interp manual page - Tcl Built-In Commands

This command makes it possible to create one or more new Tcl interpreters that co-exist with the creating interpreter in the same application. The creating interpreter is called the parent and the new interpreter is called a child. A parent can create any number of children, and each child can itself create additional children for which it is parent, resulting in a hierarchy of interpreters.

Each interpreter is independent from the others: it has its own name space for commands, procedures, and global variables. A parent interpreter may create connections between its children and itself using a mechanism called an alias. An alias is a command in a child interpreter which, when invoked, causes a command to be invoked in its parent interpreter or in another child interpreter. The only other connections between interpreters are through environment variables (the env variable), which are normally shared among all interpreters in the application, and by resource limit exceeded callbacks. Note that the name space for files (such as the names returned by the open command) is no longer shared between interpreters. Explicit commands are provided to share files and to transfer references to open files from one interpreter to another.

The interp command also provides support for safe interpreters. A safe interpreter is a child whose functions have been greatly restricted, so that it is safe to execute untrusted scripts without fear of them damaging other interpreters or the application's environment. For example, all IO channel creation commands and subprocess creation commands are made inaccessible to safe interpreters. See SAFE INTERPRETERS below for more information on what features are present in a safe interpreter. The dangerous functionality is not removed from the safe interpreter; instead, it is hidden, so that only trusted interpreters can obtain access to it. For a detailed explanation of hidden commands, see HIDDEN COMMANDS, below. The alias mechanism can be used for protected communication (analogous to a kernel call) between a child interpreter and its parent. See ALIAS INVOCATION, below, for more details on how the alias mechanism works.

A qualified interpreter name is a proper Tcl list containing a subset of its ancestors in the interpreter hierarchy, terminated by the string naming the interpreter in its immediate parent. Interpreter names are relative to the interpreter in which they are used. For example, if “a” is a child of the current interpreter and it has a child “a1”, which in turn has a child “a11”, the qualified name of “a11” in “a” is the list “a1 a11”.

The interp command, described below, accepts qualified interpreter names as arguments; the interpreter in which the command is being evaluated can always be referred to as {} (the empty list or string). Note that it is impossible to refer to a parent (ancestor) interpreter by name in a child interpreter except through aliases. Also, there is no global name by which one can refer to the first interpreter created in an application. Both restrictions are motivated by safety concerns.

THE INTERP COMMAND

The interp command is used to create, delete, and manipulate child interpreters, and to share or transfer channels between interpreters. It can have any of several forms, depending on the subcommand argument:

interp alias srcPath srcToken

Returns a Tcl list whose elements are the targetCmd and args associated with the alias represented by srcToken (this is the value returned when the alias was created; it is possible that the name of the source command in the child is different from srcToken).

interp alias srcPath srcToken {}

Deletes the alias for srcToken in the child interpreter identified by srcPath. srcToken refers to the value returned when the alias was created; if the source command has been renamed, the renamed command will be deleted.

interp alias srcPath srcCmd targetPath targetCmd ?arg arg ...?

This command creates an alias between one child and another (see the alias child command below for creating aliases between a child and its parent). In this command, either of the child interpreters may be anywhere in the hierarchy of interpreters under the interpreter invoking the command. SrcPath and srcCmd identify the source of the alias. SrcPath is a Tcl list whose elements select a particular interpreter. For example, “a b” identifies an interpreter “b”, which is a child of interpreter “a”, which is a child of the invoking interpreter. An empty list specifies the interpreter invoking the command. srcCmd gives the name of a new command, which will be created in the source interpreter. TargetPath and targetCmd specify a target interpreter and command, and the arg arguments, if any, specify additional arguments to targetCmd which are prepended to any arguments specified in the invocation of srcCmd. TargetCmd may be undefined at the time of this call, or it may already exist; it is not created by this command. The alias arranges for the given target command to be invoked in the target interpreter whenever the given source command is invoked in the source interpreter. See ALIAS INVOCATION below for more details. The command returns a token that uniquely identifies the command created srcCmd, even if the command is renamed afterwards. The token may but does not have to be equal to srcCmd.

interp aliases ?path?

This command returns a Tcl list of the tokens of all the source commands for aliases defined in the interpreter identified by path. The tokens correspond to the values returned when the aliases were created (which may not be the same as the current names of the commands).

interp bgerror path ?cmdPrefix?

