More than `po`: Debugging in lldb

More than po:
Debugging in lldb
@MicheleTitolo

What we’ll cover
• LLDB basics
• Advanced thread, breakpoint, watchpoint
• Debugging with script
• Swift

(lldb) po self.myCar
<Car: 0x7fc8206334d0>

2014 Tesla S UIDeviceRGBColorSpace 0 0 1 1

(lldb) p self.myCar.year
(NSInteger) $1 = 2014

(lldb) p *self.myCar
(Car) $1 = { 
NSObject = { 
isa = Car 
} 
_running = NO 
_make = 0x000000010077d088 @"Tesla" 
_model = 0x000000010077d0a8 @"S" 
_year = 2014 
_color = 0x00007fd531eb9e30 
_gear = Park 
}

use expr to modify
values at runtime

(lldb) po self.myCar 

2014 Tesla S UIDeviceRGBColorSpace 0 0 1 1 
(lldb) expr self.myCar.year = 2013
(NSInteger) $0 = 2013

(lldb) fr v
(ViewController *) self = 0x00007f9628520cd0 
(SEL) _cmd = "updateTipLabelsForBillAmount:" 
(float) billAmount = 33 
(float) tipPercentage = 0.200000003 
(float) tipAmount = 6.5999999 
(float) total = 39.5999985

(lldb) bt
* thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-[ViewController
viewDidLoad](self=0x00007fc3d2c21570, _cmd=0x0000000102f21cfb) + 448 at
ViewController.m:28, queue = 'com.apple.main-thread', stop reason =
breakpoint 1.1 
* frame #0: 0x0000000101a90330 Cars`-[ViewController viewDidLoad]
(self=0x00007fc3d2c21570, _cmd=0x0000000102f21cfb) + 448 at
ViewController.m:28 
frame #1: 0x000000010284f090 UIKit`-[UIViewController
loadViewIfRequired] + 738 
frame #2: 0x000000010284f28e UIKit`-[UIViewController view] + 27 
frame #3: 0x000000010276b5e9 UIKit`-[UIWindow
addRootViewControllerViewIfPossible] + 58 
frame #4: 0x000000010276b9af UIKit`-[UIWindow _setHidden:forced:] +
247 
frame #5: 0x0000000102778219 UIKit`-[UIWindow makeKeyAndVisible] + 42 
frame #6: 0x000000010271b727 UIKit`-[UIApplication
_callInitializationDelegatesForMainScene:transitionContext:] + 2732 
frame #7: 0x000000010271e43e UIKit`-[UIApplication
_runWithMainScene:transitionContext:completion:] + 1349 
frame #8: 0x000000010271d33c UIKit`-[UIApplication
workspaceDidEndTransaction:] + 179

settings set prompt [lldb]$
command alias bd breakpoint disable
command alias be breakpoint enable
command alias bdel breakpoint delete
command alias bcommand breakpoint command add
command alias commands breakpoint command list

(lldb) thread info
thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-
[ViewController viewDidLoad](self=0x00007fc3d2c21570,
_cmd=0x0000000102f21cfb) + 448 at ViewController.m:28, queue
= 'com.apple.main-thread', stop reason = breakpoint 1.1

(lldb) thread list
Process 921 stopped 
* thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-
[ViewController viewDidLoad](self=0x00007fc3d2c21570,
_cmd=0x0000000102f21cfb) + 448 at ViewController.m:28, queue
= 'com.apple.main-thread', stop reason = breakpoint 1.1 
thread #2: tid = 0x66da, 0x0000000104bcc232
libsystem_kernel.dylib`kevent64 + 10, queue =
'com.apple.libdispatch-manager' 
thread #3: tid = 0x66dc, 0x0000000104bc651a
libsystem_kernel.dylib`semaphore_wait_trap + 10, queue =
'FBSSerialQueue' 
thread #4: tid = 0x66dd, 0x0000000104bcb94a
libsystem_kernel.dylib`__workq_kernreturn + 10 
thread #5: tid = 0x66de, 0x0000000104bcb94a
libsystem_kernel.dylib`__workq_kernreturn + 10 
thread #6: tid = 0x66df, 0x0000000104bcb94a
libsystem_kernel.dylib`__workq_kernreturn + 10

