Automate GDB to print stack frame at a particular breakpoint

Question

Is it possible to have gdb attached to the PID of a running process and every time the program hits a particular breakpoint have gdb output the stackframe to an external file?

I've had a look at this and this but there is no mention on whether it is possible to attach gdb to an already running process (instead of having gdb launch it).

I can otherwise attach gdb to the PID just fine, but I'd like to automate it to run bt , store the output in an external file and then run continue . At the moment I'm doing this manually and it's a pain when I have to do it every time a breakpoint is hit.

Answer 1

Is it possible to have gdb attached to the PID of a running process??

Yes. Possible.

Updated:

Step 1:

In .gdbinit file add the following command,

define callstack
     set $Cnt = $arg0

     while($Cnt)
        commands $Cnt
        silent
        bt
        c
        end
        set $Cnt = $Cnt - 1
     end
end

Step 2: Invoke the gdb with -x <path to .gdbinit file > . Remember PID also for running process.

Step 3: Put Break points, whereever you need.

Step 4: Call the user defined command callstack and pass no .of break points.

    gdb> callstack <No.of. Break Points> 

Step 5: Now give 'c' to continue. Bcos process is already running.

For Logging I suggest to follow @VoidPointer's answer.

set pagination off
set logging file gdb.txt
set logging on 

works for me. Reference .

Answer 2

If what you need is to automate printing stack-frame using gdb when knowing PID and functions then you can try this.. (Given minimal code to be functional)

/root/.gdb_init:

set pagination off
set logging file gdb.txt
set logging on

br fun_convert
# ^^ when breaking at function fun_convert, execute `commands` till next `end`
commands
    bt
    print "Sample print command 1 \n"
    continue
end

br file.c:451
# ^^ when breaking at line 451 of file.c, execute from `commands` till next `end`
commands
    bt
    print "Sample print command 2 \n"
    continue
end

continue

Invoking GDB for PID 6474 and command file /root/.gdb_init ,

gdb -p 6474 -x /root/.gdb_init

Here, fun_convert is the function to break. This br is actual break gdb-command and you can also break at any line of file using br file.c:451 . For more break options, check gdb help. You can add any gdb-commands you need between commands and end for corresponding br . For more info on commands , check help commands on gdb .

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