How to debug a running C++ program in Linux?

Question

Hi

I have a question about debugging a running C++ program in Linux. If a programming is already running and can't be interrupted, how to do that.

I can find three ways, but I don't know too much about details, I am grateful if any one can elaborate it deeper.

1) we can use GDB by specifying the process ID

gdb -p PID

In this case, what's the difference between this and attach PID?

2) We can use pstat, however, I am using Ubuntu, no pstat, but only mpstat

it seems that mpstat does not provide too much information, and not so many options.

3) check the details information under directory ./proc

In this case, just go to the directory with the PID. However, should this be done mannually?

Answer 1

In regards to technique 1, there is no -p flag and you still need the name of the program:

gdb prog PID

There is no difference between doing that vs running gdb prog and then telling gdb attach pid.

Answer 2

Use ps -ef | grep <your program> to get the PID. Then run gdb <your program> <PID>. pstat is not a standard command. I've only used it with Solaris.

e.g.

gayan@gayan:~/FE/bin> ./fe&
[1] 5866                                 
gayan@gayan:~/FE/bin> ps -ef | grep fe
gayan     5866  5836  2 10:19 pts/3    00:00:00 ./fe
gayan     5871  5836  0 10:19 pts/3    00:00:00 grep fe
gayan@gayan:~/FE/bin> gdb fe 5866
GNU gdb (GDB; openSUSE 11.1) 6.8.50.20081120-cvs       
Copyright (C) 2008 Free Software Foundation, Inc.      
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html&gt;
This is free software: you are free to change and redistribute it.           
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"   
and "show warranty" for details.                                             
This GDB was configured as "i586-suse-linux".                                
For bug reporting instructions, please see:                                  
<http://bugs.opensuse.org/&gt;...                                               
Attaching to program: /home/gayan/FE/bin/fe, process 5866

The above was run on openSuse but should work on Ubuntu.

Answer 3

I can't find -p option in gdb man or their documentation, but it does work! I've tried it many times with older versions on RedHat and 7.0.1 on Debian.

I'm not sure how exactly it finds the exe by PID (maybe /proc/<PID>/exe), but it does. Since it's not described in their documentation, perhaps it not the most recommended way, but I haven't had any problems with it.

There's no noticeable difference between gdb -p <PID> and running gdb and in the their shell typing attach <PID>.

I personally prefer ps xa| grep myprogram for getting the PID