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Apple System Log Facility

The Apple System Log facility does not seem to be very wel known, but it is a very useful logging tool. For help during debugging as well as to help users when they encounter problems. Unfortunately apple seems to have forgotten to include ASL in Swift as of Xcode 6.1.1.

Its not too difficult to use ASL from Swift though. It can be done the following way:

Create a file "asl-bridge.h" with the following content:

#import <asl.h>
#import <Foundation/Foundation.h>

#ifndef SwiftFire_asl_bridge_h
#define SwiftFire_asl_bridge_h

int asl_bridge_log_message(int level, NSString *message);


Then add the implementation "asl-bridge.m":

#import <Foundation/Foundation.h>

#import <asl.h>
#import "asl-bridge.h"

int asl_bridge_log_message(int level, NSString *message) {
    return asl_log_message(level, "%s", [message cStringUsingEncoding:NSUTF8StringEncoding]);


In the MyProject-Bridging-Header.h include the following:

#import "asl-bridge.h"
#import <asl.h>

The above includes the ASL headers you need, and provide an interface for the logging function without generating a "unsafe" warning.

Since I don't like how the log levels in objective-c are defined I have also created a more Swifty-er log level enum definition:

enum LogLevel: Int {
    case DEBUG      = 0
    case INFO       = 1
    case NOTICE     = 2
    case WARNING    = 3
    case ERROR      = 4
    case CRITICAL   = 5
    case ALERT      = 6
    case EMERGENCY  = 7
    case NONE       = 8
    func toString() -> String {
        switch self {
        case .DEBUG:     return "DEBUG    "
        case .INFO:      return "INFO     "
        case .NOTICE:    return "NOTICE   "
        case .WARNING:   return "WARNING  "
        case .ERROR:     return "ERROR    "
        case .CRITICAL:  return "CRITICAL "
        case .ALERT:     return "ALERT    "
        case .EMERGENCY: return "EMERGENCY"
        case .NONE:      return "NONE     "
    func toAslLevel() -> Int32 {
        switch self {
        case .DEBUG:     return 7
        case .INFO:      return 6
        case .NOTICE:    return 5
        case .WARNING:   return 4
        case .ERROR:     return 3
        case .CRITICAL:  return 2
        case .ALERT:     return 1
        case .EMERGENCY: return 0
        case .NONE:      return -1

Finally we get to define the function I actually use for logging:

    private func logAslFacility(level: LogLevel, message: String) {
        asl_bridge_log_message(level.toAslLevel(), message)

Edit: See also part 2

Added 2015-05-08: To see the above in action (and more), check out my github project "SwifterLog" in the upper right corner of this page.

Happy coding

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  1. I commented saying "Where do I see the logs?" but figured out that the Console app was filtering out logs with levels lower than Warning. Hmm, we need to add more Swift API to set the filter level.. :-)

  2. Hi Andrew, see also part 2 of this post (link at the bottom of the article). There I wrote that Apple will by default filter out the levels "DEBUG" and "INFO". You can see this in the ASL configuration file located in: /etc/asl.conf
    If you want to see those levels, you have to edit that file.

    Also note that I have made SwifterLog (link in the upper right corner). When you use that, you get a "Swifter" interface to the ASL, but also can redirect output to the Xcode console or into a file.