Python Reference

The entire LLDB API is available as Python functions through a script bridging interface. This means the LLDB API's can be used directly from python either interactively or to build python apps that provide debugger features.

Additionally, Python can be used as a programmatic interface within the lldb command interpreter (we refer to this for brevity as the embedded interpreter). Of course, in this context it has full access to the LLDB API - with some additional conveniences we will call out in the FAQ.

Documentation

The LLDB API is contained in a python module named lldb. A useful resource when writing Python extensions is the lldb Python classes reference guide.

The documentation is also accessible in an interactive debugger session with the following command:

(lldb) script help(lldb)
   Help on package lldb:

   NAME
      lldb - The lldb module contains the public APIs for Python binding.

   FILE
      /System/Library/PrivateFrameworks/LLDB.framework/Versions/A/Resources/Python/lldb/__init__.py

   DESCRIPTION
...

You can also get help using a module class name. The full API that is exposed for that class will be displayed in a man page style window. Below we want to get help on the lldb.SBFrame class:

(lldb) script help(lldb.SBFrame)
   Help on class SBFrame in module lldb:

   class SBFrame(__builtin__.object)
   |  Represents one of the stack frames associated with a thread.
   |  SBThread contains SBFrame(s). For example (from test/lldbutil.py),
   |
   |  def print_stacktrace(thread, string_buffer = False):
   |      '''Prints a simple stack trace of this thread.'''
   |
...

Or you can get help using any python object, here we use the lldb.process object which is a global variable in the lldb module which represents the currently selected process:

(lldb) script help(lldb.process)
   Help on SBProcess in module lldb object:

   class SBProcess(__builtin__.object)
   |  Represents the process associated with the target program.
   |
   |  SBProcess supports thread iteration. For example (from test/lldbutil.py),
   |
   |  # ==================================================
   |  # Utility functions related to Threads and Processes
   |  # ==================================================
   |
...

Embedded Python Interpreter

The embedded python interpreter can be accessed in a variety of ways from within LLDB. The easiest way is to use the lldb command script with no arguments at the lldb command prompt:

(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> 2+3
5
>>> hex(12345)
'0x3039'
>>>

This drops you into the embedded python interpreter. When running under the script command, lldb sets some convenience variables that give you quick access to the currently selected entities that characterize the program and debugger state. In each case, if there is no currently selected entity of the appropriate type, the variable's IsValid method will return false. These variables are:

Variable	Type	Description
lldb.debugger	lldb.SBDebugger	Contains the debugger object whose script command was invoked. The lldb.SBDebugger object owns the command interpreter and all the targets in your debug session. There will always be a Debugger in the embedded interpreter.
lldb.target	lldb.SBTarget	Contains the currently selected target - for instance the one made with the file or selected by the target select <target-index> command. The lldb.SBTarget manages one running process, and all the executable and debug files for the process.
lldb.process	lldb.SBProcess	Contains the process of the currently selected target. The lldb.SBProcess object manages the threads and allows access to memory for the process.
lldb.thread	lldb.SBThread	Contains the currently selected thread. The lldb.SBThread object manages the stack frames in that thread. A thread is always selected in the command interpreter when a target stops. The thread select <thread-index> command can be used to change the currently selected thread. So as long as you have a stopped process, there will be some selected thread.
lldb.frame	lldb.SBFrame	Contains the currently selected stack frame. The lldb.SBFrame object manage the stack locals and the register set for that stack. A stack frame is always selected in the command interpreter when a target stops. The frame select <frame-index> command can be used to change the currently selected frame. So as long as you have a stopped process, there will be some selected frame.

While extremely convenient, these variables have a couple caveats that you should be aware of. First of all, they hold the values of the selected objects on entry to the embedded interpreter. They do not update as you use the LLDB API's to change, for example, the currently selected stack frame or thread.

Moreover, they are only defined and meaningful while in the interactive Python interpreter. There is no guarantee on their value in any other situation, hence you should not use them when defining Python formatters, breakpoint scripts and commands (or any other Python extension point that LLDB provides). As a rationale for such behavior, consider that lldb can run in a multithreaded environment, and another thread might call the "script" command, changing the value out from under you.

