tsv(n) 2.6 "Tcl Threading"

NAME

tsv - Part of the Tcl threading extension allowing script level manipulation of data shared between threads.

    TABLE OF CONTENTS
    SYNOPSIS
    DESCRIPTION
    ELEMENT COMMANDS
    LIST COMMANDS
    ARRAY COMMANDS
    KEYED LIST COMMANDS
    DISCUSSION
    CREDITS
    SEE ALSO
    KEYWORDS

SYNOPSIS

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 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.

Thread shared variables are not bound to any thread explicitly. That means that when a thread which created any of thread shared variables exits, the variable and associated memory is not unset/reclaimed. User has to explicitly unset the variable to reclaim the memory consumed by the variable.

ELEMENT COMMANDS

tsv::names ?pattern?

Returns names of shared variables matching optional ?pattern? or all known variables if pattern is ommited.

tsv::object varname element

Creates object accessor command for the element in the shared variable varname. Using this command, one can apply most of the other shared variable commands as method functions of the element object command. The object command is automatically deleted when the element which this command is pointing to is unset.

% tsv::set foo bar "A shared string" % set string [tsv::object foo bar] % $string append " appended" => A shared string appended

tsv::set varname element ?value?

Sets the value of the element in the shared variable varname to value and returns the value to caller. The value may be ommited, in which case the command will return the current value of the element. If the element cannot be found, error is triggered.

tsv::get varname element ?namedvar?

Retrieves the value of the element from the shared variable varname. If the optional argument namedvar is given, the value is stored in the named variable. Return value of the command depends of the existence of the optional argument namedvar. If the argument is ommited and the requested element cannot be found in the shared array, the command triggers error. If, however, the optional argument is given on the command line, the command returns true (1) if the element is found or false (0) if the element is not found.

tsv::unset varname ?element?

Unsets the element from the shared variable varname. If the optional element is not given, it deletes the variable.

tsv::exists varname element

Checks wether the element exists in the shared variable varname and returns true (1) if it does or false (0) if it doesn't.

tsv::pop varname element

Returns value of the element in the shared variable varname and unsets the element, all in one atomic operation.

tsv::move varname oldname newname

Renames the element oldname to the newname in the shared variable varname. This effectively performs an get/unset/set sequence of operations but all in one atomic step.

tsv::incr varname element ?count?

Similar to standard Tcl incr command but increments the value of the element in shared variaboe varname instead of the Tcl variable.

tsv::append varname element value ?value ...?

Similar to standard Tcl append command but appends one or more values to the element in shared variable varname instead of the Tcl variable.

tsv::lock varname arg ?arg ...?

This command concatenates passed arguments and evaluates the resulting script under the internal mutex protection. During the script evaluation, the entire shared variable is locked. For shared variable commands within the script, internal locking is disabled so no deadlock can occur. It is also allowed to unset the shared variable from within the script. The shared variable is automatically created if it did not exists at the time of the first lock operation.

% tsv::lock foo { tsv::lappend foo bar 1 tsv::lappend foo bar 2 puts stderr [tsv::set foo bar] tsv::unset foo }

LIST COMMANDS

Those command are similar to the equivalently named Tcl command. The difference is that they operate on elements of shared arrays.

tsv::lappend varname element value ?value ...?: Similar to standard Tcl lappend command but appends one or more values to the element in shared variable varname instead of the Tcl variable.
tsv::linsert varname element index value ?value ...?: Similar to standard Tcl linsert command but inserts one or more values at the index list position in the element in the shared variable varname instead of the Tcl variable.
tsv::lreplace varname element first last ?value ...?: Similar to standard Tcl lreplace command but replaces one or more values between the first and last position in the element of the shared variable varname instead of the Tcl variable.
tsv::llength varname element: Similar to standard Tcl llength command but returns length of the element in the shared variable varname instead of the Tcl variable.
tsv::lindex varname element ?index?: Similar to standard Tcl lindex command but returns the value at the index list position of the element from the shared variable varname instead of the Tcl variable.
tsv::lrange varname element from to: Similar to standard Tcl lrange command but returns values between from and to list positions from the element in the shared variable varname instead of the Tcl variable.
tsv::lsearch varname element ?options? pattern: Similar to standard Tcl lsearch command but searches the element in the shared variable varname instead of the Tcl variable.
tsv::lset varname element index ?index ...? value: Similar to standard Tcl lset command but sets the element in the shared variable varname instead of the Tcl variable.
tsv::lpop varname element ?index?: Similar to the standard Tcl lindex command but in addition to returning, it also splices the value out of the element from the shared variable varname in one atomic operation. In contrast to the Tcl lindex command, this command returns no value to the caller.
tsv::lpush varname element ?index?: This command performes the opposite of the tsv::lpop command. As its counterpart, it returns no value to the caller.

