JAL JAL - 7 months ago 29
Swift Question

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?

Answer

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 hookFunc(message: UnsafePointer<Int8>, _ length: UInt32, _ withSysLogBanner: ObjCBool) -> Void {
   print("hook")
}

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