Re: gdb arbitrarily starting threads

["William M. (Mike) Miller" <william.m.miller@gmail.com>]

On Sat, Jun 20, 2015 at 2:47 PM, Corinna Vinschen
<corinna-cygwin@cygwin.com> wrote:
> On Jun 20 09:29, William M. (Mike) Miller wrote:
>> I have a single-threaded executable that has a number of debug-print
>> routines that can be called interactively in the debugger to display
>> data structures to stderr.  Something has changed in gdb/gcc/Cygwin
>> during the last year or so (I don't update my installation very often)
>> so that when I call one of these functions, _sometimes_ gdb will start
>> a new thread and put my current thread into "running" status, so that
>> I can't step, etc. (although the thread isn't actually running).
>> Here's the scenario:
>>
>> 1) I say "gdb xxx.exe", set a breakpoint, and run.  When I hit the
>> breakpoint, "info threads" shows me two threads, one for my executable
>> and one with some _cygtls/cygthread/ntdll frames; both are stopped.
>>
>> 2) I say "up" a couple of times, then "p db_foo(p)" (where db_foo is
>> one of these debug-print routines that displays to stderr the data
>> structure pointed to by "p").  Sometimes, but not always, gdb will
>> respond with
>>
>>     [New Thread 1436.0x1f00]
>
> You don't have this under control.  Nor does Cygwin, most of the time.
>
> E.g, a debugged Windows executable has always at least two threads, one
> is your process thread, one is the thread created by the OS when the
> debugger attaches to a process.
>
> Every Cygwin process has at least two threads, your process thread, as
> well as a so-called "signal thread", the thread handling POSIX signals,
> created at every process startup by the Cygwin DLL.
>
> Certain Windows system calls create threads in your process to perform
> tasks "under the hood".
>
> Cygwin is doing the same, for instance to implement interruptible system
> calls where the underlying OS call is non-interuptible.  select(2) may
> start multiple threads at once.  Etc.
>
> There's no way around that.  Windows process handling is centered around
> cheap thread creation and termination.  Have a look into the "Details"
> view in Windows Task Manager.  Activate the "Threads" column.  Note that
> almost all processes are running with more than one thread.  Just don't
> worry about it, it's kind of system inherent.

Thanks for the reply.  My concern, however, isn't that new threads are
being created; it's that when gdb does this, it sets the state of all
threads, including my main thread, to "(running)" so that I can't
single-step through my program any more.  If I try to say "next", I
get the error:

    Cannot execute this command while the selected thread is running.

But I did nothing overt to start my thread.

It never used to do this, but it happens regularly now.

-- 
William M. (Mike) Miller | Edison Design Group
william.m.miller@gmail.com

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