This command either gets or sets the current background exception handler for the interpreter identified by path. If cmdPrefix is absent, the current background exception handler is returned, and if it is present, it is a list of words (of minimum length one) that describes what to set the interpreter's background exception handler to. See the BACKGROUND EXCEPTION HANDLING section for more details.

interp cancel ?-unwind? ?--? ?path? ?result?

Cancels the script being evaluated in the interpreter identified by path. Without the -unwind switch the evaluation stack for the interpreter is unwound until an enclosing catch command is found or there are no further invocations of the interpreter left on the call stack. With the -unwind switch the evaluation stack for the interpreter is unwound without regard to any intervening catch command until there are no further invocations of the interpreter left on the call stack. The -- switch can be used to mark the end of switches; it may be needed if path is an unusual value such as -safe. If result is present, it will be used as the error message string; otherwise, a default error message string will be used.

interp create ?-safe? ?--? ?path?

Creates a child interpreter identified by path and a new command, called a child command. The name of the child command is the last component of path. The new child interpreter and the child command are created in the interpreter identified by the path obtained by removing the last component from path. For example, if path is a b c then a new child interpreter and child command named c are created in the interpreter identified by the path a b. The child command may be used to manipulate the new interpreter as described below. If path is omitted, Tcl creates a unique name of the form interpx, where x is an integer, and uses it for the interpreter and the child command. If the -safe switch is specified (or if the parent interpreter is a safe interpreter), the new child interpreter will be created as a safe interpreter with limited functionality; otherwise the child will include the full set of Tcl built-in commands and variables. The -- switch can be used to mark the end of switches; it may be needed if path is an unusual value such as -safe. The result of the command is the name of the new interpreter. The name of a child interpreter must be unique among all the children for its parent; an error occurs if a child interpreter by the given name already exists in this parent. The initial recursion limit of the child interpreter is set to the current recursion limit of its parent interpreter.

interp debug path ?-frame ?bool??

Controls whether frame-level stack information is captured in the child interpreter identified by path. If no arguments are given, option and current setting are returned. If -frame is given, the debug setting is set to the given boolean if provided and the current setting is returned. This only affects the output of info frame, in that exact frame-level information for command invocation at the bytecode level is only captured with this setting on.

For example, with code like

proc mycontrol {... script} {
  ...
  uplevel 1 $script
  ...
}

proc dosomething {...} {
  ...
  mycontrol {
    somecode
  }
}

the standard setting will provide a relative line number for the command somecode and the relevant frame will be of type eval. With frame-debug active on the other hand the tracking extends so far that the system will be able to determine the file and absolute line number of this command, and return a frame of type source. This more exact information is paid for with slower execution of all commands.

Note that once it is on, this flag cannot be switched back off: such attempts are silently ignored. This is needed to maintain the consistency of the underlying interpreter's state.

interp delete ?path ...?

Deletes zero or more interpreters given by the optional path arguments, and for each interpreter, it also deletes its children. The command also deletes the child command for each interpreter deleted. For each path argument, if no interpreter by that name exists, the command raises an error.

interp eval path arg ?arg ...?

This command concatenates all of the arg arguments in the same fashion as the concat command, then evaluates the resulting string as a Tcl script in the child interpreter identified by path. The result of this evaluation (including all return options, such as -errorinfo and -errorcode information, if an error occurs) is returned to the invoking interpreter. Note that the script will be executed in the current context stack frame of the path interpreter; this is so that the implementations (in a parent interpreter) of aliases in a child interpreter can execute scripts in the child that find out information about the child's current state and stack frame.

interp exists path

Returns 1 if a child interpreter by the specified path exists in this parent, 0 otherwise. If path is omitted, the invoking interpreter is used.

interp expose path hiddenName ?exposedCmdName?

Makes the hidden command hiddenName exposed, eventually bringing it back under a new exposedCmdName name (this name is currently accepted only if it is a valid global name space name without any ::), in the interpreter denoted by path. If an exposed command with the targeted name already exists, this command fails. Hidden commands are explained in more detail in HIDDEN COMMANDS, below.

interp hide path exposedCmdName ?hiddenCmdName?

