I attached to my multithread application with gdb and after that type cont to continue execution.
Is there any way to stop execution at any time on cont gdb state and check what every thread do?
How to check state of every thread and get execution line number of each? (commands)
Here's what I do, (taken from here )
Create a little gdb script stackdumper.gdb that dumps the stack trace of all threads:
thread apply all backtrace
Then repeatedly attach gdb and run the dumper:
for i in $(seq 1 10) ; do
gdb -batch -x stackdumper.gdb ./a.out 123456 > stack.$i
sleep 10
done
where ./a.out is the binary you are interested and 123456 is the PID.
Adjust the sleep to match your sampling needs.
thread apply all bt
Or
info threads
t <threadid from above trace >
Followed by
where or bt
To get the backtrace for all of the stopped threads type the
thread apply all bt
command (the output is exactly the same that one might see in the MacOSX crash report box).
Usually the threads are stopped simultaneously in gdb.
Reference: http://www.delorie.com/gnu/docs/gdb/gdb_40.html
And here's about "all-stop" mode, which is default: http://sourceware.org/gdb/onlinedocs/gdb/All_002dStop-Mode.html
Is this any way to stop execution at any time on cont gdb state and check what every thread do
If you ask about ways to check what threads do without gdb the you can just run pstask <pid-of-your application>
Is it "pstack" because i don't think "pstask" is any command in linux, If it is please provide some more info
Related
got some big real time project to deal with (multiple processes (IPCs), multi Everything in short).
My working on process is started as service on Linux. I have the root access.
Here is the problem:
I'm trying to attach to a running proc, tried starting it through/with gdb but the result is the same: it stops the executable once I "touched" it with gdb or sometimes it throws:
Program received signal SIGUSR1, User defined signal 1. [Switching to Thread 0x7f9fe869f700 (LWP 2638)]
of course from there nothing can be done.
Tried:
handle all nostop
attach to launched as service (daemon) or launched as regular proc
started from gdb
thought maybe forking/multi-threaded problem - implemented in the very beginning sleep for 10 seconds - attached to it with "continue"
Guys, all I want it is to debug, hit the breakpoints, etc.
Please help! Share ideas.
Editing actual commands:
1) gdb attach myProcId. Then after reading symbols, I hit "c" which results:
Program received signal SIGUSR1, User defined signal 1.
[Switching to Thread 0x7f9fe869f700 (LWP 2638)]
0x00007f9fec09bf73 in select () from /lib64/libc.so.6
2) If I make the first line 10 seconds sleep in the code, attaching to the process, hit "c", result: it runs, shows info threads, backtrace of main, but never hits the breakpoint (for sure the code runs there - I get logs and different behaviour if I change code there), meaning the process is stuck.
3) All other combinations like gdb path/to/my/proc args list, then start. Where arg list played with different related options gdb gives us.
Maybe worth to mention: process network packets related, timers driven also.
But for me the important thing is a current snapshot on break, i don't care what will happen to the system after timers expired.
Since you mentioned that you are debugging a multiprocessing program, I think the underlying program you have is to set the breakpoint in the correct subprocess.
Try break fork and set follow-fork-mode child/parent. What you want to achieve is have gdb attached to the process that is running the code you want to debug.
Refer to this link.
Another thought is to generate a crash, since you can compile the programe. For example add a int i = *(int*)NULL and that will generate a core dump. You can then debug the core dump with gdb <program> <core dump>. You can refer to this page for how to configure core dump.
I have web daemon and request that makes it fail with SIGSEGV. So i start daemon, attach with gdb, continuing, send request and getting this:
$ gdb attach -p 630066
(gdb) c
Continuing.
Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
(gdb)
How to make gdb print stacktrace before killing application? Application do not have subprocesses, just threads.
Thanks.
Your GDB session indicates that you have not attached all threads of the multithreaded process, and some other thread (one you didn't attach) ran into SIGSEGV and terminated the entire process.
Another (somewhat unlikely) possibility is that you are using a very old version of GDB, one which still has this bug in it (the bug was fixed in 2009).
When using gdb -p NNNN you need to be careful and specify correct process id. pgrep daemon-name or ps aux | grep daemon-name should give you a good idea which process to attach.
Just enter backtrace or bt right in the gdb shell after getting SIGSEGV.