To get started with these objects and LLDB scripting, please note that almost all of the lldb Python objects are able to briefly describe themselves when you pass them to the Python print function:

(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> print lldb.debugger
Debugger (instance: "debugger_1", id: 1)
>>> print lldb.target
a.out
>>> print lldb.process
SBProcess: pid = 59289, state = stopped, threads = 1, executable = a.out
>>> print lldb.thread
SBThread: tid = 0x1f03
>>> print lldb.frame
frame #0: 0x0000000100000bb6 a.out main + 54 at main.c:16

Running a python script when a breakpoint gets hit

One very powerful use of the lldb Python API is to have a python script run when a breakpoint gets hit. Adding python scripts to breakpoints provides a way to create complex breakpoint conditions and also allows for smart logging and data gathering.

When your process hits a breakpoint to which you have attached some python code, the code is executed as the body of a function which takes three arguments:

def breakpoint_function_wrapper(frame, bp_loc, dict):
   # Your code goes here

Argument	Type	Description
frame	lldb.SBFrame	The current stack frame where the breakpoint got hit. The object will always be valid. This frame argument might not match the currently selected stack frame found in the lldb module global variable lldb.frame.
bp_loc	lldb.SBBreakpointLocation	The breakpoint location that just got hit. Breakpoints are represented by lldb.SBBreakpoint objects. These breakpoint objects can have one or more locations. These locations are represented by lldb.SBBreakpointLocation objects.
dict	dict	The python session dictionary as a standard python dictionary object.

Optionally, a Python breakpoint command can return a value. Returning False tells LLDB that you do not want to stop at the breakpoint. Any other return value (including None or leaving out the return statement altogether) is akin to telling LLDB to actually stop at the breakpoint. This can be useful in situations where a breakpoint only needs to stop the process when certain conditions are met, and you do not want to inspect the program state manually at every stop and then continue.

An example will show how simple it is to write some python code and attach it to a breakpoint. The following example will allow you to track the order in which the functions in a given shared library are first executed during one run of your program. This is a simple method to gather an order file which can be used to optimize function placement within a binary for execution locality.

We do this by setting a regular expression breakpoint that will match every function in the shared library. The regular expression '.' will match any string that has at least one character in it, so we will use that. This will result in one lldb.SBBreakpoint object that contains an lldb.SBBreakpointLocation object for each function. As the breakpoint gets hit, we use a counter to track the order in which the function at this particular breakpoint location got hit. Since our code is passed the location that was hit, we can get the name of the function from the location, disable the location so we won't count this function again; then log some info and continue the process.

Note we also have to initialize our counter, which we do with the simple one-line version of the script command.

Here is the code:

(lldb) breakpoint set --func-regex=. --shlib=libfoo.dylib
Breakpoint created: 1: regex = '.', module = libfoo.dylib, locations = 223
(lldb) script counter = 0
(lldb) breakpoint command add --script-type python 1
Enter your Python command(s). Type 'DONE' to end.
> # Increment our counter.  Since we are in a function, this must be a global python variable
> global counter
> counter += 1
> # Get the name of the function
> name = frame.GetFunctionName()
> # Print the order and the function name
> print '[%i] %s' % (counter, name)
> # Disable the current breakpoint location so it doesn't get hit again
> bp_loc.SetEnabled(False)
> # No need to stop here
> return False
> DONE

The breakpoint command add command above attaches a python script to breakpoint 1. To remove the breakpoint command:

(lldb) breakpoint command delete 1

Using the python api's to create custom breakpoints

Another use of the Python API's in lldb is to create a custom breakpoint resolver. This facility was added in r342259.

It allows you to provide the algorithm which will be used in the breakpoint's search of the space of the code in a given Target to determine where to set the breakpoint locations - the actual places where the breakpoint will trigger. To understand how this works you need to know a little about how lldb handles breakpoints.

In lldb, a breakpoint is composed of three parts: the Searcher, the Resolver, and the Stop Options. The Searcher and Resolver cooperate to determine how breakpoint locations are set and differ between each breakpoint type. Stop options determine what happens when a location triggers and includes the commands, conditions, ignore counts, etc. Stop options are common between all breakpoint types, so for our purposes only the Searcher and Resolver are relevant.

The Searcher's job is to traverse in a structured way the code in the current target. It proceeds from the Target, to search all the Modules in the Target, in each Module it can recurse into the Compile Units in that module, and within each Compile Unit it can recurse over the Functions it contains.

