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Re: Debugging agent library
- From: Yao Qi <yao at codesourcery dot com>
- To: Simon Marchi <simon dot marchi at polymtl dot ca>
- Cc: Suchakra Sharma <suchakrapani dot sharma at polymtl dot ca>, <gdb at sourceware dot org>
- Date: Wed, 8 May 2013 10:16:23 +0800
- Subject: Re: Debugging agent library
- References: <1367868074 dot 5194 dot 19 dot camel at station15 dot dorsal dot polymtl dot ca> <5188AE42 dot 50308 at codesourcery dot com> <CAFXXi0mqnd7XQQXFX5mQXF0r8dRn+crsyz8g0uip=svZpm245Q at mail dot gmail dot com>
On 05/08/2013 02:49 AM, Simon Marchi wrote:
tstart here gives me the error:
(gdb) tstart
Target returns error code '.In-process agent library not loaded in
process. Fast and static tracepoints unavailable.'
I compiled my small app using (libdagent.so is already in my ld search path):
$ gcc test.c -Wall -g -ldagent -O0
I run the gdbserver using:
$ LD_PRELOAD=/usr/local/lib/libdagent.so gdbserver :1234 ./a.out
If libdagent is linked to your program explicitly, LD_PRELOAD is not
needed any more. You have to either compile your program without
-ldagent and use LD_PRELOAD on run or compile your program with -ldagent
and run your program normally.
Then I use the same commands as you have shown in your example. info
sharedlibrary shows that the agent is loaded:
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007ffff7dddb00 0x00007ffff7df685b Yes (*) /lib64/ld-linux-x86-64.so.2
0x00007ffff7bd38c0 0x00007ffff7bd8e38 Yes /usr/local/lib/libdagent.so
0x00007ffff785ba20 0x00007ffff797c72c Yes (*) /lib64/libc.so.6
0x00007ffff7625660 0x00007ffff7630eb8 Yes (*) /lib64/libpthread.so.0
Is there something I am missing here?
If the suggestions above don't work for you, you probably have to turn
on some debugging outputs. You can start gdbserver with option
"--debug", and further, hack the variable "debug_agent" to 1 in
gdb/common/agent.c.
Also, is it possible to write our own application to control tracing,
instead of using GDB? I guess we would need to talk to the agent using
the IPA protocol, is that right?
Tracing control involves two sides, GDB and the agent. IPA protocol is
used to control the agent, however, in order to finish/control tracing,
GDB side should be controlled as well. Your tool can get the trace data
from agent, but these data needs GDB interpretation, map the address to
a symbol, for example. I am afraid you can't use your own application
to do tracing with agent.
On the other hand, I am curious on the reasons to write your own tool to
control tracing instead of using GDB. Tracing is interesting to GDB,
and you can see how much progress GDB made on tracing in the past
several years. GDB is extensible on both its c code and python script,
and you can extend it for your purpose. Last by not least,
contributions are always welcome, and people here are friendly and nice
to answer questions, AFAICS.
--
Yao (éå)