Quickies for Debugging

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General

• Am I Being Debugged? [permalink]
  

  int AmIBeingDebugged(void)
  {
          int                                        mib[4];
          struct kinfo_proc        info;
          size_t                                size;
          
          mib[0] = CTL_KERN;
          mib[1] = KERN_PROC;
          mib[2] = KERN_PROC_PID;
          mib[3] = getpid();
          size = sizeof(info);
          info.kp_proc.p_flag = 0;
          sysctl(mib,4,&info,&size,NULL,0);
          return ((info.kp_proc.p_flag & P_TRACED) == P_TRACED);
  }
  
  #define StopIfInDebugger()        __asm__ volatile ("twnei %0,0" : : "r" (AmIBeingDebugged()))

  (courtesy of arwyn on #macdev. Useful uses: controlling voluminous console output when running in gdb, or if running non-debugged, an assert routine that generates stack trace and logs it when not-being debugged, but just traps into the debugger at the point of failure when being debugged)

• Enabling core files [permalink]
  Core files are handy to have around if your program crashes with a SEGV, bus error, or if it raises an abort(). You can dig into the core file with gdb There are two ways to enable core files:
  • in your C shell do
    % limit coredumpsize unlimited
    (you can put this into your .cshrc. I don't know what to do for bash)

  • Or in your program do
    struct rlimit corelimit = { RLIM_INFINITY, RLIM_INFINITY };
setrlimit (RLIMIT_CORE, &corelimit); You may need to also add
    #include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>

  Core files seem to be getting dumped into /cores, rather than the directory the program was run from.

• NSDebug.h [permalink]
  Take a look at /System/Library/Frameworks/Foundation.framework/Headers/NSDebug.h. Lots of low-level unsupported APIs for mucking around with gory technical details.

• Triggering a low-memory warning on the device [permalink]
  You can trigger a low-memory warning in the simulator, but sometimes you use one of the myriad of APIs that aren't supported in the simulator, but you still need to track down something ultimately triggered by a low-memory warning.

  Put this somewhere convenient to trigger - in a timer, under a button, or something. Obviously, you don't want to ship with it.

  [[UIApplication sharedApplication] performSelector:@selector(_receivedMemoryNotification)];

• Turning off Panther's "Program quit - send a report to Apple" dialog [permalink]
  During development, it's annoying to get the "Your program just crashed! Send a report to Apple?". You can turn that off by entering this in the terminal:

  % defaults write com.apple.CrashReporter DialogType none

  Turn turn it back on, replace none with prompt to get the original behavior. See for details.

gdb

• Breaking on "Deallocation of a pointer not malloced" messages [permalink]
  Put a breakpoint on fprintf

• Eating Brains [permalink]
  You can turn on Zombies to catch uses of objects after they have been released. Here's an easy way to do it in the gdb console:

    % gdb build/Debug/Snorkfizzle.app/Contents/MacOS/Snorkfizzle
    (gdb) set env NSZombieEnabled=YES
    (gdb) fb -[_NSZombie methodSignatureForSelector:]
    (gdb) run
  

  Then exercise your app and trip the error.

• Get a stack trace of all threads [permalink]
  (gdb) thread apply all where

• Setting a breakpoint before shared libraries are loaded [permalink]
  Need to set a breakpoint, say on a Cocoa library function, or something in a shared library you're loading later? Set a "future breakpoint":
  (gdb) fb -[NSTextView drawRect:]

• Tracing message sends using gdb [permalink]
  

  b objc_msgSend
  comm
  silent
  printf "%c[%s %s]\n", $r3&&((id)$r3)->isa->info&2?'+':'-', $r3?((id)$r3)->isa->name:"nil", $r4
  cont
  end
  b objc_msgSend_rtp
  comm
  silent
  printf "%c[%s %s]\n", $r3&&((id)$r3)->isa->info&2?'+':'-', $r3?((id)$r3)->isa->name:"nil", $r4
  cont
  end
  

  And you'll get some output like this:

  -[NSTableColumn _bindingAdaptor]
  +[NSBinder binderClassesForObject:]
  +[NSBinder _allBinderClasses]
  +[NSDisplayPatternTitleBinder isUsableWithObject:]
  +[NSBox self]
  

  (courtesy of Rob Mayoff)

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