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Date: | Sat, 20 Jun 2015 16:02:38 -0400 |
Message-ID: | <CAH5rLZXbYNUwB_f2Vigev--NsmB3FrdhWre1uYxAr0ckN8NvmQ@mail.gmail.com> |
Subject: | Re: gdb arbitrarily starting threads |
From: | "William M. (Mike) Miller" <william DOT m DOT miller AT gmail DOT com> |
To: | cygwin AT cygwin DOT com |
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On Sat, Jun 20, 2015 at 2:47 PM, Corinna Vinschen <corinna-cygwin AT cygwin DOT 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 DOT m DOT miller AT gmail DOT com -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
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