Makes the exposed command exposedCmdName hidden, renaming it to the hidden command hiddenCmdName, or keeping the same name if hiddenCmdName is not given, in the interpreter denoted by path. If a hidden command with the targeted name already exists, this command fails. Currently both exposedCmdName and hiddenCmdName can not contain namespace qualifiers, or an error is raised. Commands to be hidden by interp hide are looked up in the global namespace even if the current namespace is not the global one. This prevents children from fooling a parent interpreter into hiding the wrong command, by making the current namespace be different from the global one. Hidden commands are explained in more detail in HIDDEN COMMANDS, below.

interp hidden path

Returns a list of the names of all hidden commands in the interpreter identified by path.

interp invokehidden path ?-option ...? hiddenCmdName ?arg ...?

Invokes the hidden command hiddenCmdName with the arguments supplied in the interpreter denoted by path. No substitutions or evaluation are applied to the arguments. Three -options are supported, all of which start with -: -namespace (which takes a single argument afterwards, nsName), -global, and --. If the -namespace flag is present, the hidden command is invoked in the namespace called nsName in the target interpreter. If the -global flag is present, the hidden command is invoked at the global level in the target interpreter; otherwise it is invoked at the current call frame and can access local variables in that and outer call frames. The -- flag allows the hiddenCmdName argument to start with a “-” character, and is otherwise unnecessary. If both the -namespace and -global flags are present, the -namespace flag is ignored. Note that the hidden command will be executed (by default) in the current context stack frame of the path interpreter. Hidden commands are explained in more detail in HIDDEN COMMANDS, below.

interp issafe ?path?

Returns 1 if the interpreter identified by the specified path is safe, 0 otherwise.

interp limit path limitType ?-option? ?value ...?

Sets up, manipulates and queries the configuration of the resource limit limitType for the interpreter denoted by path. If no -option is specified, return the current configuration of the limit. If -option is the sole argument, return the value of that option. Otherwise, a list of -option/value argument pairs must supplied. See RESOURCE LIMITS below for a more detailed explanation of what limits and options are supported.

interp marktrusted path

Marks the interpreter identified by path as trusted. Does not expose the hidden commands. This command can only be invoked from a trusted interpreter. The command has no effect if the interpreter identified by path is already trusted.

interp recursionlimit path ?newlimit?

Returns the maximum allowable nesting depth for the interpreter specified by path. If newlimit is specified, the interpreter recursion limit will be set so that nesting of more than newlimit calls to Tcl_Eval and related procedures in that interpreter will return an error. The newlimit value is also returned. The newlimit value must be a positive integer between 1 and the maximum value of a non-long integer on the platform.

The command sets the maximum size of the Tcl call stack only. It cannot by itself prevent stack overflows on the C stack being used by the application. If your machine has a limit on the size of the C stack, you may get stack overflows before reaching the limit set by the command. If this happens, see if there is a mechanism in your system for increasing the maximum size of the C stack.

interp share srcPath channelId destPath

Causes the IO channel identified by channelId to become shared between the interpreter identified by srcPath and the interpreter identified by destPath. Both interpreters have the same permissions on the IO channel. Both interpreters must close it to close the underlying IO channel; IO channels accessible in an interpreter are automatically closed when an interpreter is destroyed.

interp slaves ?path?

Returns a Tcl list of the names of all the child interpreters associated with the interpreter identified by path. If path is omitted, the invoking interpreter is used.

interp children ?path?

Synonym for . interp slaves ?path?

interp target path alias

Returns a Tcl list describing the target interpreter for an alias. The alias is specified with an interpreter path and source command name, just as in interp alias above. The name of the target interpreter is returned as an interpreter path, relative to the invoking interpreter. If the target interpreter for the alias is the invoking interpreter then an empty list is returned. If the target interpreter for the alias is not the invoking interpreter or one of its descendants then an error is generated. The target command does not have to be defined at the time of this invocation.

interp transfer srcPath channelId destPath

Causes the IO channel identified by channelId to become available in the interpreter identified by destPath and unavailable in the interpreter identified by srcPath.

child COMMAND

For each child interpreter created with the interp command, a new Tcl command is created in the parent interpreter with the same name as the new interpreter. This command may be used to invoke various operations on the interpreter. It has the following general form:

child command ?arg arg ...?

