- Reference manual
- Multithreaded applications
- Creating and destroying Prolog threads
- Monitoring threads
- Thread communication
- Thread synchronisation
- Thread support library(threadutil)
- Multithreaded mixed C and Prolog applications
- Multithreading and the XPCE graphics system
- Multithreaded applications
- Reference manual
- thread_create(:Goal, -Id)
- Shorthand for
thread_create(Goal, Id, ). See thread_create/3.
- thread_create(:Goal, -Id, +Options)
- Create a new Prolog thread (and underlying operating system thread) and
start it by executing Goal. If the thread is created
successfully, the thread identifier of the created thread is unified to Id.
Id is the alias name if the option
alias(name)is given. Otherwise it is a blob of type
thread. The anonymous blobs are subject to atom garbage collection. If a thread handle is garbage collected and the thread is not detached, it is joined if it has already terminated (see thread_join/2) and detached otherwise (see thread_detach/1).173Up to version 7.3.23, anonymous thread handles were integers. Using integers did not allow for safe checking of the thread's status as the thread may have died and the handle may have been reused and did not allow for garbage collection to take care of forgotten threads. The thread identifier blobs are printed as
), where I is the internal thread identifier and Ptr is the unique address of the identifier. The I is accepted as input argument for all thread APIs that accept a thread identifier for convenient interaction from the toplevel. See also thread_property/2.
Options is a list of options. The currently defined options are below. Stack size options can also take the value
infinite, which is mapped to the maximum stack size supported by the platform.
- Specify that the thread should only run on the specified CPUs (cores).
CpuSet is a list of integers between 0 (zero) and the known
number of CPUs (see cpu_count).
If CpuSet is empty a
domain_erroris raised. Referring to CPUs equal to or higher than the known number of CPUs returns an
This option is currently implemented for systems that provide pthread_attr_setaffinity_np(). The option is silently ignored on other systems.bugThere is currently no way to discover whether this option is supported.
- Associate an‘alias name' with the thread. This name may be used to refer to the thread and remains valid until the thread is joined (see thread_join/2). If the OS supports it (e.g., Linux), the operating system thread is named as well.
- Register AtExit as using thread_at_exit/1 before entering the thread goal. Unlike calling thread_at_exit/1 as part of the normal Goal, this ensures the AtExit is called. Using thread_at_exit/1, the thread may be signalled or run out of resources before thread_at_exit/1 is reached. See thread_at_exit/1 for details.
- Enable/disable debugging the new thread. If
true), the new thread is created with the property
debug(false)and debugging is disabled before the new thread is started. The thread debugging predicates such as tspy/1 and tdebug/0 do not signal threads with the debug property set to
false.174Currently, the flag is only used as a hint for the various debugging primitives, i.e., the system does not really enforce that the target thread stays in nodebug mode.
false(default), the thread can be waited for using thread_join/2. thread_join/2 must be called on this thread to reclaim all resources associated with the thread. If
true, the system will reclaim all associated resources automatically after the thread finishes. Please note that thread identifiers are freed for reuse after a detached thread finishes or a normal thread has been joined. See also thread_join/2 and thread_detach/1.
If a detached thread dies due to failure or exception of the initial goal, the thread prints a message using print_message/2. If such termination is considered normal, the code must be wrapped using ignore/1 and/or catch/3 to ensure successful completion.
- Inherit defaults from the given ThreadId instead of the
calling thread. This option was added to ensure that the
__thread_pool_manager(see thread_create_in_pool/4), which is created lazily, has a predictable state. The following properties are inherited:
- Enforces a maximum to the number of terms in the input queue. See
max_size(o)ption for details.
- Set the size limit for the Prolog stacks. See the Prolog flag
default is inherited from the calling thread or the thread specified
- Set the limit to which the C stack of this thread may grow. The default, minimum and maximum values are system-dependent.
The Goal argument is copied to the new Prolog engine. This implies that further instantiation of this term in either thread does not have consequences for the other thread: Prolog threads do not share data from their stacks.
