TweetFollow Us on Twitter

Simple Call Stack Logging

Volume Number: 25
Issue Number: 06
Column Tag: Programming

Simple Call Stack Logging

Who called NSLog()?

by Sengan Baring-Gould

In this article I present an extension to NSLog() which not only prints out a user specified message but also lists the function calls that led to its invocation. By the end of this article you will have a new tool you can use in your own applications, and you'll understand how it works so that you can adapt it to your needs.

Why log?

NSLog() is an important tool for debugging. It can be placed anywhere in an application to log internal state.

Many programmers prefer logging to using a debugger, as it helps them concentrate on possible causes of a problem while filtering out irrelevant information. Logging provides a complete textual record of the problem that can be studied later.

Debuggers on the other hand interrupt the developer who must record by hand all the relevant state before letting the application continue. Continuing is an unforgiving operation: once performed, prior state that was not recorded is lost.

Brian W Kernighan (one of the authors of the seminal text on the C language) said:

"As a personal choice, we tend not to use debuggers beyond getting a stack trace or the value of a variable or two. One reason is that it is easy to get lost in details of complicated data structures and control flow; we find stepping through a program less productive than thinking harder and adding output statements and self-checking code at critical places. Clicking over statements takes longer than scanning the output of judiciously-placed displays. It takes less time to decide where to put print statements than to single-step to the critical section of code, even assuming we know where that is. More important, debugging statements stay with the program; debugger sessions are transient".

However, if NSLog() is invoked from a function that is called from many other functions, determining which function call caused the bug can be very difficult. We need a record of the function calls that led to the invocation of NSLog().

Where we are headed

Our new debug function debugLog() will print out any message we want the same way NSLog() does. Following the message, it will list the function invocations that led it to be called:

2009-04-05 15:37:58.119 TestDebugLog[14442:10b] C++ constructor
        0000301f — CPP::CPP() + 33             (TestDebugLog)
        0000316c — main + 86                   (TestDebugLog)
        00001a5e — start + 54                  (TestDebugLog)

The first line follows NSLog()'s traditional format: the date, the time, the name of the application (TestDebugLog) and then the message we passed as argument: "C++ Constructor".

On the next lines, debugLog() lists the function invocations that led it to be called:

  • debugLog() was called from the C++ constructor CPP::CPP() defined in TestDebugLog.
  • CPP:CPP() was called by main also defined in TestDebugLog.
  • main was called by start also defined in TestDebugLog. (start is the function the Operating System calls when it starts an application).

The module name between parentheses specifies where a function is defined. In the following example, NSPopAutoreleasePool is shown to be defined in the Foundation framework:

2009-04-05 15:40:03.921 TestDebugLog[14462:10b] Objective-C dealloc
        000030d3 — [Objc dealloc]  + 33        (TestDebugLog)
        91117e4f — NSPopAutoreleasePool + 431  (Foundation)
        000031d1 — main + 207                  (TestDebugLog)
        00001a4a — start + 54                  (TestDebugLog)

Obtaining the list of function invocations

Obtaining the list of function invocations to print is a two step process. First, debugLog() must obtain the addresses of the functions that called it. A computer uses addresses to keep track of what it is doing, but addresses are not specified in a program's source-code.

Then debugLog() must translate these addresses into the function names that appear in the program's source code. Three different methods are required to obtain C, C++, and Objective-C function names.

1. Retrieving the list of called functions' addresses

Compilers transform source-code into machine code that computers understand. When a function is called, the caller's return address must be saved so that the CPU can continue running the caller after the function invocation completes. In the context of this discussion, we will assume that these return addresses are stored on the stack. (We will ignore specific optimizations used by the PowerPC and x86 CPUs).

Unfortunately, the stack also records other information, such as local variables and function arguments. Determining precisely which items in the stack are return addresses requires compiler specific knowledge. Conveniently, the new version of gcc which ships with Leopard provides a new function, backtrace(), which gives us the return addresses in the current stack.

Remember that inlined functions are embedded within their callers rather than being invoked. This means backtrace will not see them in the stack and they will not be listed by debugLog().

2. Function layout in memory

Functions are compiled independently by the compiler and occupy contiguous areas of memory. Therefore if we know the starting addresses of any two consecutive functions f and g, and if we have an address x which falls between f and g ( &f <= x < &g ), we know that x belongs to the function f.

One rarely has addresses that match the beginning of each function precisely. Therefore function-lookup functions are designed to return information about the preceding function when given an address. Our case matches this scenario: the return addresses provided by backtrace occur within functions. Thus we can safely ignore the difference between return addresses and starting function addresses for most of this discussion. Similarly, we'll adopt the standard convention of referring to starting function addresses as function addresses.