child is the name of the interpreter, and command and the args determine the exact behavior of the command. The valid forms of this command are:

child aliases: Returns a Tcl list whose elements are the tokens of all the aliases in child. The tokens correspond to the values returned when the aliases were created (which may not be the same as the current names of the commands).
child alias srcToken: Returns a Tcl list whose elements are the targetCmd and args associated with the alias represented by srcToken (this is the value returned when the alias was created; it is possible that the actual source command in the child is different from srcToken).
child alias srcToken {}: Deletes the alias for srcToken in the child interpreter. srcToken refers to the value returned when the alias was created; if the source command has been renamed, the renamed command will be deleted.
child alias srcCmd targetCmd ?arg ..?: Creates an alias such that whenever srcCmd is invoked in child, targetCmd is invoked in the parent. The arg arguments will be passed to targetCmd as additional arguments, prepended before any arguments passed in the invocation of srcCmd. See ALIAS INVOCATION below for details. The command returns a token that uniquely identifies the command created srcCmd, even if the command is renamed afterwards. The token may but does not have to be equal to srcCmd.
child bgerror ?cmdPrefix?: This command either gets or sets the current background exception handler for the child interpreter. If cmdPrefix is absent, the current background exception handler is returned, and if it is present, it is a list of words (of minimum length one) that describes what to set the interpreter's background exception handler to. See the BACKGROUND EXCEPTION HANDLING section for more details.
child eval arg ?arg ..?: This command concatenates all of the arg arguments in the same fashion as the concat command, then evaluates the resulting string as a Tcl script in child. The result of this evaluation (including all return options, such as -errorinfo and -errorcode information, if an error occurs) is returned to the invoking interpreter. Note that the script will be executed in the current context stack frame of child; this is so that the implementations (in a parent interpreter) of aliases in a child interpreter can execute scripts in the child that find out information about the child's current state and stack frame.
child expose hiddenName ?exposedCmdName?: This command exposes the hidden command hiddenName, eventually bringing it back under a new exposedCmdName name (this name is currently accepted only if it is a valid global name space name without any ::), in child. If an exposed command with the targeted name already exists, this command fails. For more details on hidden commands, see HIDDEN COMMANDS, below.
child hide exposedCmdName ?hiddenCmdName?: This command hides the exposed command exposedCmdName, renaming it to the hidden command hiddenCmdName, or keeping the same name if the argument is not given, in the child interpreter. If a hidden command with the targeted name already exists, this command fails. Currently both exposedCmdName and hiddenCmdName can not contain namespace qualifiers, or an error is raised. Commands to be hidden are looked up in the global namespace even if the current namespace is not the global one. This prevents children from fooling a parent interpreter into hiding the wrong command, by making the current namespace be different from the global one. For more details on hidden commands, see HIDDEN COMMANDS, below.
child hidden: Returns a list of the names of all hidden commands in child.
child invokehidden ?-option ...? hiddenName ?arg ..?: This command invokes the hidden command hiddenName with the supplied arguments, in child. No substitutions or evaluations are applied to the arguments. Three -options are supported, all of which start with -: -namespace (which takes a single argument afterwards, nsName), -global, and --. If the -namespace flag is given, the hidden command is invoked in the specified namespace in the child. If the -global flag is given, the command is invoked at the global level in the child; otherwise it is invoked at the current call frame and can access local variables in that or outer call frames. The -- flag allows the hiddenCmdName argument to start with a “-” character, and is otherwise unnecessary. If both the -namespace and -global flags are given, the -namespace flag is ignored. Note that the hidden command will be executed (by default) in the current context stack frame of child. For more details on hidden commands, see HIDDEN COMMANDS, below.
child issafe: Returns 1 if the child interpreter is safe, 0 otherwise.
child limit limitType ?-option? ?value ...?: Sets up, manipulates and queries the configuration of the resource limit limitType for the child interpreter. If no -option is specified, return the current configuration of the limit. If -option is the sole argument, return the value of that option. Otherwise, a list of -option/value argument pairs must supplied. See RESOURCE LIMITS below for a more detailed explanation of what limits and options are supported.
child marktrusted: Marks the child interpreter as trusted. Can only be invoked by a trusted interpreter. This command does not expose any hidden commands in the child interpreter. The command has no effect if the child is already trusted.
child recursionlimit ?newlimit?: Returns the maximum allowable nesting depth for the child interpreter. If newlimit is specified, the recursion limit in child will be set so that nesting of more than newlimit calls to Tcl_Eval() and related procedures in child will return an error. The newlimit value is also returned. The newlimit value must be a positive integer between 1 and the maximum value of a non-long integer on the platform.
The command sets the maximum size of the Tcl call stack only. It cannot by itself prevent stack overflows on the C stack being used by the application. If your machine has a limit on the size of the C stack, you may get stack overflows before reaching the limit set by the command. If this happens, see if there is a mechanism in your system for increasing the maximum size of the C stack.