(lldb) thread until 31
Process 1615 resuming 
(lldb)

(lldb) thread return NO
(lldb) thread return [NSNumber numberWithInt:7]
(lldb) thread return @"Hello"

(lldb) br list
Current breakpoints: 
1: file = 'Cars/Car.m', line = 50, locations = 1, resolved =
1, hit count = 0 
 
1.1: where = Cars`-[Car changeGearTo:] + 52 at Car.m:50,
address = 0x00000001026fd4e4, resolved, hit count = 0  
 
2: file = 'Cars/ViewController.m', line = 31, locations = 1,
resolved = 1, hit count = 1 
 
2.1: where = Cars`-[ViewController viewDidLoad] + 512 at
ViewController.m:31, address = 0x00000001026fcb40, resolved,
hit count = 1

(lldb) br set -f ViewController.m -l 31
Breakpoint 2: where = Cars`-[ViewController viewDidLoad] + 
512 at ViewController.m:31, address = 0x000000010e2bdb40

(lldb) br set -F "-[Car changeGearTo:]”
Breakpoint 3: where = Cars`-[Car changeGearTo:] + 20 at 
Car.m:47, address = 0x00000001073ba4c4

(lldb) br ViewController.m:31
error: command 'breakpoint' did not recognize
'ViewController .m:31' as valid (subcommand might be
invalid).

(lldb) b Car.m:63 
Breakpoint 2: where = Cars`-[Car increaseSpeedTo:] + 118 at
Car.m:63, address = 0x0000000100b7a576 
(lldb) br modify -c "speed==15" 2

(lldb) b Car.m:63 
Breakpoint 2: where = Cars`-[Car increaseSpeedTo:] + 118 at
Car.m:63, address = 0x0000000100b7a576 
(lldb) br modify -c "speed==15" 2
(lldb) c 
(lldb)

(lldb) br modify -c "speed==15 && self.gear==4” 2

(lldb) br modify -c "speed==20 && [self canChangeGearTo:4]" 2

(lldb) br modify -c "speed==15 && self.gear==4" -i 17 2

-i <count> ( --ignore-count <count> )
Set the number of times this breakpoint is skipped
before stopping.

-i <count> ( --ignore-count <count> )
Set the minimum number of times this breakpoint is skipped
before stopping.

(lldb) br delete 2
1 breakpoints deleted; 0 breakpoint locations disabled.

(lldb) breakpoint disable 2
1 breakpoints disabled.
 
... 
2: file = 'Cars/ViewController.m', line = 31, locations = 1
Options: disabled

(lldb) breakpoint enable 2
1 breakpoints enabled. 
... 
2: file = 'Cars/ViewController.m', line = 31, locations = 1,
resolved = 1, hit count = 1

(lldb) br command add -o "fr v" 2
(lldb) fr v 
(Car *) self = 0x00007fb59ad142b0 
(SEL) _cmd = "changeGearTo:" 
(Gear) gear = Neutral

(lldb) br command add 1
Enter your debugger command(s). Type 'DONE' to end. 
> fr v 
> continue 
> DONE

(lldb) br command add 1
> fr v 
> continue 
> DONE
(lldb) fr v 
(Car *) self = 0x00007f8061625490 
(SEL) _cmd = "changeGearTo:" 
(Gear) gear = Reverse 
(lldb) continue 
Command #2 'continue' continued the target.