3. Retrieving C function names

When an application is first loaded into memory, it needs to be told the addresses of the external library functions it wants to use. Because libraries are updated independently of applications, the addresses of their functions may change, although the names of their functions will not. It is the responsibility of the dynamic linker to give each application this information.

The dynamic linker reads function names from a symbol table embedded in the application and the libraries the application uses. The symbol table lists all the C function addresses and the C function names that can be accessed externally. Therefore if we know an address, we can ask the dynamic linker for the corresponding function name. The backtrace_symbols() function provides this functionality.

Because the dynamic linker only knows about externally visible function names, backtrace_symbols() always returns the nearest preceding external symbol. Static C functions are not exported and will not be given the correct name by the linker.

Most symbols will be exported as external if you compile your application in Debug Configuration. This is not true if you compile it in Release Configuration. The UNIX utility nm lists exported function names with a preceding capital T letting you check if an unexpected function name shows up.

4. Retrieving Objective-C method names

Objective-C does not use the dynamic linker. Instead it uses the Objective-C runtime, which like the dynamic linker keeps track of all known method addresses and names. (A method is a function defined within a class). However there is no equivalent to backtrace_symbols() which returns a function name when given a function address. We must build one ourselves.

The Objective-C runtime provides a function to enumerate all the Objective-C classes that can be invoked by the application, including those in the frameworks bound to the application. It also provides a function to enumerate the methods in any Objective-C class. With these components we can obtain every method's address and name.

Implementation

This code is Objective-C++ so don't forget to use the ".mm" extension for your implementation filename. We start with the necessary includes:

DebugLog Implementation File: SBGDebug.mm

#include "SBGDebug.h"
#include <cxxabi.h>
#include <map>
#include <string>
#include <execinfo.h>
#include <stdio.h>
#import <objc/Object.h>

1. Finding Objective-C methods

We want to build a method-lookup function that will return the method in which an address lies. We know how to enumerate the name and address of every Objective-C method available to the application, but this enumeration is expensive. Therefore we need a data structure in which to store the locations of the member functions.

The data structure must return the correct member function when queried with any address belonging to that member function. NSDictionary does not provide this functionality, but C++'s Standard Template Library's (STL) map container does.

map::upper_bound returns an iterator to the first element in the map whose key is larger than the queried key. For a map with method addresses as keys and method names as values, map::upper_bound will return an iterator pointing to the name of the method following the one we are looking for. Simply decrementing the iterator will make it point to the preceding method name.

// Lookup Function names
static std::map<uint32, std::string>* objectiveCMethodNames = NULL;
inline static uint32 lookupFunction(uint32 addr, const char** name)
{
   if (objectiveCMethodNames == NULL)
      return NULL;
     // Find the next function
   std::map<uint32, std::string>::iterator iter
      = objectiveCMethodNames->upper_bound( addr );
    // Go back a function: now we are looking at the right one!
   —iter;
   *name = iter->second.c_str();
   return iter->first;
}

Populating the STL map is a simple matter of iterating through all the classes known to the Objective-C runtime, and enumerating their methods.

// Add classes by stepping through their method lists.
inline static void  addObjectiveCMethod(uint32 addr, const char* name)
{   (*objectiveCMethodNames)[addr] = std::string(name); }
void addClass(Class c)
{
   unsigned int method_count;
   Method *method_list = class_copyMethodList(c, &method_count);
   for (int i = 0; i < method_count; i++)
   {
      Method      func = method_list[i];
      const char* name = sel_getName( method_getName( func ) );
      uint32      addr = (uint32) method_getImplementation( func );
      addObjectiveCMethod(   addr,
                           [[NSStringstringWith Format:@"[%s %s]",
                           class_getName(c), name] cString]);
   }
}

We could call the code that enumerates the Objective-C methods explicitly from main, but that requires remembering to add the call to each new application that uses debugLog(). Instead I can put the enumeration code into the load method of an Objective-C class which is guaranteed to be called if debugLog() is built into the application. The only gotcha is that the load method is invoked before Cocoa has created an NSAutoreleasePool. That's why the code below creates its own NSAutoreleasePool to avoid memory leaks.

