thread -
A Tcl extension implementing script level access to Tcl threading capabilities.
package require Thread ?2.4?
thread::create ?-joinable? ?script?
thread::preserve id
thread::release ?-wait? id
thread::id
thread::errorproc ?procname?
thread::unwind
thread::exit
thread::names
thread::exists
thread::send id ?-async? script ?varname?
thread::wait
thread::join id
thread::configure id ?option? ?value? ?option value?...
thread::transfer id channel
thread::cond options
thread::mutex options
tsv::names ?pattern?
tsv::object array element
tsv::set array element value
tsv::get array element ?varname?
tsv::unset array ?element?
tsv::exists array ?element?
tsv::pop array element
tsv::move array old new
tsv::incr array element ?increment?
tsv::append array element value ?value ...?
tsv::lappend array element value ?value ...?
tsv::linsert array element index value ?value ...?
tsv::lreplace array element first last ?value ...?
tsv::llength array element
tsv::lindex array element index
tsv::lrange array element first last
tsv::lsearch array element ?mode? pattern
tsv::lpop array element ?index?
tsv::lpush array element value ?index?
The thread extension creates threads that contain Tcl interpreters,
and it lets you send scripts to those threads for evaluation. It also enables you to share data
between threads in a safe, protected fashion. Additionaly, it provides script-level access
to basic thread synchronization primitives, like mutexes and condition variables.
This section describes commands for creating and destroying threads
and sending scripts to threads for evaluation.
-
thread::create ?-joinable? ?script?
- This command creates a thread that contains a Tcl interpreter.
The Tcl interpreter either evaluates the script, if
specified, or it waits in the event loop for scripts that arrive via
the thread::send command. The result of thread::create is
the ID of the thread. The result, if any, of script is
ignored. Using flag -joinable it is possible to create a
joinable thread, i.e. one upon whose exit can be waited upon (by using
thread::join command).
- thread::preserve id
- This procedure increments the thread reference counter. Each call
to this command increments the reference counter by one (1). Command returns the value
of the reference counter after the increment.
With reference counting, one can implement controlled access to a shared Tcl thread.
By incrementing the reference counter, the caller signalizes that he/she wishes to use
the thread for a longer period of time. By decrementing the counter, using the
thread::release command, caller signalizes that he/she has finished using
the thread.
- thread::release ?-wait? id
- This procedure decrements the thread reference counter. Each call to this
command decrements the reference counter by one (1). Command returns
the value of the reference counter after the decrement. When the reference counter
reaches zero (0), the target thread is terminated.
Optional flag -wait instructs the caller thread to wait for the target
thread to exit, if the effect of the command would result in termination of the
target thread, i.e. if the return result is zero (0). Without the flag, the caller
thread does not wait for the target thread to exit. Care must be taken when using
the -wait, since this may block the caller thread indefinitely.
-
thread::id
- This command returns the ID of the current thread.
-
thread::errorproc
- This command sets a handler for errors that occur in scripts sent
asynchronously, using the -async flag of the thread::send
command, to other threads. If no handler is specified, the current
handler is returned. The empty string resets the handler to default
(unspecified) value.
An uncaught error in a thread causes an error message to be sent
to the standard error channel. This default reporting scheme can be
changed by registering a procedure which is called to report the error.
The proc is called in the interpreter that invoked the
thread::errorproc command. The proc is called like this:
myerrorproc thread_id errorInfo
- thread::unwind
- This stops a prior thread::wait. Execution of the script will continue
from the thread::wait command. If thread::wait was the last command
in a thread, the thread will exit. The command usually returns empty result but
may trigger Tcl error with the message "target thread died" in some situations.
- thread::exit
- This forces a thread stuck in the thread::wait to unconditionaly
exit. This command is guaranteed to leave the program memory in the
unconsistent state, produce memory leaks and otherwise affect other
subsytem(s) of the Tcl application in an unpredictable manner.
Use of this command is deprecated in favour of the thread::unwind command
or more advanced thread reservation system implemented with thread::preserve
and thread::release commands. Use on your own risk and with extreme precaution.
The command usually returns empty result but may trigger Tcl error with the message
"target thread died" in some situations.
- thread::names
- This command returns a list of thread IDs. These are only for
threads that have been created via thread::create. If your
application creates other threads at the C level, they are not
reported by the thread::names command.
-
thread::exists
- Returns true (1) if thread given by the ID parameter exists, false (0)
otherwise. This applies only for threads that have been created via
thread::create command.
- thread::send
- This command passes a script to another thread and, optionally, waits
for the result. If the -async flag is specified, the command does
not wait for the result and it returns empty string. The target thread must
enter it's event loop in order to receive scripts sent via this command.