SAFE INTERPRETERS

A safe interpreter is one with restricted functionality, so that is safe to execute an arbitrary script from your worst enemy without fear of that script damaging the enclosing application or the rest of your computing environment. In order to make an interpreter safe, certain commands and variables are removed from the interpreter. For example, commands to create files on disk are removed, and the exec command is removed, since it could be used to cause damage through subprocesses. Limited access to these facilities can be provided, by creating aliases to the parent interpreter which check their arguments carefully and provide restricted access to a safe subset of facilities. For example, file creation might be allowed in a particular subdirectory and subprocess invocation might be allowed for a carefully selected and fixed set of programs.

A safe interpreter is created by specifying the -safe switch to the interp create command. Furthermore, any child created by a safe interpreter will also be safe.

A safe interpreter is created with exactly the following set of built-in commands:

after	append	apply	array
binary	break	catch	chan
clock	close	concat	continue
dict	eof	error	eval
expr	fblocked	fcopy	fileevent
flush	for	foreach	format
gets	global	if	incr
info	interp	join	lappend
lassign	lindex	linsert	list
llength	lrange	lrepeat	lreplace
lsearch	lset	lsort	namespace
package	pid	proc	puts
read	regexp	regsub	rename
return	scan	seek	set
split	string	subst	switch
tell	time	trace	unset
update	uplevel	upvar	variable
vwait	while

The following commands are hidden by interp create when it creates a safe interpreter:

cd	encoding	exec	exit
fconfigure	file	glob	load
open	pwd	socket	source
unload

These commands can be recreated later as Tcl procedures or aliases, or re-exposed by interp expose.

The following commands from Tcl's library of support procedures are not present in a safe interpreter:

auto_exec_ok	auto_import	auto_load
auto_load_index	auto_qualify	unknown

Note in particular that safe interpreters have no default unknown command, so Tcl's default autoloading facilities are not available. Autoload access to Tcl's commands that are normally autoloaded:

auto_mkindex	auto_mkindex_old
auto_reset	history
parray	pkg_mkIndex
::pkg::create	::safe::interpAddToAccessPath
::safe::interpCreate	::safe::interpConfigure
::safe::interpDelete	::safe::interpFindInAccessPath
::safe::interpInit	::safe::setLogCmd
tcl_endOfWord	tcl_findLibrary
tcl_startOfNextWord	tcl_startOfPreviousWord
tcl_wordBreakAfter	tcl_wordBreakBefore

can only be provided by explicit definition of an unknown command in the safe interpreter. This will involve exposing the source command. This is most easily accomplished by creating the safe interpreter with Tcl's Safe-Tcl mechanism. Safe-Tcl provides safe versions of source, load, and other Tcl commands needed to support autoloading of commands and the loading of packages.

In addition, the env variable is not present in a safe interpreter, so it cannot share environment variables with other interpreters. The env variable poses a security risk, because users can store sensitive information in an environment variable. For example, the PGP manual recommends storing the PGP private key protection password in the environment variable PGPPASS. Making this variable available to untrusted code executing in a safe interpreter would incur a security risk.

If extensions are loaded into a safe interpreter, they may also restrict their own functionality to eliminate unsafe commands. For a discussion of management of extensions for safety see the manual entries for Safe-Tcl and the load Tcl command.

A safe interpreter may not alter the recursion limit of any interpreter, including itself.

ALIAS INVOCATION

The alias mechanism has been carefully designed so that it can be used safely in an untrusted script which is being executed in a safe interpreter even if the target of the alias is not a safe interpreter. The most important thing in guaranteeing safety is to ensure that information passed from the child to the parent is never evaluated or substituted in the parent; if this were to occur, it would enable an evil script in the child to invoke arbitrary functions in the parent, which would compromise security.

