I have a program which crashes due to a segmentation fault. The core file is produced.
running the core in gdb gives me the following:
HP gdb 6.1 for HP Itanium (32 or 64 bit) and target HP-UX 11iv2 and 11iv3.
Core was generated by `gcpf1fwcApp'.
Program terminated with signal 6, Aborted.
I used the command
thread apply all bt
When I check the stack trace I get error in the main thread which is in waiting state.
However when I run the same program in GDB I get a completely different error in stack trace. Which seems more correct than the core dump.
The program has 31 threads.
Why do I get this kind of difference?
It is possible that you are simply looking at the wrong thread.
Try thread apply all where, and see if one of the threads is in fact abort()ing.
When debugging a live process, GDB will stop when a thread receives SIGABRT, and so will likely show you the relevant thread.
When debugging a core (post-mortem), GDB doesn't know which thread is relevant, and so shows them to you in whichever order the OS saved them into the core. Linux kernels save the thread which caused the process to die first, so GDB on Linux shows relevant thread from core. I am guessing that HP-UX does not do that, and so GDB shows you a "random" thread instead.
Related
I have a double-free bug. I am able to reproduce it using a debug build with Address Sanitizer (AS) detects but when I run under GDB, AS kills the GDB session.
I found this Address Sanitizer page with instructions how to keep GDB:
https://github.com/google/sanitizers/wiki/AddressSanitizerAndDebugger
but when I do:
(gdb) break __asan::ReportGenericError
at the beginning of the session, the GDB state still disappears after the problem is detected:
(gdb) bt
No stack.
the GDB state still disappears after the problem is detected
There are several possible reasons for this:
Somehow you didn't set the breakpoint correctly
It's actually a child process that is dying
Somehow the thread in which the error is detected is not attached by GDB.
To eliminate 1, use catch syscall exit_group (and possibly also catch syscall exit) -- this way GDB is sure to stop before the process disappears.
For 2, AddressSanitizer message should indicate the thread id in which the error is detected, and that id should match one of the threads in GDB info thread output.
For 3, we'd need to understand more about how that thread was created.
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
I will try to be as specific as I can, but so far I have worded this problem so poorly that Google failed to return any useful results (hence my question here).
I am attaching gdb to a multi-threaded c++ server process. All I can say is that strange things have been happening while trying to do the usual set-breakpoint-break-investigate.
First, while waiting for the breakpoint to be hit (in 'Continuing' mode), I suddenly got back the (gdb) prompt with the message:
Continuing.
[Thread 0x54d5b940 (LWP 28503) exited]
[New Thread 0x54d5b940 (LWP 28726)]
Cannot get thread event message: debugger service failed
Second, also while waiting for the breakpoint to be hit, I'm suddenly told the program has received SIGSEGV and - back to the (gdb) prompt - backtrace tells me the segfault happened in pthread_cancel(). Note the process under investigation does not normally segfault.
I clearly lack enough information about how gdb works to even begin guessing what is happening. Am I doing anything wrong? The steps I take are the same each time:
gdb attach
break 'MyFunction()'
continue
Thoughts? Thanks.
I fought with similar gdb issues for a while. My case was having lots of threads spawned that executed few functions and then exited.
It appears if a thread exits too fast and there's lots of these happening sometimes gdb cannot keep up and when it fails, it fails with style as in crashes :) I think it tries to attach to a thread that is already done as per the error message.
I see this as an issue in gdb 6.5 to 7.6 and still happening. Did not try with older versions.
My advice is look for this use case or similar. Once I changed my design to have a thread serving a queue of requests gdb works flawlessly.
Design wise is healthier to have already created threads that digest actions than always spawning new threads.
Still same code debugs without a problem on Visual Studio so I do have to say that is a small disappointment to me with regards to gdb.
I use Eclipse and looking at the GDB traces (usually enabled by default) will give you a better hint of where GDB fails. One of the buttons on the console shows you the GDB trace.