1interp(n)                    Tcl Built-In Commands                   interp(n)
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NAME

8       interp - Create and manipulate Tcl interpreters
9

SYNOPSIS

11       interp subcommand ?arg arg ...?
12______________________________________________________________________________
13

DESCRIPTION

15       This  command  makes  it  possible to create one or more new Tcl inter‐
16       preters that co-exist with the creating interpreter in the same  appli‐
17       cation.   The  creating  interpreter  is  called the master and the new
18       interpreter is called a slave.  A  master  can  create  any  number  of
19       slaves, and each slave can itself create additional slaves for which it
20       is master, resulting in a hierarchy of interpreters.
21
22       Each interpreter is independent from the others: it has  its  own  name
23       space  for commands, procedures, and global variables.  A master inter‐
24       preter may create connections between its slaves  and  itself  using  a
25       mechanism  called  an  alias.   An alias is a command in a slave inter‐
26       preter which, when invoked, causes a command to be invoked in its  mas‐
27       ter  interpreter  or in another slave interpreter.  The only other con‐
28       nections between interpreters are through  environment  variables  (the
29       env  variable), which are normally shared among all interpreters in the
30       application, and by resource limit exceeded callbacks.  Note  that  the
31       name  space  for files (such as the names returned by the open command)
32       is no longer shared between interpreters. Explicit  commands  are  pro‐
33       vided  to share files and to transfer references to open files from one
34       interpreter to another.
35
36       The interp command also provides support for safe interpreters.  A safe
37       interpreter is a slave whose functions have been greatly restricted, so
38       that it is safe to execute untrusted scripts without fear of them  dam‐
39       aging other interpreters or the application's environment. For example,
40       all IO channel creation commands and subprocess creation  commands  are
41       made  inaccessible  to  safe interpreters.  See SAFE INTERPRETERS below
42       for more information on what features are  present  in  a  safe  inter‐
43       preter.   The  dangerous  functionality  is  not  removed from the safe
44       interpreter; instead, it is hidden, so that only  trusted  interpreters
45       can obtain access to it. For a detailed explanation of hidden commands,
46       see HIDDEN COMMANDS, below.  The alias mechanism can be used  for  pro‐
47       tected  communication  (analogous  to  a  kernel  call) between a slave
48       interpreter and its master.  See  ALIAS  INVOCATION,  below,  for  more
49       details on how the alias mechanism works.
50
51       A  qualified interpreter name is a proper Tcl lists containing a subset
52       of its ancestors in the interpreter hierarchy, terminated by the string
53       naming  the  interpreter in its immediate master. Interpreter names are
54       relative to the interpreter in which they are used. For example, if “a
55       is a slave of the current interpreter and it has a slave “a1”, which in
56       turn has a slave “a11”, the qualified name of “a11” in “a” is the  list
57a1 a11”.
58
59       The  interp  command,  described  below,  accepts qualified interpreter
60       names as arguments; the interpreter in which the command is being eval‐
61       uated  can always be referred to as {} (the empty list or string). Note
62       that it is impossible to refer to a master  (ancestor)  interpreter  by
63       name  in  a slave interpreter except through aliases. Also, there is no
64       global name by which one can refer to the first interpreter created  in
65       an application.  Both restrictions are motivated by safety concerns.
66