@implementation SBGDebug
+ (void) load
{
   if (objectiveCMethodNames != NULL)
      return;
   NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
   objectiveCMethodNames = new std::map<uint32, std::string>();
   int numClasses = objc_getClassList(NULL, 0);
   if (numClasses > 0 )
   {
      Class *classes = (Class*) malloc(sizeof(Class) * numClasses);
      numClasses     = objc_getClassList(classes, numClasses);
      for (int i = 0; i < numClasses; ++i)
         addClass(classes[i]);
      free(classes);
   }
   [pool release];
}
@end

2. Deciding whether a function is C, C++ or Objective-C

The return addresses provided by backtrace() could belong to a C function, a C++ function or an Objective-C method. We need a way to decide which case we're dealing with. We start by asking both the C/C++ function-lookup function and the Objective-C method-lookup function to what function they believe a return address belongs. We will obtain two addresses a and b which should both be smaller than the return address r. Because functions are contiguous and do not intersect with each other, a and b must differ, and one of them must be lower than the other. For the same reason, the return address r cannot belong to the function with the lower address, as that function must end before the higher address. Therefore we use the name of the function that starts at the higher address:

extern "C" void debugLog(NSString* format, ...)
{
   // Print the debug message
   va_list arguments;
   va_start(arguments,format);
   NSLogv(format, arguments);
   // Dump the callstack
   uint32 callstack[128];
   int  frames = backtrace((void**) callstack, 128);
   char** strs = backtrace_symbols((void**) callstack, frames);
   for (int i = 1; i < frames; ++i)
   {
      char functionSymbol[64*1024];
      char moduleName    [64*1024];
      int  offset        = 0;
      sscanf(strs[i], "%*d %s %*s %s %*s %d", &moduleName,
                                       &functionSymbol, &offset);
      uint32 addr = callstack[i];
      if (objectiveCMethodNames)
      {
         const char* objcName;
         uint32      objcAddr = lookupFunction(addr, &objcName);
         if (      (objcAddr != 0)
               &&   (addr > objcAddr)
               &&   (addr - objcAddr < offset))
         {   printf("\t%8.8x — %s  + %d\t\t(%s)\n", addr, objcName,
                                             addr - objcAddr, moduleName);
            continue; };
      };
      int   validCppName;
      char* functionName = abi::__cxa_demangle(functionSymbol, NULL, 0,
                                                            &validCppName);
      if (validCppName == 0)
         printf(   "\t%8.8x — %s + %d\t\t(%s)\n",
                  addr, functionName, offset, moduleName);
      else
         printf(   "\t%8.8x — %s + %d\t\t(%s)\n",
                  addr, functionSymbol, offset, moduleName);
      if (functionName)
         free(functionName);
   }
   free(strs);
}

The output of backtrace_symbols() is an array of C-strings. We use sscanf() to parse them. The resulting function names are passed to the C++ demangler to convert into human-readable form if they are C++ names.

debugLog() uses printf() rather than NSLog() to avoid printing the application's name at the beginning of each line.

debugLog() is declared as extern "C" so it can be linked directly to Objective-C code. The class interface file does the same:

DebugLog Interface File: SBGDebug.h

@interface SBGDebug : NSObject
+ (void) load;
@end
#ifdef __cplusplus
extern "C" {
#endif
void debugLog(NSString* format, ...);
#ifdef __cplusplus
};
#endif

Using DebugLog

Invoke debugLog() just as you would NSLog():

Test file: TestDebugLog.mm

#import "SBGDebug.h"
// C++ test
struct CPP      { CPP(); ~CPP(); };
CPP::CPP()      { debugLog(@"C++ Constructor"); };
CPP::~CPP()   { debugLog(@"C++ Destructor"); };
// Objective C test
@interface Objc : NSObject
@end
@implementation Objc
- (id)      init
{ debugLog(@"Objective-C init"); return [super init]; };
(void)   dealloc
{ debugLog(@"Objective-C dealloc"); [super dealloc]; };
@end
// Main
int main(int argc, const char* argv[])
{
   NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
   CPP   cpp;
   Objc* objc = [[[Objc alloc] init] autorelease];
   debugLog(@"C — test arguments work too: %@", objc);
   [pool release];
   return 0;
}

Conclusion

Leopard provides all the components necessary to build a cross-platform NSLog() which can print function call traces. Although the code I provide assumes compilation to a 32-bit executable, extending it to 64 bits should be straightforward as only standard library functions are used.

You can download the entire Objective-C++ project from the MacTech ftp source archive at ftp.mactech.com/src/mactech/volume25_2009/25.06.sit.

Don't forget to compile it and run it in Debug Configuration!

