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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 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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