When the source for an alias is invoked in the child interpreter, the usual Tcl substitutions are performed when parsing that command. These substitutions are carried out in the source interpreter just as they would be for any other command invoked in that interpreter. The command procedure for the source command takes its arguments and merges them with the targetCmd and args for the alias to create a new array of arguments. If the words of srcCmd were “srcCmd arg1 arg2 ... argN”, the new set of words will be “targetCmd arg arg ... arg arg1 arg2 ... argN”, where targetCmd and args are the values supplied when the alias was created. TargetCmd is then used to locate a command procedure in the target interpreter, and that command procedure is invoked with the new set of arguments. An error occurs if there is no command named targetCmd in the target interpreter. No additional substitutions are performed on the words: the target command procedure is invoked directly, without going through the normal Tcl evaluation mechanism. Substitutions are thus performed on each word exactly once: targetCmd and args were substituted when parsing the command that created the alias, and arg1 - argN are substituted when the alias's source command is parsed in the source interpreter.

When writing the targetCmds for aliases in safe interpreters, it is very important that the arguments to that command never be evaluated or substituted, since this would provide an escape mechanism whereby the child interpreter could execute arbitrary code in the parent. This in turn would compromise the security of the system.

HIDDEN COMMANDS

Safe interpreters greatly restrict the functionality available to Tcl programs executing within them. Allowing the untrusted Tcl program to have direct access to this functionality is unsafe, because it can be used for a variety of attacks on the environment. However, there are times when there is a legitimate need to use the dangerous functionality in the context of the safe interpreter. For example, sometimes a program must be sourced into the interpreter. Another example is Tk, where windows are bound to the hierarchy of windows for a specific interpreter; some potentially dangerous functions, e.g. window management, must be performed on these windows within the interpreter context.

The interp command provides a solution to this problem in the form of hidden commands. Instead of removing the dangerous commands entirely from a safe interpreter, these commands are hidden so they become unavailable to Tcl scripts executing in the interpreter. However, such hidden commands can be invoked by any trusted ancestor of the safe interpreter, in the context of the safe interpreter, using interp invoke. Hidden commands and exposed commands reside in separate name spaces. It is possible to define a hidden command and an exposed command by the same name within one interpreter.

Hidden commands in a child interpreter can be invoked in the body of procedures called in the parent during alias invocation. For example, an alias for source could be created in a child interpreter. When it is invoked in the child interpreter, a procedure is called in the parent interpreter to check that the operation is allowable (e.g. it asks to source a file that the child interpreter is allowed to access). The procedure then it invokes the hidden source command in the child interpreter to actually source in the contents of the file. Note that two commands named source exist in the child interpreter: the alias, and the hidden command.

Because a parent interpreter may invoke a hidden command as part of handling an alias invocation, great care must be taken to avoid evaluating any arguments passed in through the alias invocation. Otherwise, malicious child interpreters could cause a trusted parent interpreter to execute dangerous commands on their behalf. See the section on ALIAS INVOCATION for a more complete discussion of this topic. To help avoid this problem, no substitutions or evaluations are applied to arguments of interp invokehidden.

Safe interpreters are not allowed to invoke hidden commands in themselves or in their descendants. This prevents them from gaining access to hidden functionality in themselves or their descendants.

The set of hidden commands in an interpreter can be manipulated by a trusted interpreter using interp expose and interp hide. The interp expose command moves a hidden command to the set of exposed commands in the interpreter identified by path, potentially renaming the command in the process. If an exposed command by the targeted name already exists, the operation fails. Similarly, interp hide moves an exposed command to the set of hidden commands in that interpreter. Safe interpreters are not allowed to move commands between the set of hidden and exposed commands, in either themselves or their descendants.

Currently, the names of hidden commands cannot contain namespace qualifiers, and you must first rename a command in a namespace to the global namespace before you can hide it. Commands to be hidden by interp hide are looked up in the global namespace even if the current namespace is not the global one. This prevents children from fooling a parent interpreter into hiding the wrong command, by making the current namespace be different from the global one.

RESOURCE LIMITS

Every interpreter has two kinds of resource limits that may be imposed by any parent interpreter upon its children. Command limits (of type command) restrict the total number of Tcl commands that may be executed by an interpreter (as can be inspected via the info cmdcount command), and time limits (of type time) place a limit by which execution within the interpreter must complete. Note that time limits are expressed as absolute times (as in clock seconds) and not relative times (as in after) because they may be modified after creation.

