interp

interp(3tcl)                 Tcl Built-In Commands                interp(3tcl)



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NAME
       interp - Create and manipulate Tcl interpreters

SYNOPSIS
       interp subcommand ?arg arg ...?
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DESCRIPTION
       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 master and the new
       interpreter is called a slave.  A master can create any number of
       slaves, and each slave can itself create additional slaves for which it
       is master, 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 master
       interpreter may create connections between its slaves and itself using
       a mechanism called an alias.  An alias is a command in a slave
       interpreter which, when invoked, causes a command to be invoked in its
       master interpreter or in another slave 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 slave 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 slave interpreter and its master.  See ALIAS INVOCATION,
       below, for more details on how the alias mechanism works.

       A qualified interpreter name is a proper Tcl lists containing a subset
       of its ancestors in the interpreter hierarchy, terminated by the string
       naming the interpreter in its immediate master. Interpreter names are
       relative to the interpreter in which they are used. For example, if “a”
       is a slave of the current interpreter and it has a slave “a1”, which in
       turn has a slave “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 master (ancestor) interpreter
       by name in a slave 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 slave
       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 slave is different from
              srcToken).

       interp alias srcPath srcToken {}
              Deletes the alias for srcToken in the slave 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 slave and another (see
              the alias slave command below for creating aliases between a
              slave and its master).  In this command, either of the slave
              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 slave of interpreter
              “a”, which is a slave 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 slave interpreter identified by path and a new
              command, called a slave command. The name of the slave command
              is the last component of path. The new slave interpreter and the
              slave 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 slave interpreter and slave
              command named c are created in the interpreter identified by the
              path a b.  The slave 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 slave command. If the -safe
              switch is specified (or if the master interpreter is a safe
              interpreter), the new slave interpreter will be created as a
              safe interpreter with limited functionality; otherwise the slave
              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 slave interpreter must be unique
              among all the slaves for its master;  an error occurs if a slave
              interpreter by the given name already exists in this master.
              The initial recursion limit of the slave 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 slave 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 slaves.
              The command also deletes the slave 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 slave 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 master interpreter) of aliases in a slave
              interpreter can execute scripts in the slave that find out
              information about the slave's current state and stack frame.

       interp exists path
              Returns 1 if a slave interpreter by the specified path exists in
              this master, 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 slaves from fooling a master 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 slave interpreters
              associated with the interpreter identified by path. If path is
              omitted, the invoking interpreter is used.

       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.

SLAVE COMMAND
       For each slave interpreter created with the interp command, a new Tcl
       command is created in the master 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:

              slave command ?arg arg ...?

       Slave 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:

       slave aliases
              Returns a Tcl list whose elements are the tokens of all the
              aliases in slave.  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).

       slave 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 slave is different from
              srcToken).

       slave alias srcToken {}
              Deletes the alias for srcToken in the slave 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.

       slave alias srcCmd targetCmd ?arg ..?
              Creates an alias such that whenever srcCmd is invoked in slave,
              targetCmd is invoked in the master.  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.

       slave bgerror ?cmdPrefix?
              This command either gets or sets the current background
              exception handler for the slave 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.

       slave 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 slave.  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 slave; this is so that the
              implementations (in a master interpreter) of aliases in a slave
              interpreter can execute scripts in the slave that find out
              information about the slave's current state and stack frame.

       slave 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 slave.  If an exposed command with the
              targeted name already exists, this command fails.  For more
              details on hidden commands, see HIDDEN COMMANDS, below.

       slave 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 slave 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
              slaves from fooling a master 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.

       slave hidden
              Returns a list of the names of all hidden commands in slave.

       slave invokehidden ?-option ...? hiddenName ?arg ..?
              This command invokes the hidden command hiddenName with the
              supplied arguments, in slave. 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 slave.  If the -global flag is given, the command is
              invoked at the global level in the slave; 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 slave.  For more details on hidden commands, see
              HIDDEN COMMANDS, below.

       slave issafe
              Returns  1 if the slave interpreter is safe, 0 otherwise.

       slave limit limitType ?-option? ?value ...?
              Sets up, manipulates and queries the configuration of the
              resource limit limitType for the slave 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.

       slave marktrusted
              Marks the slave interpreter as trusted. Can only be invoked by a
              trusted interpreter. This command does not expose any hidden
              commands in the slave interpreter. The command has no effect if
              the slave is already trusted.

       slave recursionlimit ?newlimit?
              Returns the maximum allowable nesting depth for the slave
              interpreter.  If newlimit is specified, the recursion limit in
              slave will be set so that nesting of more than newlimit calls to
              Tcl_Eval() and related procedures in slave 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 master 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 slave 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 when an untrusted script is executing in a safe slave and the
       target of the alias is a trusted master.  The most important thing in
       guaranteeing safety is to ensure that information passed from the slave
       to the master is never evaluated or substituted in the master;  if this
       were to occur, it would enable an evil script in the slave to invoke
       arbitrary functions in the master, which would compromise security.

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

       Because a master 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 slave interpreters could cause a trusted master
       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 safe slaves 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 slaves from fooling a
       master 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 master interpreter upon its slaves. 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 master 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 slave
       interpreter, that slave interpreter will have resource limits imposed
       on it that are at least as restrictive as the limits on the creating
       master interpreter. If the master interpreter of the limited master
       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 master 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]"
                  }
              }

SEE ALSO
       bgerror(3tcl), load(3tcl), safe(3tcl), Tcl_CreateSlave(3tcl),
       Tcl_Eval(3tcl), Tcl_BackgroundException(3tcl)

KEYWORDS
       alias, master interpreter, safe interpreter, slave interpreter



Tcl                                   8.6                         interp(3tcl)