- Get the Prolog thread identifier of the running thread. If the thread has an alias, the alias name is returned.
- Calls thread_join/2
and succeeds if thread Id terminated with success. Otherwise
error(is raised, where Status is the status as returned by thread_join/2.
thread_error(Id, Status), _)
- thread_join(+Id, -Status)
- Wait for the termination of the thread with the given Id.
Then unify the result status of the thread with Status. After
this call, Id becomes invalid and all resources associated
with the thread are reclaimed. Note that threads with the attribute
detached(true)cannot be joined. See also thread_property/2.
A thread that has been completed without thread_join/2 being called on it is partly reclaimed: the Prolog stacks are released and the C thread is destroyed. A small data structure representing the exit status of the thread is retained until thread_join/2 is called on the thread. Defined values for Status are:
- Set the alias name of the calling thread to Alias. An error is raised if the calling thread already has an alias or Alias is in use for a thread or message queue.
- Switch thread into detached state (see
detached(Bool)option at thread_create/3) at runtime. Id is the identifier of the thread placed in detached state. This may be the result of thread_self/1.
One of the possible applications is to simplify debugging. Threads that are created as detached leave no traces if they crash. For non-detached threads the status can be inspected using thread_property/2. Threads nobody is waiting for may be created normally and detach themselves just before completion. This way they leave no traces on normal completion and their reason for failure can be inspected.
- Terminates the thread immediately, leaving
exited(Term)as result state for thread_join/2. If the thread has the attribute
detached(true)it terminates, but its exit status cannot be retrieved using thread_join/2, making the value of Term irrelevant. The Prolog stacks and C thread are reclaimed.
The current implementation does not guarantee proper releasing of all mutexes and proper cleanup in setup_call_cleanup/3, etc. Please use the exception mechanism (throw/1) to abort execution using non-standard control.bugThe Windows port does not properly cleanup for detached threads while the cleanup for other threads is executed by the thread running thread_join/2 using the exited thread as engine. This is due to a bug in the MinGW pthread implementation.
- Run Goal when thread is started. This predicate is similar to initialization/1, but is intended for initialization operations of the runtime stacks, such as setting global variables as described in section 4.33. Goal is run on four occasions: at the call to this predicate, after loading a saved state, on starting a new thread and on creating a Prolog engine through the C interface. On loading a saved state, Goal is executed after running the initialization/1 hooks.
- Run Goal just before releasing the thread resources. This is
to be compared to at_halt/1,
but only for the current thread. These hooks are run regardless of why
the execution of the thread has been completed. When these hooks are
run, the return code is already available through
using the result of thread_self/1
as thread identifier. Note that there are two scenarios for using exit
hooks. Using thread_at_exit/1
is typically used if the thread creates a side-effect that must be
reverted if the thread dies. Another scenario is where the creator of
the thread wants to be informed when the thread ends. That cannot be
guaranteed by means of thread_at_exit/1
because it is possible that the thread cannot be created or dies almost
instantly due to a signal or resource error. The
at_exit(Goal)option of thread_create/3 is designed to deal with this scenario.
The Goal is executed with signal processing disabled. This avoids that e.g.,
thread_signal(Thread, abort)kills the exit handler rather than the thread in the case the body of Thread has just finished when the signal arrives.
- thread_setconcurrency(-Old, +New)
- Determine the concurrency of the process, which is defined as the maximum number of concurrently active threads.‘Active' here means they are using CPU time. This option is provided if the thread implementation provides pthread_setconcurrency(). Solaris is a typical example of this family. On other systems this predicate unifies Old to 0 (zero) and succeeds silently.
- thread_affinity(+ThreadID, -Current, +New)
- True when Current is unified with the current thread affinity
and the thread affinity is successfully set to New. The thread
affinity specifies the set of CPUs on which this thread is allowed
to run. The affinity is represented as a list of non-negative integers.
See also the option
This predicate is only present if this functionality can be supported and has been ported to the target operating system. Currently, only Linux support is provided.