The Searcher can be provided with a SearchFilter that it will use to restrict this search. For instance, if the SearchFilter specifies a list of Modules, the Searcher will not recurse into Modules that aren't on the list. When you pass the -s modulename flag to break set you are creating a Module-based search filter. When you pass -f filename.c to break set -n you are creating a file based search filter. If neither of these is specified, the breakpoint will have a no-op search filter, so all parts of the program are searched and all locations accepted.

The Resolver has two functions. The most important one is the callback it provides. This will get called at the appropriate time in the course of the search. The callback is where the job of adding locations to the breakpoint gets done.

The other function is specifying to the Searcher at what depth in the above described recursion it wants to be called. Setting a search depth also provides a stop for the recursion. For instance, if you request a Module depth search, then the callback will be called for each Module as it gets added to the Target, but the searcher will not recurse into the Compile Units in the module.

One other slight sublety is that the depth at which you get called back is not necessarily the depth at which the the SearchFilter is specified. For instance, if you are doing symbol searches, it is convenient to use the Module depth for the search, since symbols are stored in the module. But the SearchFilter might specify some subset of CompileUnits, so not all the symbols you might find in each module will pass the search. You don't need to handle this situation yourself, since SBBreakpoint::AddLocation will only add locations that pass the Search Filter. This API returns an SBError to inform you whether your location was added.

When the breakpoint is originally created, its Searcher will process all the currently loaded modules. The Searcher will also visit any new modules as they are added to the target. This happens, for instance, when a new shared library gets added to the target in the course of running, or on rerunning if any of the currently loaded modules have been changed. Note, in the latter case, all the locations set in the old module will get deleted and you will be asked to recreate them in the new version of the module when your callback gets called with that module. For this reason, you shouldn't try to manage the locations you add to the breakpoint yourself. Note that the Breakpoint takes care of deduplicating equal addresses in AddLocation, so you shouldn't need to worry about that anyway.

At present, when adding a scripted Breakpoint type, you can only provide a custom Resolver, not a custom SearchFilter.

The custom Resolver is provided as a Python class with the following methods:

Name	Arguments	Description
__init__	bkpt: lldb.SBBreakpoint extra_args: lldb.SBStructuredData	This is the constructor for the new Resolver. bkpt is the breakpoint owning this Resolver. extra_args is an SBStructuredData object that the user can pass in when creating instances of this breakpoint. It is not required, but is quite handy. For instance if you were implementing a breakpoint on some symbol name, you could write a generic symbol name based Resolver, and then allow the user to pass in the particular symbol in the extra_args
__callback__	sym_ctx: lldb.SBSymbolContext	This is the Resolver callback. The sym_ctx argument will be filled with the current stage of the search. For instance, if you asked for a search depth of lldb.eSearchDepthCompUnit, then the target, module and compile_unit fields of the sym_ctx will be filled. The callback should look just in the context passed in sym_ctx for new locations. If the callback finds an address of interest, it can add it to the breakpoint with the SBBreakpoint::AddLocation method, using the breakpoint passed in to the __init__ method.
__get_depth__	None	Specify the depth at which you wish your callback to get called. The currently supported options are: lldb.eSearchDepthModule lldb.eSearchDepthCompUnit lldb.eSearchDepthFunction For instance, if you are looking up symbols, which are stored at the Module level, you will want to get called back module by module. So you would want to return lldb.eSearchDepthModule. This method is optional. If not provided the search will be done at Module depth.
get_short_help	None	This is an optional method. If provided, the returned string will be printed at the beginning of the description for this breakpoint.

To define a new breakpoint command defined by this class from the lldb command line, use the command:

(lldb) breakpoint set -P MyModule.MyResolverClass

You can also populate the extra_args SBStructuredData with a dictionary of key/value pairs with:

(lldb) breakpoint set -P MyModule.MyResolverClass -k key_1 -v value_1 -k key_2 -v value_2

Although you can't write a scripted SearchFilter, both the command line and the SB API's for adding a scripted resolver allow you to specify a SearchFilter restricted to certain modules or certain compile units. When using the command line to create the resolver, you can specify a Module specific SearchFilter by passing the -s ModuleName option - which can be specified multiple times. You can also specify a SearchFilter restricted to certain compile units by passing in the -f CompUnitName option. This can also be specified more than once. And you can mix the two to specify "this comp unit in this module". So, for instance,

(lldb) breakpoint set -P MyModule.MyResolverClass -s a.out

will use your resolver, but will only recurse into or accept new locations in the module a.out.