THE INTERP COMMAND

68       The  interp  command  is  used  to create, delete, and manipulate slave
69       interpreters, and to share or transfer channels  between  interpreters.
70       It can have any of several forms, depending on the subcommand argument:
71
72       interp alias srcPath srcToken
73              Returns  a  Tcl  list  whose elements are the targetCmd and args
74              associated with the alias represented by srcToken (this  is  the
75              value  returned  when the alias was created; it is possible that
76              the name of the source command in the slave  is  different  from
77              srcToken).
78
79       interp alias srcPath srcToken {}
80              Deletes  the alias for srcToken in the slave interpreter identi‐
81              fied by srcPath.  srcToken refers to the value returned when the
82              alias  was created;  if the source command has been renamed, the
83              renamed command will be deleted.
84
85       interp alias srcPath srcCmd targetPath targetCmd ?arg arg ...?
86              This command creates an alias between one slave and another (see
87              the  alias  slave  command  below for creating aliases between a
88              slave and its master).  In this command,  either  of  the  slave
89              interpreters  may  be  anywhere in the hierarchy of interpreters
90              under the interpreter invoking the command.  SrcPath and  srcCmd
91              identify  the  source of the alias.  SrcPath is a Tcl list whose
92              elements select a particular interpreter.  For  example,  “a  b
93              identifies  an  interpreter “b”, which is a slave of interpreter
94a”, which is a slave of the  invoking  interpreter.   An  empty
95              list  specifies  the  interpreter  invoking the command.  srcCmd
96              gives the name of a new command, which will be  created  in  the
97              source  interpreter.   TargetPath and targetCmd specify a target
98              interpreter and command, and the arg arguments, if any,  specify
99              additional  arguments  to  targetCmd  which are prepended to any
100              arguments specified in the invocation of srcCmd.  TargetCmd  may
101              be  undefined at the time of this call, or it may already exist;
102              it is not created by this command.  The alias arranges  for  the
103              given  target  command  to  be invoked in the target interpreter
104              whenever the given source  command  is  invoked  in  the  source
105              interpreter.   See ALIAS INVOCATION below for more details.  The
106              command returns a token that  uniquely  identifies  the  command
107              created  srcCmd,  even if the command is renamed afterwards. The
108              token may but does not have to be equal to srcCmd.
109
110       interp aliases ?path?
111              This command returns a Tcl list of the tokens of all the  source
112              commands  for  aliases  defined in the interpreter identified by
113              path. The tokens correspond to  the  values  returned  when  the
114              aliases  were  created (which may not be the same as the current
115              names of the commands).
116
117       interp bgerror path ?cmdPrefix?
118              This command either gets or sets the current  background  excep‐
119              tion  handler for the interpreter identified by path. If cmdPre‐
120              fix is absent,  the  current  background  exception  handler  is
121              returned,  and if it is present, it is a list of words (of mini‐
122              mum length one) that describes what  to  set  the  interpreter's
123              background  exception  handler  to. See the BACKGROUND EXCEPTION
124              HANDLING section for more details.
125
126       interp cancel ?-unwind? ?--? ?path? ?result?
127              Cancels the script being evaluated in the interpreter identified │
128              by path. Without the -unwind switch the evaluation stack for the │
129              interpreter is unwound until an enclosing catch command is found │
130              or  there  are no further invocations of the interpreter left on │
131              the call stack. With the -unwind switch the evaluation stack for │
132              the  interpreter  is  unwound  without regard to any intervening │
133              catch command until there are  no  further  invocations  of  the │
134              interpreter left on the call stack. The -- switch can be used to │
135              mark the end of switches; it may be needed if path is an unusual │
136              value  such  as  -safe. If result is present, it will be used as │
137              the error message string; otherwise,  a  default  error  message │
138              string will be used.
139
140       interp create ?-safe? ?--? ?path?
141              Creates  a  slave  interpreter identified by path and a new com‐
142              mand, called a slave command. The name of the slave  command  is
143              the  last  component  of path. The new slave interpreter and the
144              slave command are created in the interpreter identified  by  the
145              path  obtained  by  removing  the  last component from path. For
146              example, if path is a b c then a new slave interpreter and slave
147              command named c are created in the interpreter identified by the
148              path a b.  The slave command may be used to manipulate  the  new
149              interpreter  as described below. If path is omitted, Tcl creates
150              a unique name of the form interpx, where x is  an  integer,  and
151              uses  it for the interpreter and the slave command. If the -safe
152              switch is specified (or if the  master  interpreter  is  a  safe
153              interpreter),  the  new  slave  interpreter will be created as a
154              safe interpreter with limited functionality; otherwise the slave
155              will  include  the  full  set of Tcl built-in commands and vari‐
156              ables. The -- switch can be used to mark the  end  of  switches;
157              it  may be needed if path is an unusual value such as -safe. The