References:

backtrace() : man backtrace.

backtrace_symbols() limitations : http://lists.apple.com/archives/darwin-dev/2009/Mar/msg00111.html.

backtrace_symbols() : man backtrace_symbols.

abi::__cxa_demangle : http://www.ib.cnea.gov.ar/~oop/biblio/libstdc++/namespaceabi.html.

Objective-C runtime: http://developer.apple.com/documentation/Cocoa/Reference/ObjCRuntimeRef/ObjCRuntimeRef.pdf.

The Standard Template Library: http://www.sgi.com/tech/stl/


Dr. Sengan Baring-Gould is a Boulder, Colorado-based independent Mac OS X developer and writer. He is available for consulting and specializes in Algorithms, AI, Cocoa, Debugging tools, High performance code, and UIs. He can be reached at sengan@ansemond.com.

 
AAPL
$100.96
Apple Inc.
-0.83
MSFT
$47.52
Microsoft Corpora
+0.84
GOOG
$596.08
Google Inc.
+6.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

Audio Hijack Pro 2.11.3 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Airfoil 4.8.9 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more

Latest Forum Discussions

See All

View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »
Tap Army Review
Tap Army Review By Jennifer Allen on September 19th, 2014 Our Rating: :: SHOOT EM ALLUniversal App - Designed for iPhone and iPad Mindless but fun, Tap Army is a lane-based shooter that should help you relieve some stress.   | Read more »
Monsters! Volcanoes! Loot! Epic Island f...
Monsters! Volcanoes! Loot! | Read more »
Plunder Pirates: Tips, Tricks, Strategie...
Ahoy There, Seadogs: Interested in knowing our thoughts on all this plundering and pirating? Check out our Plunder Pirates Review! Have you just downloaded the rather enjoyable pirate-em-up Plunder Pirates and are in need of some assistance? Never... | Read more »
Goat Simulator Review
Goat Simulator Review By Lee Hamlet on September 19th, 2014 Our Rating: :: THE GRUFFEST OF BILLY GOATSUniversal App - Designed for iPhone and iPad Unleash chaos as a grumpy goat in this humorous but short-lived casual game.   | Read more »
A New and Improved Wunderlist is Here fo...
A New and Improved Wunderlist is Here for iOS 8 Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Evernote Update for iOS 8 Adds Web Clipp...
Evernote Update for iOS 8 Adds Web Clipping, Quick Notes, and More Posted by Ellis Spice on September 19th, 2014 [ permalink ] | Read more »
Apple Names Ultimate Productivity Bundl...
Apple Names Ultimate Productivity Bundle by Readdle as the Essential Bundle on the App Store Posted by Jessica Fisher on September 19th, 2014 [ permalink | Read more »

Price Scanner via MacPrices.net

iFixIt Tears Down iPhone 6; Awards Respectabl...
iFixit notes that even the smaller 4.7″ iPhone 6 is a giant among iPhones; so big that Apple couldn’t fit it into the familiar iPhone form factor. In a welcome reversal of a recent trend to more or... Read more
Phone 6 Guide – Tips Book For Both iPhone 6...
iOS Guides has announced its latest eBook: iPhone 6 Guide. Brought to you by the expert team at iOS Guides, and written by best-selling technology author Tom Rudderham, iPhone 6 Guide is packed with... Read more
How to Upgrade iPhone iPad to iOS 8 without D...
PhoneClean, a iPhone cleaner utility offered by iMobie Inc., reveals a solution for upgrading iPhone and iPad to iOS 8 without deleting photos, apps, the new U2 album or anything. Thanks to more than... Read more
Inpaint 6 – Photo Retouching Tool Gets Faster...
TeoreX has announced Inpaint 6, a simple retouching tool for end users that helps remove scratches, watermarks, and timestamps as well as more complex objects like strangers, unwanted elements and... Read more
Worldwide PC Monitor Market Sees Growth in To...
Worldwide PC monitor shipments totaled 32.5 million units in the second quarter of 2014 (2Q14), a year-over-year decline of -2.9%, according to the International Data Corporation (IDC) Worldwide... Read more
Updated Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Mac Pros available for up to $260 off MSRP
Adorama has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 4-core Mac Pro: $2839.99, $160 off MSRP - 6-core Mac Pro: $3739.99, $260... Read more
13-inch 2.6GHz/256GB Retina MacBook Pros avai...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Previous-generation 15-inch 2.0GHz Retina Mac...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
21″ 2.7GHz iMac available for $1179, save $12...
Adorama has 21″ 2.7GHz Hawell iMacs on sale for $1179.99 including free shipping. Their price is $120 off MSRP. NY and NJ sales tax only. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.