ARRAY COMMANDS

This command supports most of the options of the standard Tcl array command. In addition to those, it allows binding a shared variable to some persisten storage databases. Currently the only persistent option supported is the famous GNU Gdbm database. This option has to be selected during the package compilation time. The implementation provides hooks for defining other persistency layers, if needed.

tsv::array set varname list: Does the same as standard Tcl array set.
tsv::array get varname ?pattern?: Does the same as standard Tcl array get.
tsv::array names varname ?pattern?: Does the same as standard Tcl array names.
tsv::array size varname: Does the same as standard Tcl array size.
tsv::array reset varname list: Does the same as standard Tcl array set but it clears the varname and sets new values from the list atomically.
tsv::array bind varname handle: Binds the varname to the persistent storage handle. The format of the handle is <handler>:<address>. For the built-in GNU Gdbm persistence layer, the format of the handle is "gdbm:<path>" where <path> is the path to the Gdbm database file.
tsv::array unbind varname: Unbinds the shared array from its bound persistent storage.
tsv::array isbound varname: Returns true (1) if the shared varname is bound to some persistent storage or zero (0) if not.

KEYED LIST COMMANDS

Keyed list commands are borrowed from the TclX package. Keyed lists provide a structured data type built upon standard Tcl lists. This is a functionality similar to structs in the C programming language.

A keyed list is a list in which each element contains a key and value pair. These element pairs are stored as lists themselves, where the key is the first element of the list, and the value is the second. The key-value pairs are referred to as fields. This is an example of a keyed list:

{{NAME {Frank Zappa}} {JOB {musician and composer}}}

Fields may contain subfields; `.' is the separator character. Subfields are actually fields where the value is another keyed list. Thus the following list has the top level fields ID and NAME, and subfields NAME.FIRST and NAME.LAST:

{ID 106} {NAME {{FIRST Frank} {LAST Zappa}}}

There is no limit to the recursive depth of subfields, allowing one to build complex data structures. Keyed lists are constructed and accessed via a number of commands. All keyed list management commands take the name of the variable containing the keyed list as an argument (i.e. passed by reference), rather than passing the list directly.

tsv::keyldel varname keylist key: Delete the field specified by key from the keyed list keylist in the shared variable varname. This removes both the key and the value from the keyed list.
tsv::keylget varname keylist key ?retvar?: Return the value associated with key from the keyed list keylist in the shared variable varname. If the optional retvar is not specified, then the value will be returned as the result of the command. In this case, if key is not found in the list, an error will result.

If retvar is specified and key is in the list, then the value is returned in the variable retvar and the command returns 1 if the key was present within the list. If key isn't in the list, the command will return 0, and retvar will be left unchanged. If {} is specified for retvar, the value is not returned, allowing the Tcl programmer to determine if a key is present in a keyed list without setting a variable as a side-effect.
tsv::keylkeys varname keylist ?key?: Return the a list of the keys in the keyed list keylist in the shared variable varname. If key is specified, then it is the name of a key field who's subfield keys are to be retrieved.
tsv::keylset varname keylist key value ?key value..?: Set the value associated with key, in the keyed list keylist to value. If the keylist does not exists, it is created. If key is not currently in the list, it will be added. If it already exists, value replaces the existing value. Multiple keywords and values may be specified, if desired.

DISCUSSION

The current implementation of thread shared variables allows for easy and convenient access to data shared between different threads. Internally, the data is stored in Tcl objects and all package commands operate on internal data representation, thus minimizing shimmering and improving performance. Special care has been taken to assure that all object data is properly locked and deep-copied when moving objects between threads.

Due to the internal design of the Tcl core, there is no provision of full integration of shared variables within the Tcl syntax, unfortunately. All access to shared data must be performed with the supplied package commands. Also, variable traces are not supported. But even so, benefits of easy, simple and safe shared data manipulation outweights imposed limitations.

CREDITS

Thread shared variables are inspired by the nsv interface found in AOLserver, a highly scalable Web server from America Online.

KEYWORDS

locking, synchronization, thread shared data, threads