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interp(n)		     Tcl Built-In Commands		     interp(n)

______________________________________________________________________________

NAME
       interp - Create and manipulate Tcl interpreters

SYNOPSIS
       interp subcommand ?arg arg ...?
_________________________________________________________________

DESCRIPTION
       This  command  makes  it	 possible to create one or more new Tcl inter‐
       preters that co-exist with the creating interpreter in the same	appli‐
       cation.	 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 inter‐
       preter may create connections between its slaves	 and  itself  using  a
       mechanism  called  an  alias.   An alias is a command in a slave inter‐
       preter which, when invoked, causes a command to be invoked in its  mas‐
       ter  interpreter	 or in another slave interpreter.  The only other con‐
       nections 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  pro‐
       vided  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  dam‐
       aging 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	inter‐
       preter.	 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  pro‐
       tected  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 eval‐
       uated  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 identi‐
	      fied 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 inter‐
	      preter and command, and the arg arguments, if any, specify addi‐
	      tional  arguments	 to targetCmd which are prepended to any argu‐
	      ments 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	 error │
	      handler  for the interpreter identified by path. If cmdPrefix is │
	      absent, the current background error 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	 error │
	      to. See the BACKGROUND ERROR HANDLING section for more details.

       interp create ?-safe? ?--? ?path?
	      Creates  a  slave	 interpreter identified by path and a new com‐
	      mand, 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 vari‐
	      ables. 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 effects the out‐
	      put 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 exe‐
	      cution of all commands.

       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 implementa‐
	      tions (in a master interpreter) of aliases  in  a	 slave	inter‐
	      preter  can  execute scripts in the slave that find out informa‐
	      tion 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 com‐
	      mand 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 hid‐
	      ing  the	wrong command, by making the current namespace be dif‐
	      ferent 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 inter‐
	      preter identified by path.

       interp invokehidden path ?-option ...? hiddenCmdName ?arg ...?
	      Invokes the hidden command hiddenCmdName with the arguments sup‐
	      plied  in	 the  interpreter denoted by path. No substitutions or
	      evaluation are applied to the arguments. Three -options are sup‐
	      ported,  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 unnec‐
	      essary.  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 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 issafe ?path?
	      Returns 1 if the interpreter identified by the specified path is
	      safe, 0 otherwise.

       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 inter‐
	      preter 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, rela‐
	      tive 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 avail‐
	      able 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	deter‐
       mine  the  exact behavior of the command.  The valid forms of this com‐
       mand 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	srcTo‐
	      ken).

       slave alias srcToken {}
	      Deletes the alias for srcToken in the slave interpreter.	srcTo‐
	      ken 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 com‐
	      mand 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 error │
	      handler for the slave interpreter. If cmdPrefix is  absent,  the │
	      current  background  error  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 error to. See │
	      the BACKGROUND ERROR 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 cur‐
	      rent context stack frame of slave; this is so that the implemen‐
	      tations  (in  a master interpreter) of aliases in a slave inter‐
	      preter can execute scripts in the slave that find	 out  informa‐
	      tion 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  tar‐
	      geted 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 com‐
	      mand fails.  Currently both exposedCmdName and hiddenCmdName can
	      not  contain  namespace qualifiers, or an error is raised.  Com‐
	      mands 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 sup‐
	      plied 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 hiddenCmd‐
	      Name  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 inter‐
	      preter.  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 subpro‐
       cess 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 inter‐
       preter 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	  con‐
       cat	continue	      dict	  eof	      error	  eval
       expr	   fblocked    fcopy	   fileevent
       flush	   for	       foreach	   format
       gets	   global      if	   incr
       info	   interp      join	   lappend	      lassign	  lin‐
       dex	linsert	    list  llength     lrange	  lrepeat     lreplace
       lsearch	   lset	       lsort	   namespace			 pack‐
       age     pid	   proc	       puts	  read	      regexp	  reg‐
       sub	rename		       return	   scan	       seek	   set
       split	   string      subst	   switch
       tell	   time	       trace	   unset
       update	   uplevel     upvar	   variable vwait	while The fol‐
       lowing commands are hidden by interp create  when  it  creates  a  safe
       interpreter:	 cd	     encoding	 exec	     exit     fconfig‐
       ure  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_qual‐
       ify    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::interpCre‐
       ate ::safe::interpConfigure   ::safe::interpDelete ::safe::interpFindI‐
       nAccessPath	::safe::interpInit   ::safe::setLogCmd	    tcl_endOf‐
       Word	   tcl_findLibrary   tcl_startOfNextWord  tcl_startOfPrevious‐
       Word  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 mech‐
       anism.	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 envi‐
       ronment 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	inter‐
       preter, 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  com‐
       mand  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 functional‐
       ity 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 manage‐
       ment, must be performed on these windows within	the  interpreter  con‐
       text.

       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  com‐
       mand 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 inter‐
       preter 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 evalu‐
       ating 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 them‐
       selves 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 com‐
       mands, in either themselves or their descendants.

       Currently, the names of hidden commands cannot contain namespace quali‐
       fiers, 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 inter‐
       preter 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 com‐ │
       mand) 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 abso‐ │
       lute  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	 call‐ │
       backs  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 gen‐ │
       erated at that point and normal processing of errors within the	inter‐ │
       preter (by the catch command) is disabled, so the error propagates out‐ │
       wards (building a stack-trace as it goes) to the point where  the  lim‐ │
       ited 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 particu‐ │
       lar 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 particu‐ │
	      lar  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 │
	      error, it is reported through  the  background  error  mechanism │
	      (see BACKGROUND ERROR 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  fre‐ │
	      quently (out of the points when the Tcl interpreter is in a con‐ │
	      sistent 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	 over‐ │
	      head  (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  sec‐ │
	      ond  unless specified at a sub-second level using the -millisec‐ │
	      onds option. This option may be the empty	 string,  which	 indi‐ │
	      cates that a time limit is not set for the interpreter.

       -value This  option  specifies  the  number of commands that the inter‐ │
	      preter 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  lim‐ │
       ited master as necessary.					       │

BACKGROUND ERROR HANDLING						       │
       When  an	 error	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 error is instead reported through the background error │
       handling mechanism.  Every interpreter has a background	error  handler │
       registered;  the default error handler arranges for the bgerror command │
       in the interpreter's global namespace to be  called,  but  other	 error │
       handlers	 may  be  installed  and process background errors in substan‐ │
       tially different ways.						       │

       A background error 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 error message string, and the second will a dictionary │
       of  return  options  (this  is also the sort of information that can be │
       obtained by trapping a normal error using catch of course.) The result‐ │
       ing  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 invo‐
       cation 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(n), load(n), safe(n), Tcl_CreateSlave(3)

KEYWORDS
       alias, master interpreter, safe interpreter, slave interpreter

Tcl				      7.6			     interp(n)
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