Redirect lldb output to file

Question

I'm using lldb inside Xcode, and one of my variables contains a huge chunk of JSON data. Using po myVar isn't much helpful to analyse this data, as it will output in the tiny Xcode debug console.

Is there a way to redirect lldb output to a file ?

I saw here that such a feature seems to be available on gdb as :

(gdb) set logging on
(gdb) set logging file /tmp/mem.txt
(gdb) x/512bx 0xbffff3c0
(gdb) set logging off

and is "translated" in lldb as :

(lldb) memory read --outfile /tmp/mem.txt --count 512 0xbffff3c0
(lldb) me r -o/tmp/mem.txt -c512 0xbffff3c0
(lldb) x/512bx -o/tmp/mem.txt 0xbffff3c0

However, the memory read command will not help in my case, and apparently, --outfile is not available for the print command.

Answer 1

You can use a Python script to do so (and much more), as explained here:

LLDB Python scripting in Xcode

Create a file named po.py in a directory of your choice (for example "~/.lldb"):

import lldb

def print_to_file(debugger, command, result, dict):
  #Change the output file to a path/name of your choice
  f=open("/Users/user/temp.txt","w")
  debugger.SetOutputFileHandle(f,True);
  #Change command to the command you want the output of
  command = "po self"
  debugger.HandleCommand(command)

def __lldb_init_module (debugger, dict):
  debugger.HandleCommand('command script add -f po.print_to_file print_to_file ')

Then in lldb write:

command script import ~/.lldb/po.py
print_to_file

Answer 2

Here is a slight modification incorporating some of the comments from above:

def toFile(debugger, command, result, dict):
    f=open("/Users/user/temp.txt","w")
    debugger.SetOutputFileHandle(f,True);
    debugger.HandleCommand(command)  
    f.close()
    debugger.SetOutputFileHandle(sys.stdout, True)

This allows the command to be supplied as an argument, and reverts the output file handle to stdout after the command is run.

Answer 3

Assuming that you have a variable named jsonData (which has a Data type) you can save it to a file with this command:

expr jsonData.write(to: URL(fileURLWithPath: "/tmp/datadump.bin"))

Alternatively instead of above command you could dump memory used by this variable to a file as in the example below:

(lldb) po jsonData
▿ Optional<Data>
  ▿ some : 32547 bytes
    - count : 32547
    ▿ pointer : 0x00007fe8b69bb410
      - pointerValue : 140637472797712

(lldb) memory read --force --binary --outfile /tmp/datadump.bin --count 32547 0x00007fe8b69bb410
32547 bytes written to '/tmp/datadump.bin'

Answer 4

I found session save <filename> to be a much better, easier option than those listed here. It's not quite the same as you can't use it (to my knowledge selectively) but for generating logs, it's quite handy.