GDB Tracepoint

Set Tracepoint

Observe the program without interrupting.

  trace [PROBE_MODIFIER] [LOCATION] [thread THREADNUM]
    [-force-condition] [if CONDITION]

Set actions to be taken at a trace.

  actions [tracepoint]
  > collect
  > while-stepping
  > end

Set data item to collect.

  collect expressions...

Set stepping commands.

  > while-stepping N
    > collect ...
    > end

Set passcount.

  passcount count [tracepoint]


Sample: Setup a tracepoint. Trace only work within remote mode, so need a gdbserver.

#include <unistd.h>

int main()
{
  int s = 0;
  int i = 0;
  while (1)
  {
    s += i;
    i += 1;
    sleep(3);
  }
}
  

gdbserver --multi :12345
  
(gdb) target extended-remote :12345
(gdb) set remote exec-file demo
(gdb) file demo
(gdb) start


(gdb) trace demo.c:10
(gdb) actions
> collect i, s
> while-stepping 2
  > collect i
  > end
> end
(gdb) passcount 20 2


(gdb) info tracepoints


Collect Traces

Start trace.

  tstart

Stop trace.

  tstop

Show status.

  tstatus

Collect trace.

(gdb) tstart
(gdb) continue
(gdb) tstop

(gdb) tstatus
Trace stopped by a tstop.
Collected 24 trace frames.


Analyze Traces

Select trace frame by index.

  tfind start # frame 0
  tfind n     # frame n
  tfind       # next frame
  tfind -     # prev frame

Select trace frame by filter.

  tfind tracepoint 
  tfind line         
  tfind pc         
  tfind range       
  tfind outside

Leave trace frame, back to live debug.

  tfind end

Print collected data.

  tdump

Convenience variables.

  (int) $trace_frame
  (int) $tracepoint
  (int) $trace_line
  (char []) $trace_file
  (char []) $trace_func

Scan through the trace frames.

(gdb) tfind start
(gdb) while $trace_frame != -1
> frame
> tdump
> tfind
> end

Found trace frame 0, tracepoint 2
#0  0x00000000004006cc in main () at demo.c:10
Data collected at tracepoint 2, trace frame 0:
i = 16
s = 136

Found trace frame 1, tracepoint 2
#0  0x00000000004006d0 in main () at demo.c:10
Data collected at tracepoint 2, trace frame 1:
i = 16

Found trace frame 2, tracepoint 2
#0  0x00000000004006d4 in main () at demo.c:10
Data collected at tracepoint 2, trace frame 2:
i = 16

Found trace frame 3, tracepoint 2
#0  0x00000000004006cc in main () at demo.c:10
Data collected at tracepoint 2, trace frame 3:
i = 17
s = 153

Save Trace Data

Save trace data in client.

  tsave filename

Save trace data in server.

  tsave -r filename

Load trace data.

  target tfile filename

Sample: Save and load trace data.

(gdb) tsave demo.trace
(gdb) file demo
(gdb) target tfile demo.trace

(gdb) tfind start
Found trace frame 0, tracepoint 1
#0  0x00000000004006cc in main () at demo.c:10

(gdb) tdump
Data collected at tracepoint 1, trace frame 0:
i = 16
s = 136