Carpe diem (Felix's blog)

I am a happy developer

C/ObjC Block and Debugger

In the last post, we discussed block byref internal structures and showed the sample code of how to printf it. However, we don’t want to write that much helper functions every time! Thanks for Big Nerd Ranch: Advanced Mac OSX Programming, we can now load gdb/lldb plugins and no need to do that anymore! ;D

GDB

Add these line into your ~.gdbinit file. It was originally created by Big Nerd Ranch Inc. I renamed the functions from dump-block-literal and dump-block-byref to pblock and pbyref.

define pblock
    printf "%s", (const char*)_Block_dump($arg0)
end

document pblock
    Dumps runtime information about the supplied block reference
    Argument is the name or address of a block literal
end

define pbyref
    printf "%s", (const char*)_Block_byref_dump((char*)&$arg0 - 2*sizeof(int) - 2*sizeof(void*))
end

document pbyref
    Dumps runtime information about the supplied __block variable
    Argument is a pointer to the variable embeded in a block byref structure
end

To use it, you just simply type pbyref and pblock followed by a variable in gdb.

(gdb) pbyref x
byref data block 0x7fff5fbff480 contents:
  forwarding: 0x7fff5fbff480
  flags: 0x0
  size: 32

(gdb) pblock localBlock
^0x7fff5fbff450 (new layout) =
isa: stack Block
flags: HASSIGNATURE HASHELP
refcount+deallocating: 0
invoke: 0x100001430
descriptor: 0x100002070
descriptor->reserved: 0
descriptor->size: 40
descriptor->copy helper: 0x100001470
descriptor->dispose helper: 0x1000014b0
descriptor->signature: 0x100001c29 'v8@?0'
descriptor->layout: 0x0 '(null)'

LLDB

I made a lldb version this weekend. You can find it on Github. I tried to use command alias or command regex at first, but it just won’t work. So I just write a python plugin for lldb and load it in .lldbinit. The code is quite trivial:

blockHelper.pysource 
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#!/usr/bin/env python

import lldb
import commands

def pbyref(debugger, command, result, internal_dict):
    cmd = "expr (void)printf(\"%s\",(const char*)_Block_byref_dump((char*)&" + \
    command + "- 2*sizeof(int) - 2*sizeof(void *)));"
    lldb.debugger.HandleCommand(cmd)

def pblock(debugger, command, result, internal_dict):
    cmd = "expr (void)printf(\"%s\",(const char*)_Block_dump(" + command + "));"
    lldb.debugger.HandleCommand(cmd)

def __lldb_init_module(debugger, internal_dict):
    debugger.HandleCommand('command script add -f blockHelper.pbyref pbyref')
    debugger.HandleCommand('command script add -f blockHelper.pblock pblock')
    print 'The "pbyref" command has been installed and is ready of use.'
    print 'The "pblock" command has been installed and is ready of use.'

INSTALL

  • Download this git repostory to your home directory as .lldb.
git clone https://github.com/dryman/lldb-block-helper.git ~/.lldb
  • add this line into your ~/.lldbinit.
command script import ~/.lldb/blockHelper.py

USAGE

(lldb) pbyref x
<no result>
byref data block 0x100713f90 contents:
  forwarding: 0x100713f90
  flags: 0x1000004
  size: 32

(lldb) pblock localBlock
<no result>
^0x7fff5fbff480 (new layout) =
isa: stack Block
flags: HASSIGNATURE HASHELP
refcount+deallocating: 0
invoke: 0x100001430
descriptor: 0x100002070
descriptor->reserved: 0
descriptor->size: 40
descriptor->copy helper: 0x100001470
descriptor->dispose helper: 0x1000014b0
descriptor->signature: 0x100001c29 'v8@?0'
descriptor->layout: 0x0 '(null)'

References

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