lldb-for-gdb-users.txt   [plain text]

Here's a short precis of how to run lldb if you are familiar with the
gdb command set:

1) LLDB Command Structure:

First some details on lldb command structure to help orient you...

Unlike gdb's command set, which is rather free-form, we tried to make
the lldb command syntax fairly structured.  The commands are all of the

<noun> <verb> [-options [option-value]] [argument [argument...]]

The command line parsing is done before command execution, so it is
uniform across all the commands.  The command syntax is very simple,
basically arguments, options and option values are all white-space
separated.  If you need to put a backslash or double-quote character
in an argument you back-slash it in the argument.  That makes the
command syntax more regular, but it also means you may have to
quote some arguments in lldb that you wouldn't in gdb.

Options can be placed anywhere on the command line, but if the arguments
begin with a "-" then you have to tell lldb that you're done with options
using the "--" option.  So for instance, the "process launch" command takes
the "-s" option to mean "stop the process at the first instruction".  It's 
arguments are the arguments you are passing to the program.  So if you wanted
to pass an argument that contained a "-" you would have to do:

(lldb) process launch -- -program_arg value

We also tried to reduce the number of special purpose argument
parsers, which sometimes forces the user to be a little more explicit
about stating their intentions.  The first instance you'll note of
this is the breakpoint command.  In gdb, to set a breakpoint, you
would just say:

(gdb) break foo.c:12


(gdb) break foo

if foo is a function.  As time went on, the parser that tells foo.c:12
from foo from foo.c::foo (which means the function foo in the file
foo.c) got more and more complex and bizarre, and especially in C++
there are times where there's really no way to specify the function
you want to break on.  The lldb commands are more verbose but also precise.  
So you say:

(lldb) breakpoint set -f foo.c -l 12

to set a file & line breakpoint.  To set a breakpoint on a function
by name, you do:

(lldb) breakpoint set -n foo

This can allow us to be more expressive, so you can say:

(lldb) breakpoint set -M foo

to break on all C++ methods named foo, or:

(lldb) breakpoint set -S alignLeftEdges:

to set a breakpoint on all ObjC selectors called alignLeftEdges:.  It
also makes it easy to compose specifications, like:

(lldb) breakpoint set -s foo.dylib -n foo

for all functions called foo in the shared library foo.dylib.  Suggestions
on more interesting primitives of this sort are also very welcome.

So for instance:

(lldb) breakpoint set -n "-[SKTGraphicView alignLeftEdges:]"

Just like gdb, the lldb command interpreter does a shortest unique
string match on command names, so the previous command can also be

(lldb) b s -n "-[SKTGraphicView alignLeftEdges:]"

lldb also supports command completion for source file names, symbol
names, file names, etc. Completion is initiated by a hitting a <TAB>.
Individual options in a command can have different completers, so for
instance the -f option in "breakpoint" completes to source files, the
-s option to currently loaded shared libraries, etc...  We can even do 
things like if you specify -s, and are completing on -f, we will only
list source files in the shared library specified by -s...

The individual commands are pretty extensively documented, using
the "help" command.  And there is an "apropos" command that will
search the help for a particular word and dump a summary help string
for each matching command.

Finally, there is a mechanism to construct aliases for commonly used
commands.  So for instance if you get annoyed typing

(lldb) b s -f foo.c -l 12

you can do:

(lldb) command alias bfl breakpoint set -f %1 -l %2
(lldb) bfl foo.c 12

We have added a few aliases for commonly used commands (e.g. "step",
"next" and "continue") but we haven't tried to be exhaustive because
in our experience it is more convenient to make the basic commands
unique down to a letter or two, and then learn these sequences than
fill the namespace with lots of aliases, and then have to type them
all the way out.

However, users are free to customize lldb's command set however they
like, and since lldb reads the file ~/.lldbinit at startup, you can
store all your aliases there and they will be generally available to
you.  Your aliases are also documented in the help command so you can
remind yourself of what you've set up.

lldb also has a built-in Python interpreter, which is accessible by
the "script" command.  All the functionality of the debugger is
available as classes in the Python interpreter, so the more complex
commands that in gdb you would introduce with the "define" command can
be done by writing Python functions using the lldb-Python library,
then loading the scripts into your running session and accessing them
with the "script" command.  

2) A typical session:

a) Setting the program to debug:

As with gdb, you can start lldb and specify the file you wish to debug
on the command line:

$ lldb /Projects/Sketch/build/Debug/Sketch.app
Current executable set to '/Projects/Sketch/build/Debug/Sketch.app' (x86_64).

or you can specify it after the fact with the "file" command:

(lldb) file /Projects/Sketch/build/Debug/Sketch.app
Current executable set to '/Projects/Sketch/build/Debug/Sketch.app' (x86_64).

b) Setting breakpoints:

We've discussed how to set breakpoints above.  You can use "help break set" 
to see all the options for breakpoint setting.  For instance, we might do:

(lldb) b s -S alignLeftEdges:
Breakpoint created: 1: name = 'alignLeftEdges:', locations = 1, resolved = 1

You can find out about the breakpoints you've set with:

(lldb) break list
Current breakpoints:
1: name = 'alignLeftEdges:', locations = 1, resolved = 1
  1.1: where = Sketch`-[SKTGraphicView alignLeftEdges:] + 33 at /Projects/Sketch/SKTGraphicView.m:1405, address = 0x0000000100010d5b, resolved, hit count = 0 

Note that each "logical" breakpoint can have multiple "locations".
The logical breakpoint has an integer id, and it's locations have an
id within their parent breakpoint (the two are joined by a ".",
e.g. 1.1 in the example above.)  

