All-Stop Mode (Debugging with GDB)

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#### 5.5.1 All-Stop Mode

In all-stop mode, whenever your program stops under GDB for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.

Conversely, whenever you restart the program, all threads start executing. This is true even when single-stepping with commands like `step` or `next`.

In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target’s operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.

You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.

Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message such as ‘`[Switching to Thread n]`’ to identify the thread.

On some OSes, you can modify GDB’s default behavior by locking the OS scheduler to allow only a single thread to run.

`set scheduler-locking mode`

  Set the scheduler locking mode. It applies to normal execution, record mode, and replay mode. mode can be one of the following:

  `off`

    There is no locking and any thread may run at any time.

  `on`

    Only the current thread may run when the inferior is resumed. New threads created by the resumed thread are held stopped at their entry point, before they execute any instruction.

  `step`

    Behaves like `on` when stepping, and `off` otherwise. Threads other than the current never get a chance to run when you step, and they are completely free to run when you use commands like ‘`continue`’, ‘`until`’, or ‘`finish`’.

    This mode optimizes for single-stepping; it prevents other threads from preempting the current thread while you are stepping, so that the focus of debugging does not change unexpectedly. However, unless another thread hits a breakpoint during its timeslice, GDB does not change the current thread away from the thread that you are debugging.

  `replay`

    Behaves like `on` in replay mode, and `off` in either record mode or during normal execution. This is the default mode.

`show scheduler-locking`

  Display the current scheduler locking mode.

By default, when you issue one of the execution commands such as `continue`, `next` or `step`, GDB allows only threads of the current inferior to run. For example, if GDB is attached to two inferiors, each with two threads, the `continue` command resumes only the two threads of the current inferior. This is useful, for example, when you debug a program that forks and you want to hold the parent stopped (so that, for instance, it doesn’t run to exit), while you debug the child. In other situations, you may not be interested in inspecting the current state of any of the processes GDB is attached to, and you may want to resume them all until some breakpoint is hit. In the latter case, you can instruct GDB to allow all threads of all the inferiors to run with the `set schedule-multiple` command.

`set schedule-multiple`

  Set the mode for allowing threads of multiple processes to be resumed when an execution command is issued. When `on`, all threads of all processes are allowed to run. When `off`, only the threads of the current process are resumed. The default is `off`. The `scheduler-locking` mode takes precedence when set to `on`, or while you are stepping and set to `step`.

`show schedule-multiple`

  Display the current mode for resuming the execution of threads of multiple processes.

Next: Non-Stop Mode, Up: Thread Stops [Contents][Index]