GDB: Is there some way to construct "Local" convenient variables?

Question

I have an real-time OO program (nevertheless written in C) that I am trying to debug. I have an issue that some objects out of thousands get corrupted during a specific window of time, every now and then. I want to have one breakpoint at the start of the window, which automatically sets a watchpoint on a member variable, and then have that watchpoint removed at the end of the window by another breakpoint. The trouble is I need some way of tying a watchpoint number to a given object. If I could construct a convenience variable by some mechanism so that, for example, if $var=28, then set $x${var}=watch -l foo would be the equivalent of set $x28=watch -l foo. (which doesn't actually work) This would allow me to do this:

breakpoint obj_init+23
command
  $var = *obj
  $x${var} = watch -l foo
  continue
done

breakpoint obj_final
command
  $var = *obj
  delete $x${var}
  continue
done

So I don't (hopefully) overrun the number of available hardware watchpoints.

Does anyone know how I might achieve this without trying to write a python extension? (My python is very rusty.)

did you try valgrind? seems easier to setup and usually help in data corruption problems — OznOg
– OznOg, Commented Apr 20, 2021 at 16:58
I don't think it's possible without python. Also, I agree that you should try it with valgrind or sanitizers first. — ssbssa
– ssbssa, Commented Apr 21, 2021 at 10:51
valgrind was useless. Mostly because I'm running a gstreamer pipeline consisting of glib objects under python, and they ALL do strange things to memory handling. I was not able to get a useful run after a full day of trying, and yes, I've read the valgrind docs for each of the frameworks in question. — swestrup
– swestrup, Commented Apr 21, 2021 at 17:08
As for sanitizers I don't think they are (yet) enabled on my platform with gcc. I need to run these tests on CentOS and its years behind on compilers. — swestrup
– swestrup, Commented Apr 21, 2021 at 17:10

Mark Plotnick · Accepted Answer · 2021-04-21 14:39:04Z

1

You can use GDB's eval command to set and use variables whose names are composed of the results of numeric or string expressions.

You can think of eval as doing a printf of its arguments and then executing the resulting string. eval "set var $x%d = 1", 5 will run the command set var $x5 = 1.

The other piece of info you need is that the watch command, as with all breakpoint commands, will set the convenience variable $bpnum to the breakpoint number.

break obj_init+23
commands
  set var $var = *obj
  watch -l foo
  eval "set var $x%d = $bpnum", $var
  continue
done

break obj_final
commands
  set var $var = *obj
  eval "delete $x%d", $var
  continue
done

answered Apr 21, 2021 at 14:39

Mark Plotnick

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1 Comment

swestrup Over a year ago

Thanks! I was unaware of the 'eval' command!

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