27.6.5.8 Multithreaded Debugging

In GDB’s all-stop mode, whenever your program stops, all execution threads stop. Likewise, whenever you restart the program, all threads start executing. See All-Stop Mode in The GNU debugger. For some multi-threaded targets, GDB supports a further mode of operation, called non-stop mode, in which you can examine stopped program threads in the debugger while other threads continue to execute freely. See Non-Stop
Mode in The GNU debugger. Versions of GDB prior to 7.0 do not support non-stop mode, and it does not work on all targets.

The variable gdb-non-stop-setting determines whether Emacs runs GDB in all-stop mode or non-stop mode. The default is t, which means it tries to use non-stop mode if that is available. If you change the value to nil, or if non-stop mode is unavailable, Emacs runs GDB in all-stop mode. The variable takes effect when Emacs begins a debugging session; if you change its value, you should restart any active debugging session.

When a thread stops in non-stop mode, Emacs usually switches to that thread. If you don’t want Emacs to do this switch if another stopped thread is already selected, change the variable gdb-switch-when-another-stopped to nil.

Emacs can decide whether or not to switch to the stopped thread depending on the reason which caused the stop. Customize the variable gdb-switch-reasons to select the stop reasons which will cause a thread switch.

The variable gdb-stopped-functions allows you to execute your functions whenever some thread stops.

In non-stop mode, you can switch between different modes for GUD execution control commands.

Non-stop/A

When gdb-gud-control-all-threads is t (the default value), interruption and continuation commands apply to all threads, so you can halt or continue all your threads with one command using gud-stop-subjob and gud-cont, respectively. The ‘ Go’ button is shown on the tool bar when at least one thread is stopped, whereas ‘ Stop’ button is shown when at least one thread is running.

Non-stop/T

When gdb-gud-control-all-threads is nil, only the current thread is stopped/continued. ‘ Go’ and ‘ Stop’ buttons on the GUD tool bar are shown depending on the state of current thread.

You can change the current value of gdb-gud-control-all-threads from the tool bar or from ‘ GUD->GDB-MI’ menu.

Stepping commands always apply to the current thread.

In non-stop mode, you can interrupt/continue your threads without selecting them. Hitting i in threads buffer interrupts thread under point, c continues it, s steps through. More such commands may be added in the future.

Note that when you interrupt a thread, it stops with the ‘ signal received’ reason. If that reason is included in your gdb-switch-reasons (it is by default), Emacs will switch to that thread.