To explore stack trace for each separate thread, start with info thread, then choose the thread you need, for example thread 3 and then type bt to see the stack trace for that thread.
If I have some multithreaded process and want to trace it with gdb using attach command, to which thread it will connect (e.g. current running or main)? I know that I can discover it with info threads but I want to know which thread it will choose by default.
For Linux, all of the threads are stopped by the ptrace command when gdb attaches.
It has been my experience that gdb defaults to the main thread for C/C++ applications. If you attach to a process and do a 'bt' it will list the stack for 'main'.
Information is available for all threads however. gdb can look at the thread(s) information in the /proc filesystem. The proc contains information about each thread in the tasks area. Details about the stack address is located in the stat file as well as the maps file. Details are also available regarding the register values for each thread.
Along the lines of your question, I've often wondered why stepping through a multithreaded application will cause gdb to jump from thread to thread. I think that gdb is still at the mercy of the kernel scheduler so that a step on a thread may lead to a different thread getting the CPU resource and a breakpoint being triggered.
On Linux, where thread ids exist in the same space as process ids, it appears you can run gdb -p tid to attach to the thread with given tid and its owning process, without knowing the pid. Because the main thread of a process has tid == pid, it makes sense that running gdb -p pid connects to the main thread.
Example code that connects gdb to the currently executing thread, e.g. for generating a pretty stack trace: https://github.com/facebook/rocksdb/pull/11150
Without getting into to to much detail, I'm working on a program that consists of several separate processes all running on embedded QNX RTOS. They don't have a parent-child relationship, they are all spawned using spawnlp(P_NOWAIT, ...) and they all communicate with each other using the IPC mechanism provided by the OS.
When I'm debugging with GDB and I hit a breakpoint in the process I'm working in, all of my threads are paused, which is great. But is there a way to also have it pause execution of my other processes? Right now what's happening is all the other processes keep on truckin' while my process is paused and so all the IPC queues get full etc. etc.
Thanks in advance,
HF
You can associate a list of GDB commands with each breakpoint. So when you hit a breakpoint in process A, you can for example send a SIGTRAP to process B, which should drop it into the debugger:
(gdb) b main
Breakpoint 1 at 0x804834a: file testA.c, line 40.
(gdb) command
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
>shell kill -s TRAP `pidof testB`
>end
(gdb)
More info at Breakpoint Command Lists
I'm a little new to GDB. I'm hoping someone can help me with something that should be quite simple, I've used Google/docs but I'm just missing something.
What is the 'normal' way folks debug threaded apps with GDB? I'm using pthreads. I'm wanting to watch only one thread - the two options I see are
a) tell the debugger somehow to attach to a particular thread, such that stepping wont result in jumping threads on each context switch
b) tell the debugger to suspend/free any 'uninteresting' threads
I'd prefer to go route b) - reading the help for GDB I dont see a command for this, tips?
See documentation for set scheduler-locking on.
Beware: if you suspend other threads, and if one of them holds a lock, and if your interesting thread needs that lock at some point while stepping, you'll deadlock.
What is the 'normal' way folks debug threaded apps
You can never debug thread correctness, you can only design it in. In my experience, most of debugging of threaded apps is putting in assertions, and examining state of the world when one of the assertions is violated.
First, you need to enable comfortable for multi-threading debugger behavior with the following commands. No idea why it's disabled by default.
set target-async 1
set non-stop on
I personally put those commands into .gdbinit file. They make your every command to be applied only to the currently focused thread. Note: the thread might be running, so you have to pause it.
To see the focused thread execute the thread.
To switch to another thread append the number of the thread, e.g. thread 2.
To see all threads with their numbers issue info thread.
To apply a command to a particular thread issue something like thread apply threadnum command. E.g. thread apply 4 bt will apply backtrace command to a thread number 4. thread apply all continue continues all paused threads.
There is a small problem though — many commands needs the thread to be paused. I know a few ways of doing that:
interrupt command: interrupts the thread execution, accepts a number of a thread to pause, without an argument breaks the focused one.
Setting a breakpoint somewhere. Note that you may set a breakpoint to a particular thread, so that other threads will ignore it, like break linenum thread threadnum. E.g. break 25 thread 4.
You may also find very useful that you can set a list of commands to be executed when a breakpoint hit through the command commands — so e.g. you may quickly print interesting values, then continue execution.