(lldb) br command list 3
Breakpoint 3: 
Breakpoint commands: 
fr v

a watchpoint tracks
a value over time

[self.myCar addObserver:self 
forKeyPath:@"speed" 
options:NSKeyValueObservingOptionNew 
context:KVOContext];

[self.myCar addObserver:self 
forKeyPath:@"speed" 
options:NSKeyValueObservingOptionNew 
context:KVOContext]; 
 
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object change:(NSDictionary *)change
context:(void *)context 
{ 
if (car.speed > 1000) { 
NSLog(@"WHY AM I GOING SO FAST %ld", 
(long)car.speed); 
} 
}

- (void)setSpeed:(NSInteger)speed
{
_speed = speed;
if (_speed > 1000) {
NSLog(@"WHY AM I GOING SO FAST %ld",
(long)_speed);
}
}

breakpoint: place
watchpoint: value

(lldb) watch set var self.speed 
error: "self" is a pointer and . was used to attempt to 
access "speed". Did you mean "self->speed"?
(lldb) watch set var self->speed 
error: "speed" is not a member of "(Car *const) self"
(lldb) watch set var _speed

Watchpoint created: Watchpoint 1: addr = 0x7fb9c145d768 size 
= 8 state = enabled type = w 
watchpoint spec = ‘_speed' 
new value: 0

new value: 0
(lldb) watch modify -c '(_speed==15)'

new value: 0
(lldb) c 
Process 2150 resuming

new value: 0
(lldb) c 
Watchpoint 1 hit: 
old value: 0 
new value: 15 
(lldb)

(lldb) watch set var _speed 
Watchpoint created: Watchpoint 1: addr = 0x7fa3b2e05278 size 
new value: 0 

new value: 0 
(lldb) watch command add 1

new value: 0 
> p _speed 
> continue 
> DONE

new value: 0 
> p _speed 
> continue 
> DONE
(lldb) c 

new value: 0 
> p _speed 
> continue 
> DONE
(lldb) c 
(NSInteger) $16 = 15 
Command #2 'continue' continued the target.

Stopped due to an error evaluating condition of watchpoint
Watchpoint 1: addr = 0x7f84d940a180 size = 8 state =
disabled type = w: "(_gear==First)"
error: use of undeclared identifier 'First'
error: 1 errors parsing expression
Watchpoint 1 hit:
old value: Park
new value: First

watch delete
watch enable
watch disable

(lldb) watch disable 1 
1 watchpoints disabled.
(lldb) watch enable 1 
1 watchpoints enabled.

rdar://20054190
(Apple closed the previous one !)

>>> if count > 1:
... print "hello"
File "<console>", line 2
print "hello"
^
IndentationError: expected an indented block
>>>

(lldb) help script
Pass an expression to the script interpreter for evaluation and
return the results. Drop into the interactive interpreter if no
expression is given. This command takes 'raw' input (no need to
quote stuff).
Syntax: script [<script-expression-for-evaluation>]

(lldb) script 
Python Interactive Interpreter. To exit, type 'quit()',
'exit()'.

(lldb) script 
Python Interactive Interpreter. To exit, type 'quit()',
'exit()'.
>>> print lldb.debugger
Debugger (instance: "debugger_47", id: 47)
>>> print lldb.target
Cars
>>> print lldb.process
SBProcess: pid = 6425, state = stopped, threads = 5,
executable = Cars
>>> print lldb.thread
SBThread: tid = 0x3813e
>>> print lldb.frame
frame #0: 0x0000000103075979 Cars`-[ViewController
viewDidLoad](self=0x00007f8173f23680,
_cmd=0x0000000104507cfb) + 57 at ViewController.m:24
>>> quit()

def breakpoint_func(frame, bp_loc, dict):
# implementation here
# return false to skip

current stack frame

breakpoint location

python session dict

python session dictuseless

only stop when
-changeGearTo:
is in the call stack

find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame  
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()

find_in_stack = [
def
names = set([frame.GetFunctionName()
return False
quit()
stack symbol