When a limit is exceeded for an interpreter, first any handler callbacks defined by parent interpreters are called. If those callbacks increase or remove the limit, execution within the (previously) limited interpreter continues. If the limit is still in force, an error is generated at that point and normal processing of errors within the interpreter (by the catch command) is disabled, so the error propagates outwards (building a stack-trace as it goes) to the point where the limited interpreter was invoked (e.g. by interp eval) where it becomes the responsibility of the calling code to catch and handle.

LIMIT OPTIONS

Every limit has a number of options associated with it, some of which are common across all kinds of limits, and others of which are particular to the kind of limit.

-command: This option (common for all limit types) specifies (if non-empty) a Tcl script to be executed in the global namespace of the interpreter reading and writing the option when the particular limit in the limited interpreter is exceeded. The callback may modify the limit on the interpreter if it wishes the limited interpreter to continue executing. If the callback generates an exception, it is reported through the background exception mechanism (see BACKGROUND EXCEPTION HANDLING). Note that the callbacks defined by one interpreter are completely isolated from the callbacks defined by another, and that the order in which those callbacks are called is undefined.
-granularity: This option (common for all limit types) specifies how frequently (out of the points when the Tcl interpreter is in a consistent state where limit checking is possible) that the limit is actually checked. This allows the tuning of how frequently a limit is checked, and hence how often the limit-checking overhead (which may be substantial in the case of time limits) is incurred.
-milliseconds: This option specifies the number of milliseconds after the moment defined in the -seconds option that the time limit will fire. It should only ever be specified in conjunction with the -seconds option (whether it was set previously or is being set this invocation.)
-seconds: This option specifies the number of seconds after the epoch (see clock seconds) that the time limit for the interpreter will be triggered. The limit will be triggered at the start of the second unless specified at a sub-second level using the -milliseconds option. This option may be the empty string, which indicates that a time limit is not set for the interpreter.
-value: This option specifies the number of commands that the interpreter may execute before triggering the command limit. This option may be the empty string, which indicates that a command limit is not set for the interpreter.

Where an interpreter with a resource limit set on it creates a child interpreter, that child interpreter will have resource limits imposed on it that are at least as restrictive as the limits on the creating parent interpreter. If the parent interpreter of the limited parent wishes to relax these conditions, it should hide the interp command in the child and then use aliases and the interp invokehidden subcommand to provide such access as it chooses to the interp command to the limited parent as necessary.

BACKGROUND EXCEPTION HANDLING

When an exception happens in a situation where it cannot be reported directly up the stack (e.g. when processing events in an update or vwait call) the exception is instead reported through the background exception handling mechanism. Every interpreter has a background exception handler registered; the default exception handler arranges for the bgerror command in the interpreter's global namespace to be called, but other exception handlers may be installed and process background exceptions in substantially different ways.

A background exception handler consists of a non-empty list of words to which will be appended two further words at invocation time. The first word will be the interpreter result at time of the exception, typically an error message, and the second will be the dictionary of return options at the time of the exception. These are the same values that catch can capture when it controls script evaluation in a non-background situation. The resulting list will then be executed in the interpreter's global namespace without further substitutions being performed.

CREDITS

The safe interpreter mechanism is based on the Safe-Tcl prototype implemented by Nathaniel Borenstein and Marshall Rose.

EXAMPLES

Creating and using an alias for a command in the current interpreter:

interp alias {} getIndex {} lsearch {alpha beta gamma delta}
set idx [getIndex delta]

Executing an arbitrary command in a safe interpreter where every invocation of lappend is logged:

set i [interp create -safe]
interp hide $i lappend
interp alias $i lappend {} loggedLappend $i
proc loggedLappend {i args} {
    puts "logged invocation of lappend $args"
    interp invokehidden $i lappend {*}$args
}
interp eval $i $someUntrustedScript

Setting a resource limit on an interpreter so that an infinite loop terminates.

set i [interp create]
interp limit $i command -value 1000
interp eval $i {
    set x 0
    while {1} {
        puts "Counting up... [incr x]"
    }
}

KEYWORDS

alias, parent interpreter, safe interpreter, child interpreter