Re: [PATCH] Improve attach on Windows

Pedro Alves
2026-04-02T12:22:38Z

Hi!

I'm looking at the windows non-stop series again, trying to figure out where we were.

There was one problem exposed by the AdaCore testsuite that I wasn't able to reproduce locally, so I don't know what to do about it.

More below.

I'll look at the rest of the series, rebase, and see what can be merged already.

On 2025-06-11 23:06, Pedro Alves wrote:

> On 2025-06-05 18:57, Tom Tromey wrote:
>>>>>>> "Pedro" == Pedro Alves <pedro@palves.net> writes:
>>
>> Pedro> I've pushed the series to the users/palves/windows-non-stop-v2 branch
>> Pedro> on sourceware.org, for your convenience.
>>
>> I merged this into the local tree and ran the internal AdaCore test
>> suite on Windows 2016.
>
> Thank you very much for doing this. Much appreciated.
>
>>
>> There were three "failures".
>>
>> One of them is actually an improvement, where the test works around a
>> current issue.
>>
>
> One step forward, two steps back, I guess.
>
> Tackling this incrementally. I've looked at the first one:
>
>> One of them seems to be a problem with the test. This test "attach"es
>> to a running process without a "file", and the test doesn't seem to
>> match the new output:
>>
>> (gdb) attach 1228
>> Attaching to process 1228
>> [Switching to Thread 1228.0x580]
>> 0x00007ffe3c756714 in ntdll!ZwDelayExecution ()
>> from C:\Windows\SYSTEM32\ntdll.dll
>>
>
> I guess your test wasn't expecting that the current frame is printed?
>
> See the patch below, on top of current master. You are now seeing the current
> frame being printed, unlike what I claim in the commit log of the patch below,
> because with target-non-stop on, the Windows backend is a target_wait_no_wait==false
> target, like Linux.
>
> But even with the non-stop series, with "maint set target-non-stop off",
> (or with Windows older than Windows 10,) then target_wait_no_wait==true, and
> GDB doesn't print the frame.
>
> IMO, this is just a preexisting bug that the non-stop series happens to hide.
>
> Let me know what you think of the patch.
>
> And yes, the fact that run control communicates with normal_stop via
> globals is ugly and fragile to say the least.
>
> From 0cb83571c91ba42b3de519b034509092a526dc65 Mon Sep 17 00:00:00 2001
> From: Pedro Alves <pedro@palves.net>
> Date: Wed, 11 Jun 2025 22:05:23 +0100
> Subject: [PATCH] Improve attach on Windows

I think this one is a clear and easy improvement.
I've retested it, and I see the same progression in gdb.base/attach.exp, so I've now pushed it.