find_in_stack = [
def
return False
quit()
stack symbol
function declaration

find_in_stack = [
def
return False
quit()
stack symbol
global accessor

find_in_stack = [
def
return False
quit()
stack symbol
global accessor
all functions in stack

find_in_stack = [
def
return False
quit()
all_ignored = set(find_in_stack) 
stack symbol
global accessor
check if symbol we want
is in this stack

find_in_stack = [
def
return False
quit()
if len(ignored_here) == 0: 
return False 
stack symbol
global accessor
is in this stack
if it isn’t, continue

find_in_stack = [
def
return False
quit()
return False 
quit()
stack symbol
global accessor
is in this stack
if it isn’t, continue

(lldb) br command add -s python 2

Enter your Python command(s). Type 'DONE' to end. 
def function(frame,bp_loc,internal_dict): 
"""frame: the SBFrame for the location at which you
stopped 
bp_loc: an SBBreakpointLocation for the breakpoint
location information 
internal_dict: an LLDB support object not to be
used"""

Enter your Python command(s). Type 'DONE' to end. 
def function(frame,bp_loc,internal_dict): 
"""frame: the SBFrame for the location at which you
stopped 
bp_loc: an SBBreakpointLocation for the breakpoint
location information 
internal_dict: an LLDB support object not to be
used"""
global find_in_stack 
find_in_stack = ['-[Car changeGearTo:]'] 
names = set([frame.GetFunctionName() for frame in
frame.GetThread()]) 
ignored_here = all_ignored.intersection(names) 
return False 
DONE

(lldb) br command add -s python 2 -o “print 1+1”

Use a python function to better
describe a class

def Car_Summary(value,unused):
make = value.GetChildMemberWithName("_make")
model = value.GetChildMemberWithName("_model")
makeSummary = make.GetSummary()
modelSummary = model.GetSummary()
return makeSummary + " " + modelSummary
quit()

def
make = value.GetChildMemberWithName(
model = value.GetChildMemberWithName(
quit()
value is the frame

def
quit()
make = value.GetChildMemberWithName("_make") 
value is the frame
get the variables we want

def
quit()
makeSummary = make.GetSummary() 
value is the frame
printable
summaries

def
quit()
value is the frame
printable
summaries
return a string

def
quit()
 
quit()
value is the frame
printable
summaries
return a string

• Only break after another breakpoint has
been hit
• Check multiple threads for a symbol
• Data formatters for everything
• Custom LLDB commands
More Use Cases

<SwiftCars.Car: 0x7fe8bb456a60> 
(lldb) p *self.myCar
error: <EXPR>:1:1: error: '*' is not a prefix unary operator 
*self.myCar 
^

(lldb) fr v -F self.myCar
self.myCar = 0x00007fe8bb456a60 
self.myCar = 
self.myCar.isa = SwiftCars.Car 
self.myCar.make._core._owner = nil 
self.myCar.model._core._owner = nil 
self.myCar.color = 0x00007fe8bb452c80 
self.myCar.color.isa = UICachedDeviceRGBColor 
self.myCar.color._systemColorName = 0x00007fe8bb4424c0
“blueColor" 
self.myCar.color.redComponent = 0 
self.myCar.color.greenComponent = 0 
self.myCar.color.blueComponent = 1 
self.myCar.color.alphaComponent = 1 
self.myCar.color.cachedColor = 0x0000000000000000 
self.myCar.color.cachedColorOnceToken = 0 
self.myCar.running.value = 0 
self.myCar.gear = Park

find_in_stack = ['SwiftCars.Car.changeGearTo']
App name

Resources
• WWDC ’13 Session 413
• WWDC ’14 Sessions 409 & 410
• http://lldb.llvm.org/
• http://www.objc.io/issue-19/lldb-debugging.html
• http://blog.ittybittyapps.com/blog/2013/11/07/
integrating-reveal-without-modifying-your-xcode-
project/
• https://github.com/facebook/chisel

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