This is done by default for threads created without a startup script. Threads
can enter the event loop explicitly by calling thread::wait or
vwait.
Optional varname specifies name of the variable to store
the result of the script. Without the -async flag, the command
returns the evaluation code, similarily to the standard Tcl catch
command.
If, however, the -async flag is specified, the command returns
immediately and caller can later vwait on varname to get the
result of the passed script.
- thread::wait
- This enters the event loop so a thread can receive messages from thread::send.
This is equivalent to vwait unusedvariable
except that thread::unwind will
not unblock a vwait.
-
thread::join
- This command waits for the thread with ID id to exit and
then returns it's exit code. Errors will be returned for threads which
are not joinable or already waited upon by another thread.
-
thread::configure
- This command configures various low-level aspects of the thread with ID
id in the similar way as the standard Tcl command fconfigure
configures some Tcl channel options.
Options currently supported are: -eventmark and -unwindonerror.
The -eventmark option, when set, limits the number of asynchronously
posted scripts to the thread event loop. The thread::send -async
command will block until the number of pending scripts in the event
loop does not drop below the value configured with -eventmark.
Default value for the -eventmark is 0 (zero) which effectively
disables the checking, i.e. allows for unlimited number of posted scripts.
The boolean -unwindonerror option, when set, causes the target thread
to unwind if the result of the script processing resulted in error.
Default value for the -unwindonerror is 0 (false), i.e. thread continues
to process scripts after one of the posted scripts fails.
-
thread::transfer
- This moves the specified channel from the
current thread and interpreter to the main interpreter of the thread
with the given id. After the move the current interpreter has no
access to the channel anymore, but the main interpreter of the target
thread will be able to use it from now on.
The command waits until the other thread has incorporated the
channel. Because of this it is possible to deadlock the participating
threads by commanding the other through a synchronous
thread::send to transfer a channel to us. This easily extends
into longer loops of threads waiting for each other.
Other restrictions: The channel in question must not be shared among
multiple interpreters running in the sending thread. This
automatically excludes the special channels for standard input, output
and error.
This section describes commands used for script-level access to
most common and basic thread synchronization primitives: mutexes and
condition variables.
- thread::mutex options
- This command provides script-level access to mutexes. Mutexes are most
common thread synchronization primitives. They are used to synchronize
access from two or more threads to one or more shared resources. Care has to
be taken when using mutexes in an multithreading program. Improper use of
mutexes may lead to various deadlock situations.
The thread::mutex command supports following subcommands and options:
- thread::mutex create
- Creates the mutex and returns it's opaque handle. This handle
should be used for any future reference to the newly created mutex.
-
thread::mutex destroy mutex
- Destroys the mutex mutex. Extreme care has to be taken that
nobody is using the mutex, otherwise unexpected errors may happen.
-
thread::mutex lock mutex
- Locks the mutex mutex. Locking the mutex may deadlock the program
if same thread attempts to lock the same mutex twice without unlocking it
in between.
-
thread::mutex unlock mutex
- Unlocks the mutex mutex so some other thread may lock it again.
- thread::cond options
- This command provides script-level access to condition variables. A condition
variable creates a safe environment for the program to test some condition, sleep
on it when false and be awakened when it might have become true. A condition variable
is always used in the conjuction with a mutex.
The thread::cond supports following subcommands and options:
- thread::cond create
- Creates the condition variable and returns it's opaque handle. This handle
should be used for any future reference to newly created condition variable.
-
thread::cond destroy cond
- Destroys condition variable cond. Extreme care has to be taken that nobody
is using (i.e. waiting on) the condition variable, otherwise unexpected
errors may happen.
-
thread::cond notify cond
- Wakes up all threads waiting on the condition variable cond.
-
thread::cond wait cond mutex ?ms?
- This command is used to suspend program execution until the condition
variable cond has been signalled (see thread::cond notify) or the
optional timer has expired. The mutex must be locked by the calling
thread on entrance to thread::cond wait. While waiting on the cond,
the command releases mutex. Before returning to the calling
thread, the command re-acquires mutex again. Unlocking the mutex
and waiting on the condition variable cond is done atomically.
The ms command option, if given, must be an integer specifying time
interval in milliseconds the command waits to be signalled. Otherwise
the command waits forever.
In multithreading programs, there are many situations where a thread has
to wait for some event to happen until it is allowed to proceed.
This is usually accomplished by repeatedly testing a condition under the
mutex protection and waiting on the condition variable until the condition
evaluates to true:
set mutex [thread::mutex create]
set cond [thread::cond create]
thread::mutex lock
while {<some_condition_is_true>} {
thread::cond wait $cond $mutex
}
thread::mutex unlock
Repeated testing of the condition is needed since the condition variable
may get signalled without the condition being actually changed (spurious
thread wake-ups, for example).