Another option for creating scripted breakpoints is to use the SBTarget.CreateBreakpointFromScript API. This one has the advantage that you can pass in an arbitrary SBStructuredData object, so you can create more complex parametrizations. SBStructuredData has a handy SetFromJSON method which you can use for this purpose. Your __init__ function gets passed this SBStructuredData object. This API also allows you to directly provide the list of Modules and the list of CompileUnits that will make up the SearchFilter. If you pass in empty lists, the breakpoint will use the default "search everywhere,accept everything" filter.

Using the python API' to create custom stepping logic

A slightly esoteric use of the Python API's is to construct custom stepping types. LLDB's stepping is driven by a stack of "thread plans" and a fairly simple state machine that runs the plans. You can create a Python class that works as a thread plan, and responds to the requests the state machine makes to run its operations.

There is a longer discussion of scripted thread plans and the state machine, and several interesting examples of their use in:

https://github.com/llvm/llvm-project/blob/master/lldb/examples/python/scripted_step.py

And for a MUCH fuller discussion of the whole state machine, see:

https://github.com/llvm/llvm-project/blob/master/lldb/include/lldb/Target/ThreadPlan.h

If you are reading those comments it is useful to know that scripted thread plans are set to be "MasterPlans", and not "OkayToDiscard".

To implement a scripted step, you define a python class that has the following methods:

Name	Arguments	Description
__init__	thread_plan: lldb.SBThreadPlan	This is the underlying SBThreadPlan that is pushed onto the plan stack. You will want to store this away in an ivar. Also, if you are going to use one of the canned thread plans, you can queue it at this point.
explains_stop	event: lldb.SBEvent	Return True if this stop is part of your thread plans logic, false otherwise.
is_stale	None	If your plan is no longer relevant (for instance, you were stepping in a particular stack frame, but some other operation pushed that frame off the stack) return True and your plan will get popped.
should_step	None	Return True if you want lldb to instruction step one instruction, or False to continue till the next breakpoint is hit.
should_stop	event: lldb.SBEvent	If your plan wants to stop and return control to the user at this point, return True. If your plan is done at this point, call SetPlanComplete on your thread plan instance. Also, do any work you need here to set up the next stage of stepping.

To use this class to implement a step, use the command:

(lldb) thread step-scripted -C MyModule.MyStepPlanClass

Or use the SBThread.StepUsingScriptedThreadPlan API. The SBThreadPlan passed into your __init__ function can also push several common plans (step in/out/over and run-to-address) in front of itself on the stack, which can be used to compose more complex stepping operations. When you use subsidiary plans your explains_stop and should_stop methods won't get called until the subsidiary plan is done, or the process stops for an event the subsidiary plan doesn't explain. For instance, step over plans don't explain a breakpoint hit while performing the step-over.

Create a new lldb command using a Python function

Python functions can be used to create new LLDB command interpreter commands, which will work like all the natively defined lldb commands. This provides a very flexible and easy way to extend LLDB to meet your debugging requirements.

To write a python function that implements a new LLDB command define the function to take four arguments as follows:

def command_function(debugger, command, result, internal_dict):
    # Your code goes here

Optionally, you can also provide a Python docstring, and LLDB will use it when providing help for your command, as in:

def command_function(debugger, command, result, internal_dict):
    """This command takes a lot of options and does many fancy things"""
    # Your code goes here

Since lldb 3.5.2, LLDB Python commands can also take an SBExecutionContext as an argument. This is useful in cases where the command's notion of where to act is independent of the currently-selected entities in the debugger.

This feature is enabled if the command-implementing function can be recognized as taking 5 arguments, or a variable number of arguments, and it alters the signature as such:

def command_function(debugger, command, exe_ctx, result, internal_dict):
    # Your code goes here

Argument	Type	Description
debugger	lldb.SBDebugger	The current debugger object.
command	python string	A python string containing all arguments for your command. If you need to chop up the arguments try using the shlex module's shlex.split(command) to properly extract the arguments.
exe_ctx	lldb.SBExecutionContext	An execution context object carrying around information on the inferior process' context in which the command is expected to act Optional since lldb 3.5.2, unavailable before
result	lldb.SBCommandReturnObject	A return object which encapsulates success/failure information for the command and output text that needs to be printed as a result of the command. The plain Python "print" command also works but text won't go in the result by default (it is useful as a temporary logging facility).
internal_dict	python dict object	The dictionary for the current embedded script session which contains all variables and functions.

