169 sigsuspend(&set);
(gdb)
As you see ,after executing syssuspend(&set); gdb will hang ,how can I have a chance to type any commands?
If you want to debug several processes running the same executable (a.out), you want the (relatively) new GDB multi-inferior support.
Related
Is there any gcc option I can set that will give me the line number of the segmentation fault?
I know I can:
Debug line by line
Put printfs in the code to narrow down.
Edits:
bt / where on gdb give No stack.
Helpful suggestion
I don't know of a gcc option, but you should be able to run the application with gdb and then when it crashes, type where to take a look at the stack when it exited, which should get you close.
$ gdb blah
(gdb) run
(gdb) where
Edit for completeness:
You should also make sure to build the application with debug flags on using the -g gcc option to include line numbers in the executable.
Another option is to use the bt (backtrace) command.
Here's a complete shell/gdb session
$ gcc -ggdb myproj.c
$ gdb a.out
gdb> run --some-option=foo --other-option=bar
(gdb will say your program hit a segfault)
gdb> bt
(gdb prints a stack trace)
gdb> q
[are you sure, your program is still running]? y
$ emacs myproj.c # heh, I know what the error is now...
Happy hacking :-)
You can get gcc to print you a stacktrace when your program gets a SEGV signal, similar to how Java and other friendlier languages handle null pointer exceptions. See my answer here for more details:
how to generate a stacktace when my C++ app crashes ( using gcc compiler )
The nice thing about this is you can just leave it in your code; you don't need to run things through gdb to get the nice debug output.
If you compile with -g and follow the instructions there, you can use a command-line tool like addr2line to get file/line information from the output.
Run it under valgrind.
you also need to build with debug flags on -g
You can also open the core dump with gdb (you need -g though).
If all the preceding suggestions to compile with debugging (-g) and run under a debugger (gdb, run, bt) are not working for you, then:
Elementary: Maybe you're not running under the debugger, you're just trying to analyze the postmortem core dump. (If you start a debug session, but don't run the program, or if it exits, then when you ask for a backtrace, gdb will say "No stack" -- because there's no running program at all. Don't forget to type "run".) If it segfaulted, don't forget to add the third argument (core) when you run gdb, otherwise you start in the same state, not attached to any particular process or memory image.
Difficult: If your program is/was really running but your gdb is saying "No stack" perhaps your stack pointer is badly smashed. In which case, you may be a buffer overflow problem somewhere, severe enough to mash your runtime state entirely. GCC 4.1 supports the ProPolice "Stack Smashing Protector" that is enabled with -fstack-protector-all. It can be added to GCC 3.x with a patch.
There is no method for GCC to provide this information, you'll have to rely on an external program like GDB.
GDB can give you the line where a crash occurred with the "bt" (short for "backtrace") command after the program has seg faulted. This will give you not only the line of the crash, but the whole stack of the program (so you can see what called the function where the crash happened).
The No stack problem seems to happen when the program exit successfully.
For the record, I had this problem because I had forgotten a return in my code, which made my program exit with failure code.
I want to write a script that will read a process memory, and display its contents with some modification/format.
It would be create if i could run a c program inside gdb and send gdb commands from this program. Do you know if it is possible ?
It would be great if i could run a c program inside gdb and send gdb commands from this program.
That's easy:
(gdb) shell /tmp/a.out > /tmp/gdb.script
(gdb) source /tmp/gdb.script
If your gdb is built with Python support, much better scripting support is available. Start here.
I'm using gcc 4.9.2 & gdb 7.2 in Solaris 10 on sparc. The following was tested after compiling/linking with -g, -ggdb, and -ggdb3.
When I attach to a process:
~ gdb
/snip/
(gdb) attach pid_goes_here
... it is not loading symbolic information. I started with netbeans which starts gdb without specifying the executable name until after the attach occurs, but I've eliminated netbeans as the cause.
I can force it to load the symbol table under netbeans if I do one of the following:
Attach to the process, then in the debugger console do one of the following:
(gdb) detach
(gdb) file /path/to/file
(gdb) attach the_pid_goes_here
or
(gdb) file /path/to/file
(gdb) sharedlibrary .
I want to know if there's a more automatic way I can force this behavior. So far googling has turned up zilch.
I want to know if there's a more automatic way I can force this behavior.
It looks like a bug.
Are you sure that the main executable symbols are loaded? This bug says that attach pid without giving the binary doesn't work on Solaris at all.
In any case, it's supposed to work automatically, so your best bet to make it work better is probably to file a bug, and wait for it to be fixed (or send a patch to fix it yourself :-)
I'm doing an OS class that's based on xv6 and I wrote a program that needs to run on it.
I know that I can debug kernel code with make qemu-gdb but I'm not sure how to debug my own user program.
Lets say I want to debug cat, how would I go about doing that?
Thanks
P.S. isn't there an xv6 tag? should this question even go here?
From the xv6 top-level dir:
Run the emulator in debug mode (assuming no X11): make qemu-nox-gdb
In other terminal just run the debugger loading the kernel symbols with:
gdb kernel This is important, otherwise the debugger will be confused between kernel and and user program symbols, for example main()
From the gdb interface run: (gdb) target remote localhost:26000
where 26000 is the TCP port that the step #1 report at the end (this might change).
Load the user exec with (gdb)file user_program
Place a breakpoint (gdb) break main and continue with (gdb) continue
etc...
file cat, break main, continue
semi reference running and debugging xv6
I'm trying to debug a server I wrote with gdb as it segfaults under very specific and rare conditions.
Is there any way I can make gdb run in the background (via quiet or batch mode?), follow children (as my server is a daemon and detaches from the main PID) and automatically dump the core and the backtrace (to a designated file) once the program crashes?
Assuming you have appropriate permissions, you can have gdb attach to any process. You can do it on the command line with:
gdb /path/to/binary _pid_
or from within gdb with the attach command:
attach _pid_
So, once your daemon has started, you can use either of these techniques to attach to the final PID your daemon is running as. Attaching gdb stops the process which you are tracing so you will need to issue a "continue" to restart it.
I don't know a direct way to get gdb to run arbitrary commands when the program crashes. Here is one workaround I can think of:
Create and register a signal handlers for SIGSEGV.
Tell gdb not to stop on that signal (handle SIGSEGV nostop)
Set a breakpoint at the first line of your signal handler.
Assign commands to the breakpoint from step 3
Why not just run the process interactively in a persistent screen session? Why must it be a daemon when debugging? Or just run gdb in the screen session and attach it to the running process (e.g. gdb /path/to/binary -p PID_of_binary) after it forks.
First, I'd setup your shell / environment to give you a core dump. In bash:
ulimit -c unlimited
Once you have the core dump, you can use gdb to examine the stack trace:
gdb /path/to/app /path/to/core/file
I'm not really a gdb expert but two things come to mind
Tracepoints which might give you the necessary information as your program runs or
Use gdb's remote debugging facility to debug your program while it's running as a daemon.
How to generate a stacktrace when my gcc C++ app crashes answer for this question should do what you want. (assuming you can make changes in your code)
You might want to take a look at how Samba facilitates debugging; it has a configurable "panic action" that can suspend the application, notify the developer, spawn gdb, etc., and is run as part of its signal handler. See lib/util/fault.c in the Samba source tree.
My practice: comment out daemon function call, rebuild binary, then use gdb to run.