This section describes commands implementing thread shared variables. A thread
shared variable is very similar to a Tcl array but in contrast to a Tcl array
it is created in thread-shared memory and can be accessed from many threads at
the same time. Important feature of thread shared variable is that each access
to the variable is internaly protected by a mutex so script programmer does not have
to take care about locking the variable himself.
- tsv::names
- Returns names of shared variables matching optional pattern or all known
variables if pattern is ommited.
- tsv::object
- Creates object accessor command for the element in the given shared
array. Using this command, one can apply most of the other shared variable
commands as method functions of the element object command:
% tsv::set foo bar "A shared string"
% set string [tsv::object foo bar]
% $string append " appended"
=> A shared string appended
- tsv::set
- Sets the value of the element in the shared array to
value and returns the value. The value may be ommited, and
the command will return the current value of the element. If the
element cannot be found, error is triggered.
- tsv::get
- Retrieves a value of the element located in the shared array.
The command triggers error if the element is not found. If the optional
varname is given, the value is stored in the named variable. In this case,
the command returns true (1) if element is found or false (0) if the
element is not found.
- tsv::unset
- Deletes the element in the shared array. If the element
is not given, it deletes the whole array.
- tsv::exists
- Checks wether the element exists in the shared array.
If the element is not given it tests the existence of the
array itself. Returns true (1) if the item exists,
false (0) if not.
- tsv::pop
- Returns value of the element in the shared array variable
and unsets the element in one atomic operation.
- tsv::move
- Renames the element old to new in the shared array.
This effectively performs an get/unset/set sequence of operations
but in one atomic step.
- tsv::incr
- Similar to standard Tcl incr but increments the value of the
element in shared array instead of the Tcl variable.
- tsv::append
- Similar to standard Tcl append but appends one or more values
to the element in the shared array instead of the Tcl variable.
- tsv::lappend
- Similar to standard Tcl lappend but appends one or more values
to the list element in the shared array instead of the Tcl variable.
- tsv::linsert
- Similar to standard Tcl linsert but inserts one or more values at the
index list position in the list element in the shared array
instead of the Tcl variable.
- tsv::lreplace
- Similar to standard Tcl lreplace but replaces one or more values
from the list element in the shared array instead of the Tcl variable.
- tsv::llength
- Similar to standard Tcl llength but returns length of the list
element in the shared array instead of the Tcl variable.
- tsv::lindex
- Similar to standard Tcl lindex but returns value at the index
list position from the list element in the shared array
instead of the Tcl variable.
- tsv::lrange
- Similar to standard Tcl lrange but returns values between first
and last list position from the list element in the shared array
instead of the Tcl variable.
- tsv::lsearch
- Similar to standard Tcl lsearch but searches the list element
in the shared array instead of the Tcl variable.
- tsv::lpop
- Splices out the value at the index list position from the list element
in the shared array. If index is not specified, it defaults to zero.
- tsv::lpush
- Inserts the value at the index list position in the list element
in the shared array. If index is not specified, it defaults to zero.
- tsv::array
- This command supports most of the options of the standard Tcl array
command like:
- tsv::array set
- Does the same as standard Tcl array set
- tsv::array get
- Does the same as standard Tcl array get
- tsv::array names
- Does the same as standard Tcl array names
- tsv::array size
- Does the same as standard Tcl array size
- tsv::array reset
- Does the same as standard Tcl array set but
it clears the array and sets new values atomically.
The fundamental threading model in Tcl is that there can be one or
more Tcl interpreters per thread, but each Tcl interpreter should only
be used by a single thread which created it.
A "shared memory" abstraction is awkward to provide in Tcl because Tcl
makes assumptions about variable and data ownership. Therefore this extension
supports a simple form of threading where the main thread can manage several
background, or "worker" threads. For example, an event-driven server can pass
requests to worker threads, and then await responses from worker threads or
new client requests. Everything goes through the common Tcl event loop, so
message passing between threads works naturally with event-driven I/O,
vwait on variables, and so forth. For the transfer of bulk information
it is possible to move channels between the threads.
In addition, this extension provides simple but effective way of
threads to access thread-shared data without the need of explicit locking.
This is implemented with shared variable arrays. Shared variable array
is a data structure similar to Tcl array but internaly protected with mutex.
Several threads can access array elements without synchronizing themselves.
Script programmer can create any number of such arrays and access them
or their elements from any thread, without explicit locking.
For advanced multithreading scripts, script-level access to two basic
synchronization primitives, mutex and condition variables, is also supported.
Shared variable arrays are inspired by the nsv interface found in
AOLserver 3.+ highly scalable Web server from America Online.
Guide to the Tcl threading model
threads, events, message passing, synchronization, shared variables, mutex, condition variable