Since lldb 3.7, Python commands can also be implemented by means of a class which should implement the following interface:

class CommandObjectType:
    def __init__(self, debugger, session_dict):
        this call should initialize the command with respect to the command interpreter for the passed-in debugger
    def __call__(self, debugger, command, exe_ctx, result):
        this is the actual bulk of the command, akin to Python command functions
    def get_short_help(self):
        this call should return the short help text for this command[1]
    def get_long_help(self):
        this call should return the long help text for this command[1]

[1] This method is optional.

As a convenience, you can treat the result object as a Python file object, and say

print >>result, "my command does lots of cool stuff"

SBCommandReturnObject and SBStream both support this file-like behavior by providing write() and flush() calls at the Python layer.

One other handy convenience when defining lldb command-line commands is the command command script import which will import a module specified by file path, so you don't have to change your PYTHONPATH for temporary scripts. It also has another convenience that if your new script module has a function of the form:

def __lldb_init_module(debugger, internal_dict):
    # Command Initialization code goes here

where debugger and internal_dict are as above, that function will get run when the module is loaded allowing you to add whatever commands you want into the current debugger. Note that this function will only be run when using the LLDB command command script import, it will not get run if anyone imports your module from another module. If you want to always run code when your module is loaded from LLDB or when loaded via an import statement in python code you can test the lldb.debugger object, since you imported the module at the top of the python ls.py module. This test must be in code that isn't contained inside of any function or class, just like the standard test for __main__ like all python modules usually do. Sample code would look like:

if __name__ == '__main__':
    # Create a new debugger instance in your module if your module
    # can be run from the command line. When we run a script from
    # the command line, we won't have any debugger object in
    # lldb.debugger, so we can just create it if it will be needed
    lldb.debugger = lldb.SBDebugger.Create()
elif lldb.debugger:
    # Module is being run inside the LLDB interpreter
    lldb.debugger.HandleCommand('command script add -f ls.ls ls')
    print 'The "ls" python command has been installed and is ready for use.'

Now we can create a module called ls.py in the file ~/ls.py that will implement a function that can be used by LLDB's python command code:

#!/usr/bin/python

import lldb
import commands
import optparse
import shlex

def ls(debugger, command, result, internal_dict):
    print >>result, (commands.getoutput('/bin/ls %s' % command))

# And the initialization code to add your commands
def __lldb_init_module(debugger, internal_dict):
    debugger.HandleCommand('command script add -f ls.ls ls')
    print 'The "ls" python command has been installed and is ready for use.'

Now we can load the module into LLDB and use it

% lldb
(lldb) command script import ~/ls.py
The "ls" python command has been installed and is ready for use.
(lldb) ls -l /tmp/
total 365848
-rw-r--r--@  1 someuser  wheel         6148 Jan 19 17:27 .DS_Store
-rw-------   1 someuser  wheel         7331 Jan 19 15:37 crash.log

A more interesting template has been created in the source repository that can help you to create lldb command quickly:

https://github.com/llvm/llvm-project/blob/master/lldb/examples/python/cmdtemplate.py

A commonly required facility is being able to create a command that does some token substitution, and then runs a different debugger command (usually, it po'es the result of an expression evaluated on its argument). For instance, given the following program:

#import <Foundation/Foundation.h>
NSString*
ModifyString(NSString* src)
{
      return [src stringByAppendingString:@"foobar"];
}

int main()
{
      NSString* aString = @"Hello world";
      NSString* anotherString = @"Let's be friends";
      return 1;
}

you may want a pofoo X command, that equates po [ModifyString(X) capitalizedString]. The following debugger interaction shows how to achieve that goal:

(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> def pofoo_funct(debugger, command, result, internal_dict):
...   cmd = "po [ModifyString(" + command + ") capitalizedString]"
...   lldb.debugger.HandleCommand(cmd)
...
>>> ^D
(lldb) command script add pofoo -f pofoo_funct
(lldb) pofoo aString
$1 = 0x000000010010aa00 Hello Worldfoobar
(lldb) pofoo anotherString
$2 = 0x000000010010aba0 Let's Be Friendsfoobar

Using the lldb.py module in Python

LLDB has all of its core code build into a shared library which gets used by the lldb command line application. On macOS this shared library is a framework: LLDB.framework and on other unix variants the program is a shared library: lldb.so. LLDB also provides an lldb.py module that contains the bindings from LLDB into Python. To use the LLDB.framework to create your own stand-alone python programs, you will need to tell python where to look in order to find this module. This is done by setting the PYTHONPATH environment variable, adding a path to the directory that contains the lldb.py python module. The lldb driver program has an option to report the path to the lldb module. You can use that to point to correct lldb.py:

For csh and tcsh:

% setenv PYTHONPATH `lldb -P`

For sh and bash:

% export PYTHONPATH=`lldb -P`

Alternately, you can append the LLDB Python directory to the sys.path list directly in your Python code before importing the lldb module.