Thanks,
Pedro Alves

>
> Unlike most targets, on Windows, when you attach, GDB doesn't print
> the current stack frame. Vis:
>
> On GNU/Linux:
>
> attach 3340347
> Attaching to program: /home/pedro/gdb/build/gdb/testsuite/outputs/gdb.base/attach/attach, process 3340347
> Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...
> Reading symbols from /usr/lib/debug/.build-id/d5/197096f709801829b118af1b7cf6631efa2dcd.debug...
> Reading symbols from /lib64/ld-linux-x86-64.so.2...
> Reading symbols from /usr/lib/debug/.build-id/9c/b53985768bb99f138f48655f7b8bf7e420d13d.debug...
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
> 0x00005b3bf29be174 in main () at /home/pedro/gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/attach.c:19
> 19 while (! should_exit)
> (gdb) PASS: gdb.base/attach.exp: do_attach_tests: attach1, after setting file
>
> On Cygwin:
>
> (gdb) attach 6692
> Attaching to program: /home/alves/gdb/build-cygwin-testsuite/outputs/gdb.base/attach/attach, process 6692
> [New Thread 6692.0x2e60]
> [New Thread 6692.0x2e9c]
> [New Thread 6692.0xd6c]
> [New Thread 6692.0x137c]
> [New Thread 6692.0x1270]
> (gdb) FAIL: gdb.base/attach.exp: do_attach_tests: attach1, after setting file
>
> On Linux, GDB prints the frame because after the target_attach, GDB
> goes back to the event loop, to wait for an initial stop event. The
> stop event arrives, and we process it, which sets the stop_print_frame
> global, and then we get to normal_stop, which prints the frame iff
> stop_print_frame is set, which it is.
>
> Windows OTOH, is a target_attach_no_wait target, so after
> target_attach, there is no going back to event loop. In
> infcmd.c:attach_command, we go straight to attach_post_wait which
> takes us to normal_stop. But this time, nothing set
> stop_print_frame to true, so no frame is printed. Actually, if the
> global happened to be true due to an earlier event from debugging a
> previous inferior, then we will print the frame.
>
> This patch makes GDB's behavior consistent, by making sure the globals
> normal_stop looks at are in a good state in the target_attach_no_wait
> path.
>
> With that alone, GDB now prints the frame:
>
> (gdb) attach 2915
> Attaching to program: /usr/bin/sleep.exe, process 2832
> [New Thread 2832.0x2a68]
> [New Thread 2832.0xb1c]
> [New Thread 2832.0x8ac]
> [Switching to Thread 2832.0x8ac]
> 0x00007ffec51d4a71 in ntdll!DbgBreakPoint () from C:/Windows/SYSTEM32/ntdll.dll
>
> This is still not ideal, IMHO, as the current thread is the thread
> that Windows injects to attach:
>
> (gdb) info threads
> Id Target Id Frame
> 1 Thread 2832.0x2100 "sleep" 0x00007ffec51d18d7 in ntdll!ZwWaitForMultipleObjects () from C:/Windows/SYSTEM32/ntdll.dll
> 2 Thread 2832.0x2a68 "sig" 0x00007ffec51d0e47 in ntdll!ZwReadFile () from C:/Windows/SYSTEM32/ntdll.dll
> 3 Thread 2832.0xb1c 0x00007ffec51d49d7 in ntdll!ZwWaitForWorkViaWorkerFactory () from C:/Windows/SYSTEM32/ntdll.dll
> * 4 Thread 2832.0x8ac 0x00007ffec51d4a71 in ntdll!DbgBreakPoint () from C:/Windows/SYSTEM32/ntdll.dll
>
> Automatically switching to main thread is IMHO more useful. That
> results in very similar output than what we see on Linux:
>
> attach 5164
> Attaching to program: /home/alves/gdb/build-cygwin-testsuite/outputs/gdb.base/attach/attach, process 5164
> [New Thread 5164.0x87c]
> [New Thread 5164.0x28f0]
> [New Thread 5164.0x376c]
> [New Thread 5164.0x2db4]
> [New Thread 5164.0xce4]
> main () at /home/alves/gdb/src/gdb/testsuite/gdb.base/attach.c:19
> 19 while (! should_exit)
> (gdb)
>
> If we do this, then we can simplify gdb.base/attach.exp a bit by
> removing a couple Cygwin special cases.
>
> The patch does all that, which results in the following
> gdb.base/attach.exp progressions:
>
> -FAIL: gdb.base/attach.exp: do_attach_tests: attach1, after setting file
> -FAIL: gdb.base/attach.exp: do_attach_tests: attach2, with no file
> -FAIL: gdb.base/attach.exp: do_attach_tests: load file manually, after attach2 (re-read) (got interactive prompt)
> -FAIL: gdb.base/attach.exp: do_attach_tests: attach when process' a.out not in cwd
> -FAIL: gdb.base/attach.exp: do_attach_failure_tests: first attach
> +PASS: gdb.base/attach.exp: do_attach_tests: attach1, after setting file
> +PASS: gdb.base/attach.exp: do_attach_tests: attach2, with no file
> +PASS: gdb.base/attach.exp: do_attach_tests: attach when process' a.out not in cwd
