.

See also Does Swift not work with function pointers?.

Update for Swift 2: It is now possible to pass a Swift closure
to a C function taking a function pointer parameter. Also
SCNetworkReachabilityCreateWithName()
does not return an unmanaged object anymore:

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
}, &context) 

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(), kCFRunLoopCommonModes)

Update for Swift 3 (Xcode 8):

let host = "google.com"
var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
let reachability = SCNetworkReachabilityCreateWithName(nil, host)!

SCNetworkReachabilitySetCallback(reachability, { (_, flags, _) in
    print(flags)
    }, &context)

SCNetworkReachabilityScheduleWithRunLoop(reachability, CFRunLoopGetMain(),
                                         CFRunLoopMode.commonModes.rawValue)

Using LLVM C API in a Swift Xcode Project

Try installing LLVM pre-compiled by simply running brew install llvm using Homebrew.

NOTE: I strongly recommend using a Swift wrapper such as LLVMSwift, in which case you should follow its installation instructions from here on. But if you would like to directly access LLVM yourself, then read on.

Add /usr/local/opt/llvm/include to your header search paths and /usr/local/opt/llvm/lib to your library search paths under the desired target of your project under "Build Settings":

And drag /usr/local/opt/llvm/lib/libLLVM.dylib (open in Finder with open -R '/usr/local/opt/llvm/lib/libLLVM.dylib') to "Linked Frameworks and Libraries" under "General" (and make it "Required" as shown):

Finally, create an Objective-C Bridging Header (steps 1-2 in this tutorial if you are not sure how) and include whichever headers you need (for example, #include ):

And you're all set! Simply use any LLVM class as you normally would in Swift code.

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