Re: Printing bitmap objects in the debugger

On Wed, Sep 14, 2016 at 3:46 PM, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
wrote:

>
>
> lately I'm using LVM debugger (which probably does not have something
> equivalent),
>

And I was so clueless about lldb's powerful scripting interface. For
example, you can write something like this in bms_utils.py:

import lldb

def print_bms_members (bms):
words = bms.GetChildMemberWithName("words")
nwords = int(bms.GetChildMemberWithName("nwords").GetValue())

ret = 'nwords = {0} bitmap: '.format(nwords,)
for i in range(0, nwords):
ret += hex(int(words.GetChildAtIndex(0, lldb.eNoDynamicValues,
True).GetValue()))

return ret

And then do this while attached to lldb debugger:

Process 99659 stopped
* thread #1: tid = 0x59ba69, 0x00000001090b012f
postgres`bms_add_member(a=0x00007fe60a0351f8, x=10) + 15 at
bitmapset.c:673, queue = 'com.apple.main-thread', stop reason = breakpoint
1.1
frame #0: 0x00000001090b012f
postgres`bms_add_member(a=0x00007fe60a0351f8, x=10) + 15 at bitmapset.c:673
670 int wordnum,
671 bitnum;
672
-> 673 if (x < 0)
674 elog(ERROR, "negative bitmapset member not allowed");
675 if (a == NULL)
676 return bms_make_singleton(x);
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> from bms_utils import *
>>> bms = lldb.frame.FindVariable ("a")
>>> print print_bms_members(bms)
nwords = 1 bitmap: 0x200

The complete API reference is available here
http://lldb.llvm.org/python_reference/index.html

Looks like an interesting SoC project to write useful lldb/gdb scripts to
print internal structures for ease of debugging :-)

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services