> +PASS: gdb.base/attach.exp: do_attach_failure_tests: first attach
>
> Change-Id: I359bdb25660c9a4d5d873e8771cfd1cd2a54c97b
> ---
> gdb/infcmd.c | 5 ++++-
> gdb/infrun.c | 13 +++++++++++++
> gdb/infrun.h | 3 +++
> gdb/testsuite/gdb.base/attach.exp | 26 ++++++--------------------
> gdb/windows-nat.c | 7 +++++++
> 5 files changed, 33 insertions(+), 21 deletions(-)
>
> diff --git a/gdb/infcmd.c b/gdb/infcmd.c
> index e9b58ce5521..cbf2373193c 100644
> --- a/gdb/infcmd.c
> +++ b/gdb/infcmd.c
> @@ -2722,7 +2722,10 @@ attach_command (const char *args, int from_tty)
> return;
> }
> else
> - attach_post_wait (from_tty, mode);
> + {
> + set_normal_stop_state_just_attached ();
> + attach_post_wait (from_tty, mode);
> + }
>
> disable_commit_resumed.reset_and_commit ();
> }
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 2e02642c52a..41ab3342ee3 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -407,6 +407,19 @@ static process_stratum_target *target_last_proc_target;
> static ptid_t target_last_wait_ptid;
> static struct target_waitstatus target_last_waitstatus;
>
> +/* See infrun.h. */
> +
> +void
> +set_normal_stop_state_just_attached ()
> +{
> + stop_print_frame = true;
> + stopped_by_random_signal = 0;
> +
> + target_waitstatus status;
> + status.set_ignore ();
> + set_last_target_status (nullptr, minus_one_ptid, status);
> +}
> +
> void init_thread_stepping_state (struct thread_info *tss);
>
> static const char follow_fork_mode_child[] = "child";
> diff --git a/gdb/infrun.h b/gdb/infrun.h
> index b9b64aca45a..b707314fba6 100644
> --- a/gdb/infrun.h
> +++ b/gdb/infrun.h
> @@ -418,5 +418,8 @@ struct scoped_enable_commit_resumed
> bool m_prev_enable_commit_resumed;
> };
>
> +/* Set up state for normal_stop after we just attached, on
> + target_attach_no_wait targets. */
> +extern void set_normal_stop_state_just_attached ();
>
> #endif /* GDB_INFRUN_H */
> diff --git a/gdb/testsuite/gdb.base/attach.exp b/gdb/testsuite/gdb.base/attach.exp
> index 0d1550a0541..484fa776ae4 100644
> --- a/gdb/testsuite/gdb.base/attach.exp
> +++ b/gdb/testsuite/gdb.base/attach.exp
> @@ -159,16 +159,9 @@ proc_with_prefix do_attach_failure_tests {} {
>
> # Verify that we can't double attach to the process.
>
> - set test "first attach"
> - gdb_test_multiple "attach $testpid" "$test" {
> - -re "Attaching to program.*`?$escapedbinfile'?, process $testpid.*main.*at .*$srcfile:.*$gdb_prompt $" {
> - pass "$test"
> - }
> - -re "Attaching to program.*`?$escapedbinfile\.exe'?, process $testpid.*\[Switching to thread $testpid\..*\].*$gdb_prompt $" {
> - # Response expected on Cygwin.
> - pass "$test"
> - }
> - }
> + gdb_test "attach $testpid" \
> + "Attaching to program.*`?${escapedbinfile}(\.exe)?'?, process $testpid.*main.*at .*$srcfile:.*" \
> + "first attach"
>
> gdb_test "add-inferior" "Added inferior 2.*" "add empty inferior 2"
> gdb_test "inferior 2" "Switching to inferior 2.*" "switch to inferior 2"
> @@ -252,16 +245,9 @@ proc_with_prefix do_attach_tests {} {
> }
> }
>
> - set test "attach1, after setting file"
> - gdb_test_multiple "attach $testpid" "$test" {
> - -re "Attaching to program.*`?$escapedbinfile'?, process $testpid.*main.*at .*$srcfile:.*$gdb_prompt $" {
> - pass "$test"
> - }
> - -re "Attaching to program.*`?$escapedbinfile\.exe'?, process $testpid.*\[Switching to thread $testpid\..*\].*$gdb_prompt $" {
> - # Response expected on Cygwin
> - pass "$test"
> - }
> - }
> + gdb_test "attach $testpid" \
> + "Attaching to program.*`?${escapedbinfile}(\.exe)?'?, process $testpid.*main.*at .*$srcfile:.*" \
> + "attach1, after setting file"
>
> # Verify that we can "see" the variable "should_exit" in the
> # program, and that it is zero.
> diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
> index 939c5813aa7..0477731d40c 100644
> --- a/gdb/windows-nat.c
> +++ b/gdb/windows-nat.c
> @@ -1980,6 +1980,13 @@ windows_nat_target::attach (const char *args, int from_tty)
> #endif
>
> do_initial_windows_stuff (pid, 1);
> +
> + /* The thread that reports the initial breakpoint, and thus ends up
> + as the selected thread when we get here, was injected into the
> + inferior by DebugActiveProcess. Switch to the main thread, which
> + is normally more useful to the user than the injected thread. */
> + switch_to_thread (first_thread_of_inferior (current_inferior ()));
> +
> target_terminal::ours ();
> }
>
>
> base-commit: eb6c9310ee4d6cbde509d251fafb54ae45f5a5bf