10.5 library(threadutil): Interactive thread utilities

This library provides utilities that are primarily intended for interactive usage in a threaded Prolog environment. It allows for inspecting threads, manage I/O of background threads (depending on the environment) and manipulating the debug status of threads.

threads

List currently known threads with their status. For each thread it lists the id, class, status, debug status, CPU time and current stack usage. If a thread is listed with Debug set to‘X’, it cannot be debugged. If the Debug is show as‘V’, it is running in debug mode (see debug/0) and responds to spy points and break points.

join_threads

Join all terminated threads. For normal applications, dealing with terminated threads must be part of the application logic, either detaching the thread before termination or making sure it will be joined. The predicate join_threads/0 is intended for interactive sessions to reclaim resources from threads that died unexpectedly during development.

[det]with_stopped_threads(:Goal, Options)

Stop all threads except the caller while running once(Goal). Note that this is in the thread user utilities as this is not something that should be used by normal applications. Notably, this may deadlock if the current thread requires input from some other thread to complete Goal or one of the stopped threads has a lock. Options:

stop_nodebug_threads(+Boolean)

If true (default false), also stop threads created with the debug(false) option.

except(+List)

Do not stop threads from this list.

bug

Note that the threads are stopped when they process signals. As signal handling may be delayed, this implies they need not be stopped before Goal starts.

[semidet]thread_has_console

True when the calling thread has an attached console.

See also

attach_console/0

[det]attach_console

[det]attach_console(+Title)

Create a new console and make the standard Prolog streams point to it. If not provided, the title is built using the thread id. Does nothing if the current thread already has a console attached.

[det]interactor

[det]interactor(+Title)

Run a Prolog toplevel in another thread with a new console window. If Title is given, this will be used as the window title. As of SWI-Prolog version 10, the console is provided by the XPCE graphics library using library(epilog).

[det]tspy(:Spec)

[det]tspy(:Spec, +ThreadOrClass)

Trap the graphical debugger on reaching Spec. The predicate tspy/0 enabled debug mode in all threads using tdebug/0 while tspy/1 enables debug mode using tdebug/1.

[det]tdebug

[det]tdebug(+ThreadOrClass)

[det]tnodebug

[det]tnodebug(+ThreadOrClass)

Enable or disable a thread or group of threads for debugging using the graphical tracer. A group of threads is addressed based on the class property of a thread set by thread_create/3 or set_thread/2. This implies loading the graphical tracer and switching the thread to debug mode using debug/0. New threads created inherit their debug mode from the thread that created them.

Thread classes have been introduced in SWI-Prolog 10.0.2/10.1.5. This allows for more selective debugging as well as ensuring debugging works in newly created threads. For example, the HTTP server creates all its worker threads in the class http. Using query below, we reliable make sure spy points are trapped in HTTP handler threads, regardless of whether the worker existed or is lazily created and regardless of whether the user switched to nodebug mode while tracing a previous event (see debug_reset_from_class/0).

?- tdebug(http).

[det]tbacktrace(+Thread)

[det]tbacktrace(+Thread, +Options)

Print a backtrace for Thread to the stream user_error of the calling thread. This is achieved by inserting an interrupt into Thread using call_in_thread/2. Options:

depth(+MaxFrames)

Number of stack frames to show. Default is the current Prolog flag backtrace_depth or 20.

Other options are passed to get_prolog_backtrace/3.

bug

call_in_thread/2 may not process the event.

[det]tprofile(+Thread)

Profile the operation of Thread until the user hits a key.

[det]thread_alias(+Alias)

Set the alias for a thread.

deprecated

Use set_thread/2 using alias(Alias).