How to script gdb (with python)? Example add breakpoints, run, what breakpoint did we hit?

Question

I'm trying to create a little unit test with gdb, for a embedded mcu that is controlled by OpenOCD (that gives me control over my target via a gdb server).

So I would like to automate this with some scripting of gdb.

I would like to write some kind of script for gdb that more or less does this:

1. Add a couple of breakpoints
2. Start the program
3. When we stop, where did it stop (get the frame info)
4. Quit.

Any ideas?

A example on how to do this in python gdb scripting would be nice.

Thanks Johan

---

Note:

Let's say that we have this basic structure, that more or less goes into test_failed() or test_success() depending on what the function start_test() returns.

void test_failed() {    
    while(1);    
}

void test_success() {    
    while(1);    
}

int main(void) {    
    int status = start_test();    

    if( status > 0 ) {    
        test_failed();    
    }    
    test_success();

    while(1);    
}

To do this manually in gdb is very strait forward,

(gdb) break test_success
Breakpoint 1 at 0x20: file src/main.c, line 9.
(gdb) break test_failed
Breakpoint 2 at 0x18: file src/main.c, line 5.
(gdb) cont
Continuing.

Breakpoint 1, test_success () at src/main.c:9
9       while(1);
(gdb) frame
#0  test_success () at src/main.c:9
9       while(1);
(gdb) 

So the next step I tried was to add those gdb commands into a gdb startup script that more or less just looked like this.

break test_success
break test_failed
target remote localhost:3333
cont 
frame

and start it with

arm-none-eabi-gdb --batch --command=commands.gdb main.elf

And this kind of works, but it is not very nice. How do I do this with the "new and cool" python scripts, that gdb seem to support.

Answer 1

FYI recent gdb versions are scriptable in Python. You can call python code from the gdb command line. This opens a whole new world, check the relevant documentation. From the command line run:

 dnf/yum/apt-get install gdb-doc
 info gdb extending python

If you do not like the text-based info browser, here is one (among many?) alternative, graphical browser:

yelp 'info:gdb' # , go to "Extending"

Here is a sample gdb-python script. It attaches gdb to the first "your_program" found running.

#!/usr/bin/python

import subprocess
import string

def backquotes(cmdwords):
        output = subprocess.Popen(cmdwords, stdout=subprocess.PIPE).communicate()[0]
        return output.strip()

pid = backquotes(['pgrep', 'your_program'])

gdb.execute("attach " + str(pid))

Answer 2

A reduced example that I'm currently using:

class DebugPrintingBreakpoint(gdb.Breakpoint):
    debugging_IDs = frozenset({37, 153, 420})
    def stop(self):
        top = gdb.newest_frame()
        someVector = top.read_var('aVectorVar')
        # Access the begin() & end() pointer of std::vector in GNU Standard C++ lib
        first = someVector['_M_impl']['_M_start']
        last = someVector['_M_impl']['_M_finish']
        values = []
        while first != last:
            values.append(int(first.dereference()['intID']))
            first = first + 1
        if not set(values) & debugging_IDs:
            return False # skip: none of the items we're looking for can be found by ID in the vector on the stack
        print("Found other accompanying IDs: {}".format(values))
        return True # drop to gdb's prompt
# Ensure shared libraries are loaded already
gdb.execute("start")
# Set our breakpoint, which happens to reside in some shared lib, hence the "start" previously
DebugPrintingBreakpoint("source.cpp:42")
gdb.execute("continue")

You can execute this script from gdb's prompt like this:

(gdb) source script.py

Or from the command-line:

$ gdb --command script.py ./executable.elf

See the complete GDB Python API docs for further info.