I'd like to to debug a multiprocess C++ project with GDB, specifically I'd like to know if there is a way to achieve the following
Attach multiple processes to a single instance of GDB while letting all the processes run
Setting up a breakpoint in the source code of one of the processes stops all the attached processes
The ideal solution would be something similar to what is offered by the Visual Studio debugger as described here.
At the moment I'm able to attach multiple processes to a GDB instance but then only the current selected inferior is executed while the others are stopped and waiting for a continue command.
In order to be able to run inferiors in the background, one needs to issue this gdb command
set target-async on
after start up and before running anything. With this option in effect, one ca issue
continue&
(or just c&) and this will send the inferior to the background, giving an opportunity to switch to run another one.
Stopping all inferiors at once is a bit more difficult. There is no built-in command for that. Fortunately gdb is scriptable and it is possible to attach a script to a breakpoint. Once the breakpoint is hit, the commands are executed. Put inferior n and interrupt commands in the script for each inferior. It is probably more convenient to do that from a Python script, something like
(gdb) python
>inf = gdb.inferiors()
>for i in inf:
> gdb.execute("inferior %d" % i.num)
> gdb.execute("interrupt")
Related
I have multiple different processes communicating over IPC and when debugging a single process using gdb, whenever a breakpoint is hit, I am trying to send a message to other processes. Is there a way to automatically invoke a function/piece of code (NotifyAll()) whenever a breakpoint is hit without manually running commands and invoking the function in the gdb console.
Basically, whenever a gdb debugger is attached to one of these processes, I want gdb to know that it should invoke NotifyAll() whenever a breakpoint (application-wide) is hit.
Yes, this can be done using the Python scripting capabilities in gdb.
In particular you want to add a listener to gdb.events.stop that checks for a breakpoint stop event, then calls your function. It's possible (I don't know offhand) that you'll have to defer the calling of the function by posting an event to the gdb event loop.
To make this work with the minimum of manual intervention, use the gdb script auto-loading feature to associate this Python script with your application. This will require users to trust the script (read about add-auto-load-safe-path), but that's all.
Note that doing things like this is potentially confusing to people trying to debug your application. For example, setting a breakpoint in the RPC code will cause problems unless your script takes extra care.
I'm currently debugging syslinux (a boot loader) through the gdb stub of qemu.
Recently, I wrote some gdb commands that (un)load the debug symbols everytime a module is dynamically (un)loaded. In order not to disrupt the execution, I ended the commands with continue.
break com32/lib/sys/module/elf_module.c:282
commands
silent
python
name = gdb.parse_and_eval("module->name").string()
addr = int(str(gdb.parse_and_eval("module->base_addr")), 0)
gdb.execute("load-syslinux-module %s 0x%08x" % (name, addr))
end
continue
end
However, when stepping through the code line by line, if the next or step command makes the execution hit the breakpoint, the breakpoints takes precedence, the commands are executed, including the continue. And the execution continue irrespectively of the line-by-line debugging I was doign. This also happen if I try to step over the function that has this breakpoint.
How can I keep (un)loading the debug symbols on the fly while not interfering with the debugging?
Is there an alternative to the continue command? Maybe using breakpoints isn't the right way? I'd take any solution.
This can't be done from the gdb CLI. However, it is easy to do from Python.
In Python the simplest way is to define one's own gdb.Breakpoint subclass, and define the stop method on it. This method can do the work you like, then return False to tell gdb to continue.
The stop facility was designed to avoid the problems with cont in commands. See the documentation for more details.
So I am relatively new to coding so please forgive improper vocab. What I am basically trying to do is create a script for, or perhaps enter commands into, GDB so that it can run my code with the input file of a test case over and over. Basically, I am working on a project right now that makes heavy usage of semaphores and mutexes, and somewhere, every once in a blue moon, my code breaks due to race conditions. If I could have gdb run my test case continuously until my code reached a seg fault, this would be ideal.
PS- Please be specific as to what I must do, I am not great at dissecting answers that have heavy technical answers.
Thank You!
The simplest solution is expect script. Expect is a program to automate interactions with programs that expose a text terminal interface.
Examples are available at http://en.wikipedia.org/wiki/Expect
The script should be like
#!/usr/bin/expect
# start gdb
spawn gdb yourprogram
while {1} {
# wait for gdb to start, expect the (gdb) to appear
expect "(gdb)"
# send command to run your program
send "run your_args\n"
expect {
"Program exited normally." {continue} # just run again
"(Some error message)" {interact} # start to debug
}
}
You can use GDB scripts in order to automate your GDB sessions.The GDB macro coding language consists of gdb commands along with basic looping statements and conditional statements.
You can find information about it here
http://www.adacore.com/adaanswers/gems/gem-119-gdb-scripting-part-1/
What are the best ways to automate a GDB debugging session?
I am trying to debug a service. The usual procedure is to start the service and attach gdb to the process. But I want to debug the code when the service is still starting up. It takes a while for gdb to load the libraries, and the required code has executed before I can put the breakpoints. Any idea how to do it? Thanks!
Let's assume your service is called "myservice.exe"
If you can get on the box that the code is actually running on, then I would do the following:
$ gdb myservice.exe
(gdb) break myclass:myfunction
(gdb) run
This should get you what you want.
Note: if you can't run gdb directly, then put a "sleep" statement for 1 minute at the very start (before the part you want to debug) - that should allow you to connect before it starts the sensitive code.
How do I stop a GDB execution without a breakpoint?
Just use a regular interrupt Ctrl-c will work just fine. GDB just forwards the SIGINT to the debugging process which then dies. GDB will catch the non-standard exit and break the process there, so you can still examine all the threads, their stacks and current values of variables. This works fine, though you would be better off using break points. The only time I find myself doing this is, if I think I've gotten into some sort of infinite loop.
GUI applications don't react to ^C and ^Break the way console applications do. Since these days most non-trivial projects tend to be GUI applications or libraries primarily used in GUI applications, you have two options:
Send SIGSTOP to the application from a separate terminal. This is cumbersome.
If you press ^C or ^Break on the GDB prompt, GDB will terminate but the application will remain running. You can then run GDB again to attach to it using the -p command-line switch. This loses debugger state.
In both cases, you might find this helpful: tasklist | grepProcessName| sed -e 's/ProcessName*\([0-9]*\).*/gdbModuleName-pid=\1/' > rungdb.sh You can modify this for use in shell scripts, makefiles or to send a signal instead of attaching GDB.
info threads will help you figure out which thread you want to look at. Then use threadThreadNumber to switch to it.
Start a shell, find the process ID using ps and send it SIGSTOP or SIGINT by using the kill command (e.g. kill -INT pid).
Just type BREAK without any arguments.
Break, when called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame
Ctrl + Z seems to work for me (but only in some cases - I'm not sure why).