GDB: setting complex break point with condition involving variable from previous frame

Question

Is it possible to set a complex breakpoint which has condition which involves check on the argument passed to the outer function(frame).

eg.

  1 #0  sample::_processMessage (this=0xa5c8c0, data=0x7fffe5ae31db "\027w\270߸\023\032\212\v", line=0x7fffe4799db8 "224.4.2.197:60200", should_process=true) a sample.cpp:426
  2 #1  0x00007ffff682f05d in sample::_process (this=0xa5c8c0, should_process=true, line=0x7fffe4799db8 "224.4.2.197:60200", data=0x7fffe5ae31db "\027w\270߸\023\032\212\v", sn=31824) a sample.cpp:390
  3 #2  0x00007ffff6836744 in sample::drain (this=0xa5c8c0, force=true) at sample.cpp:2284
  4 #3  0x00007ffff682ed81 in sample::process (this=0xa5c8c0, mdData=0x7fffe67914e0) at sample.cpp:354

Here I want to set a break point on sample.cpp:356,which is on in the function process-frame#3, if the _process or frame #1 at the time hitting breakpoint has sn == 31824

so actually break point is at function _process but I want pause the execution in the function process

Thanks in advance

Answer 1

I don't know if it's possible to create conditional breakpoints that reference an outer frame, but you could use breakpoint commands to achieve a similar result.

Here's an example gdb session:

(gdb) break some-location
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>silent
>up
>if (sn != 31824)
 >continue
 >end
>end

Now every time gdb hits the breakpoint it will automatically move up a frame, check sn and continue if the value is not correct. This will not be any (or much) slower than a conditional breakpoint, and the only real downside is that it will print out a line each time the breakpoint is hit, even if gdb then continues.

The silent in the command list cuts down on some of the normal output that is produced when a breakpoint is hit, this can be removed to get a more verbose experience.