Also the breakpoints remain "live" so that if another shared library
were to be loaded that had another implementation of the
"alignLeftEdges:" selector, the new location would be added to
breakpoint 1 (e.g. a "1.2" breakpoint would be set on the newly loaded

The other piece of information in the breakpoint listing is whether the
breakpoint location was "resolved" or not.  A location gets resolved when
the file address it corresponds to gets loaded into the program you are
debugging.  For instance if you set a breakpoint in a shared library that 
then gets unloaded, that breakpoint location will remain, but it will no 
longer be "resolved".

One other thing to note for gdb users is that lldb acts like gdb with:

(gdb) set breakpoint pending on

That is, lldb should always make a breakpoint from your specification, even
if it couldn't find any locations that match the specification.  You can tell
whether the expression was resolved or not by checking the locations field
in "breakpoint list", and we report the breakpoint as "pending" when you
set it so you can tell you've made a typo more easily, if that was indeed 
the reason no locations were found:

(lldb) b s -f no_such_file.c -l 10000000
Breakpoint created: 1: file ='no_such_file.c', line = 10000000, locations = 0 (pending)

You can delete, disable, set conditions and ignore counts either on all the
locations generated by your logical breakpoint, or on particular locations
your specification resolved to.  For instance if we wanted to add a command
to print a backtrace when we hit this breakpoint we could do:

(lldb) b command add -c 1.1
Enter your debugger command(s).  Type 'DONE' to end.
> bt

The "-c" option specifies that the breakpoint command is a set of lldb
commmand interpreter commands.  Use "-s" if you want to implement your
breakpoint command using the Python interface instead.

c) Running the program:

Then you can either launch the process with the command:

(lldb) process launch

or its alias:

(lldb) r

Or you can attach to a process by name with:

(lldb) process attach -n Sketch

the "attach by name"  also supports the "-w" option which waits for the
next process of that name to show up, and attaches to that.  You can also
attach by PID:

(lldb) process attach -p 12345
Process 46915 Attaching
(lldb) Process 46915 Stopped
1 of 3 threads stopped with reasons:
* thread #1: tid = 0x2c03, 0x00007fff85cac76a, where = libSystem.B.dylib`__getdirentries64 + 10, stop reason = signal = SIGSTOP, queue = com.apple.main-thread

Note that we tell you that "1 of 3 threads stopped with reasons" and
then list those threads.  In a multi-threaded environment it is very
common for more than one thread to hit your breakpoint(s) before the
kernel actually returns control to the debugger.  In that case, you
will see all the threads that stopped for some interesting reason
listed in the stop message.

d) Controlling execution:

After launching, we can continue until we hit our breakpoint.  The primitive
commands for process control all exist under the "thread" command:

(lldb) thread continue
Resuming thread 0x2c03 in process 46915
Resuming process 46915

At present you can only operate on one thread at a time, but the
design will ultimately support saying "step over the function in
Thread 1, and step into the function in Thread 2, and continue Thread
3" etc.  When we eventually support keeping some threads running while
others are stopped this will be particularly important.  For
convenience, however, all the stepping commands have easy aliases.  
So "thread continue" is just "c", etc.

The other program stepping commands are pretty much the same as in gdb.  
You've got:

  1. (lldb) thread step-in
     The same as gdb's "step" -- there is also the alias "s" in lldb

  2. (lldb) thread step-over
     The same as gdb's "next" -- there is also the alias "n" in lldb

  3. (lldb) thread step-out
     The same as gdb's "finish" -- there is also the alias "f" in lldb

And the "by instruction" versions:

(lldb) thread step-inst
(lldb) thread step-over-inst

Finally, there's:

(lldb) thread until 100

which runs the thread in the current frame till it reaches line 100 in
this frame or stops if it leaves the current frame.  This is a pretty 
close equivalent to gdb's "until" command.

One thing here that might be a little disconcerting to gdb users here is that
when you resume process execution, you immediately get a prompt back.  That's
because the lldb interpreter remains live when you are running the target.
This allows you to set a breakpoint, etc without having to explicitly interrupt
the program you are debugging.  We're still working out all the operations
that it is safe to do while running.  But this way of operation will set us
up for "no stop" debugging when we get to implementing that.

If you want to interrupt a running program do:

(lldb) process interrupt

To find out the state of the program, use:

(lldb) process status
Process 47958 is running.