Now your python scripts are ready to import the lldb module. Below is a python script that will launch a program from the current working directory called "a.out", set a breakpoint at "main", and then run and hit the breakpoint, and print the process, thread and frame objects if the process stopped:

#!/usr/bin/python

import lldb
import os

def disassemble_instructions(insts):
    for i in insts:
        print i

# Set the path to the executable to debug
exe = "./a.out"

# Create a new debugger instance
debugger = lldb.SBDebugger.Create()

# When we step or continue, don't return from the function until the process
# stops. Otherwise we would have to handle the process events ourselves which, while doable is
#a little tricky.  We do this by setting the async mode to false.
debugger.SetAsync (False)

# Create a target from a file and arch
print "Creating a target for '%s'" % exe

target = debugger.CreateTargetWithFileAndArch (exe, lldb.LLDB_ARCH_DEFAULT)

if target:
    # If the target is valid set a breakpoint at main
    main_bp = target.BreakpointCreateByName ("main", target.GetExecutable().GetFilename());

    print main_bp

    # Launch the process. Since we specified synchronous mode, we won't return
    # from this function until we hit the breakpoint at main
    process = target.LaunchSimple (None, None, os.getcwd())

    # Make sure the launch went ok
    if process:
        # Print some simple process info
        state = process.GetState ()
        print process
        if state == lldb.eStateStopped:
            # Get the first thread
            thread = process.GetThreadAtIndex (0)
            if thread:
                # Print some simple thread info
                print thread
                # Get the first frame
                frame = thread.GetFrameAtIndex (0)
                if frame:
                    # Print some simple frame info
                    print frame
                    function = frame.GetFunction()
                    # See if we have debug info (a function)
                    if function:
                        # We do have a function, print some info for the function
                        print function
                        # Now get all instructions for this function and print them
                        insts = function.GetInstructions(target)
                        disassemble_instructions (insts)
                    else:
                        # See if we have a symbol in the symbol table for where we stopped
                        symbol = frame.GetSymbol();
                        if symbol:
                            # We do have a symbol, print some info for the symbol
                            print symbol

Writing lldb frame recognizers in Python

Frame recognizers allow for retrieving information about special frames based on ABI, arguments or other special properties of that frame, even without source code or debug info. Currently, one use case is to extract function arguments that would otherwise be unaccesible, or augment existing arguments.

Adding a custom frame recognizer is done by implementing a Python class and using the 'frame recognizer add' command. The Python class should have a 'get_recognized_arguments' method and it will receive an argument of type lldb.SBFrame representing the current frame that we are trying to recognize. The method should return a (possibly empty) list of lldb.SBValue objects that represent the recognized arguments.

An example of a recognizer that retrieves the file descriptor values from libc functions 'read', 'write' and 'close' follows:

class LibcFdRecognizer(object):
  def get_recognized_arguments(self, frame):
    if frame.name in ["read", "write", "close"]:
      fd = frame.EvaluateExpression("$arg1").unsigned
      value = lldb.target.CreateValueFromExpression("fd", "(int)%d" % fd)
      return [value]
    return []

The file containing this implementation can be imported via 'command script import' and then we can register this recognizer with 'frame recognizer add'. It's important to restrict the recognizer to the libc library (which is libsystem_kernel.dylib on macOS) to avoid matching functions with the same name in other modules:

(lldb) command script import .../fd_recognizer.py
(lldb) frame recognizer add -l fd_recognizer.LibcFdRecognizer -n read -s libsystem_kernel.dylib

When the program is stopped at the beginning of the 'read' function in libc, we can view the recognizer arguments in 'frame variable':

(lldb) b read
(lldb) r
Process 1234 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.3
    frame #0: 0x00007fff06013ca0 libsystem_kernel.dylib`read
(lldb) frame variable
(int) fd = 3