Search code examples
cswiftfunction-pointersiphone-privateapi

Representing NULL Function Pointers to C Functions in Swift

Consider the private-yet-sort-of-documented Cocoa C functions _NSLogCStringFunction() and _NSSetLogCStringFunction(). _NSLogCStringFunction() returns a function pointer to the C function used by the Objective-C runtime behind-the-scenes for NSLog(), and _NSSetLogCStringFunction() allows developers to specify their own C function for logging. More information on both of these functions can be found in this Stack Overflow question and this WebObjects support article.

In C, I can pass in a NULL function pointer to _NSSetLogCStringFunction():

extern void _NSSetLogCStringFunction(void(*)(const char*, unsigned, BOOL));

_NSSetLogCStringFunction(NULL); // valid

However, I'm running into some issues when I try to do this in pure Swift:

/// Represents the C function signature used under-the-hood by NSLog
typealias NSLogCStringFunc = (UnsafePointer<Int8>, UInt32, Bool) -> Void

/// Sets the C function used by NSLog
@_silgen_name("_NSSetLogCStringFunction")
func _NSSetLogCStringFunction(_: NSLogCStringFunc) -> Void

_NSSetLogCStringFunction(nil) // Error: nil is not compatible with expected argument type 'NSLogCStringFunc' (aka '(UnsafePointer<Int8>, UInt32, Bool) -> ()')

If I try bypass this compile-time warning with unsafeBitCast, my program just crashes with EXC_BAD_INSTRUCTION (as expected, since the signature is wrong):

let nullPtr: UnsafePointer<Void> = nil
let nullFuncPtr = unsafeBitCast(nullPtr, NSLogCStringFunc.self)
_NSSetLogCStringFunction(nullFuncPtr) // crash

How do I represent a NULL function pointer to (void *) or (void(*)(const char *, unsigned, BOOL))/(UnsafePointer<Int8>, UInt32, Bool) -> Void in Swift?

Solution

  • The Swift mapping of the (Objective-)C declaration

    extern void _NSSetLogCStringFunction(void(*)(const char*, unsigned, BOOL));

    is 

    public func _NSSetLogCStringFunction(_: (@convention(c) (UnsafePointer<Int8>, UInt32, ObjCBool) -> Void)!)

    The easiest solution would be to put the Objective-C extern declaration into an Objective-C header file and include that from the bridging header.

    Alternatively, in pure Swift it should be

    typealias NSLogCStringFunc = @convention(c) (UnsafePointer<Int8>, UInt32, ObjCBool) -> Void

@_silgen_name("_NSSetLogCStringFunction")
func _NSSetLogCStringFunction(_: NSLogCStringFunc!) -> Void

    In either case, the function parameter is an implicitly unwrapped optional, and you can call it with nil. Example:

    func myLogger(message: UnsafePointer<Int8>, _ length: UInt32, _ withSysLogBanner: ObjCBool) -> Void {
    print(String(format:"myLogger: %s", message))
}

_NSSetLogCStringFunction(myLogger) // Set NSLog hook.
NSLog("foo")
_NSSetLogCStringFunction(nil) // Reset to default.
NSLog("bar")

    Output:

    myLogger: foo
2016-04-28 18:24:05.492 prog[29953:444704] bar