This is very convenient, but it does have the down-side that debugging
programs that use stdin is no longer as straightforward.  For now, you
have to specify another tty to use as the program stdout & stdin using
the appropriate options to "process launch", or start your program in
another terminal and catch it with "process attach -w".  We will come
up with some more convenient way to juggle the terminal back & forth
over time.

e) Examining program state:

Once you've stopped, lldb will choose a current thread, usually the
one that stopped "for a reason", and a current frame in that thread.
Many the commands for inspecting state work on this current

To inspect the current state of your process, you can start with the

(lldb) thread list
Process 46915 state is Stopped
* thread #1: tid = 0x2c03, 0x00007fff85cac76a, where = libSystem.B.dylib`__getdirentries64 + 10, stop reason = signal = SIGSTOP, queue = com.apple.main-thread
  thread #2: tid = 0x2e03, 0x00007fff85cbb08a, where = libSystem.B.dylib`kevent + 10, queue = com.apple.libdispatch-manager
  thread #3: tid = 0x2f03, 0x00007fff85cbbeaa, where = libSystem.B.dylib`__workq_kernreturn + 10

The * indicates that Thread 1 is the current thread.  To get a
backtrace for that thread, do:

(lldb) thread backtrace
thread #1: tid = 0x2c03, stop reason = breakpoint 1.1, queue = com.apple.main-thread
  frame #0: 0x0000000100010d5b, where = Sketch`-[SKTGraphicView alignLeftEdges:] + 33 at /Projects/Sketch/SKTGraphicView.m:1405
  frame #1: 0x00007fff8602d152, where = AppKit`-[NSApplication sendAction:to:from:] + 95
  frame #2: 0x00007fff860516be, where = AppKit`-[NSMenuItem _corePerformAction] + 365
  frame #3: 0x00007fff86051428, where = AppKit`-[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:] + 121
  frame #4: 0x00007fff860370c1, where = AppKit`-[NSMenu performKeyEquivalent:] + 272
  frame #5: 0x00007fff86035e69, where = AppKit`-[NSApplication _handleKeyEquivalent:] + 559
  frame #6: 0x00007fff85f06aa1, where = AppKit`-[NSApplication sendEvent:] + 3630
  frame #7: 0x00007fff85e9d922, where = AppKit`-[NSApplication run] + 474
  frame #8: 0x00007fff85e965f8, where = AppKit`NSApplicationMain + 364
  frame #9: 0x0000000100015ae3, where = Sketch`main + 33 at /Projects/Sketch/SKTMain.m:11
  frame #10: 0x0000000100000f20, where = Sketch`start + 52

You can also provide a list of threads to backtrace, or the keyword
"all" to see all threads:

(lldb) thread backtrace all

Next task is inspecting data:

The most convenient way to inspect a frame's arguments and local variables is:

(lldb) frame variable 
self = (SKTGraphicView *) 0x0000000100208b40
_cmd = (struct objc_selector *) 0x000000010001bae1
sender = (id) 0x00000001001264e0
selection = (NSArray *) 0x00000001001264e0
i = (NSUInteger) 0x00000001001264e0
c = (NSUInteger) 0x00000001001253b0

You can also choose particular variables to view:

(lldb) frame variable self
(SKTGraphicView *) self = 0x0000000100208b40

The frame variable command is not a full expression parser but it
does support some common operations like dereferencing:

(lldb) fr v *self
(SKTGraphicView *) self = 0x0000000100208b40
  (NSView) NSView = {
    (NSResponder) NSResponder = {

and structure element references:

(lldb) frame variable self.isa
(struct objc_class *) self.isa = 0x0000000100023730

The frame variable command will also perform "object printing" operations on
variables (currently we only support NSPrintForDebugger) with:

(lldb) fr v -o self
(SKTGraphicView *) self = 0x0000000100208b40
<SKTGraphicView: 0x100208b40>

You can select another frame to view with:

(lldb) frame select 9
frame #9: 0x0000000100015ae3, where = Sketch`main + 33 at /Projects/Sketch/SKTMain.m:11
  10      int main(int argc, const char *argv[]) {
  11 ->       return NSApplicationMain(argc, argv);
  12          }

Another neat trick that the variable list does is array references, so:

(lldb) fr v argv[0]
(char const *) argv[0] = 0x00007fff5fbffaf8 "/Projects/Sketch/build/Debug/Sketch.app/Contents/MacOS/Sketch"

If you need to view more complex data or change program data, you can
use the general "expression" command.  It takes an expression and
evaluates it in the scope of the currently selected frame.  For instance:

(lldb) expr self
$0 = (SKTGraphicView *) 0x0000000100135430
(lldb) expr self = 0x00
$1 = (SKTGraphicView *) 0x0000000000000000
(lldb) frame var self
(SKTGraphicView *) self = 0x0000000000000000

You can also call functions:

(lldb) expr (int) printf ("I have a pointer 0x%llx.\n", self)
$2 = (int) 22
I have a pointer 0x0.

One thing to note from this example is that lldb commands can be defined to
take "raw" input.  "expression" is one of these.  So in the expression command,
you don't have to quote your whole expression, nor backslash protect quotes,

Finally, the results of the expressions are stored in persistent variables
(of the form $[0-9]+) that you can use in further expressions, like:

(lldb) expr self = $0
$4 = (SKTGraphicView *) 0x0000000100135430