158              result of the command is the name of the  new  interpreter.  The
159              name  of a slave interpreter must be unique among all the slaves
160              for its master;  an error occurs if a slave interpreter  by  the
161              given name already exists in this master.  The initial recursion
162              limit of the slave interpreter is set to the  current  recursion
163              limit of its parent interpreter.
164
165       interp debug path ?-frame ?bool??
166              Controls  whether  frame-level  stack information is captured in
167              the slave interpreter identified by path.  If no  arguments  are
168              given,  option  and  current setting are returned.  If -frame is
169              given, the debug setting is set to the given boolean if provided
170              and the current setting is returned.  This only effects the out‐
171              put of info frame, in that  exact  frame-level  information  for
172              command  invocation  at the bytecode level is only captured with
173              this setting on.
174
175              For example, with code like
176
177                     proc mycontrol {... script} {
178                       ...
179                       uplevel 1 $script
180                       ...
181                     }
182
183                     proc dosomething {...} {
184                       ...
185                       mycontrol {
186                         somecode
187                       }
188                     }
189
190              the standard setting will provide a relative line number for the
191              command  somecode  and  the relevant frame will be of type eval.
192              With frame-debug active on the other hand the  tracking  extends
193              so  far  that  the system will be able to determine the file and
194              absolute line number of this command, and return a frame of type
195              source. This more exact information is paid for with slower exe‐
196              cution of all commands.
197
198              Note that once it is on, this flag cannot be switched back  off:
199              such  attempts  are silently ignored. This is needed to maintain
200              the consistency of the underlying interpreter's state.
201
202       interp delete ?path ...?
203              Deletes zero or more interpreters given  by  the  optional  path
204              arguments, and for each interpreter, it also deletes its slaves.
205              The command also deletes the slave command for each  interpreter
206              deleted.  For each path argument, if no interpreter by that name
207              exists, the command raises an error.
208
209       interp eval path arg ?arg ...?
210              This command concatenates all of the arg arguments in  the  same
211              fashion  as  the  concat  command,  then evaluates the resulting
212              string as a Tcl script in the slave  interpreter  identified  by
213              path.  The  result  of  this  evaluation  (including  all return
214              options, such as -errorinfo and -errorcode  information,  if  an
215              error  occurs)  is  returned  to the invoking interpreter.  Note
216              that the script will be executed in the  current  context  stack
217              frame  of  the path interpreter; this is so that the implementa‐
218              tions (in a master interpreter) of aliases  in  a  slave  inter‐
219              preter  can  execute scripts in the slave that find out informa‐
220              tion about the slave's current state and stack frame.
221
222       interp exists path
223              Returns 1 if a slave interpreter by the specified path exists in
224              this  master,  0  otherwise.  If  path  is omitted, the invoking
225              interpreter is used.
226
227       interp expose path hiddenName ?exposedCmdName?
228              Makes the hidden command hiddenName exposed, eventually bringing
229              it  back under a new exposedCmdName name (this name is currently
230              accepted only if it is a valid global name  space  name  without
231              any ::), in the interpreter denoted by path.  If an exposed com‐
232              mand with the targeted name already exists, this command  fails.
233              Hidden commands are explained in more detail in HIDDEN COMMANDS,
234              below.
235
236       interp hide path exposedCmdName ?hiddenCmdName?
237              Makes the exposed command exposedCmdName hidden, renaming it  to
238              the  hidden  command  hiddenCmdName, or keeping the same name if
239              hiddenCmdName is not given, in the interpreter denoted by  path.
240              If  a hidden command with the targeted name already exists, this
241              command fails.  Currently both exposedCmdName and  hiddenCmdName
242              can  not  contain  namespace  qualifiers, or an error is raised.
243              Commands to be hidden by interp hide are looked up in the global
244              namespace  even  if the current namespace is not the global one.
245              This prevents slaves from fooling a master interpreter into hid‐
246              ing  the  wrong command, by making the current namespace be dif‐
247              ferent from the global one.  Hidden commands  are  explained  in
248              more detail in HIDDEN COMMANDS, below.
249
250       interp hidden path
251              Returns a list of the names of all hidden commands in the inter‐
252              preter identified by path.
253
254       interp invokehidden path ?-option ...? hiddenCmdName ?arg ...?
255              Invokes the hidden command hiddenCmdName with the arguments sup‐
256              plied  in  the  interpreter denoted by path. No substitutions or
257              evaluation are applied to the arguments. Three -options are sup‐
258              ported,  all  of  which  start with -: -namespace (which takes a
259              single argument afterwards, nsName), -global, and  --.   If  the
260              -namespace flag is present, the hidden command is invoked in the
261              namespace called nsName  in  the  target  interpreter.   If  the
262              -global  flag  is  present, the hidden command is invoked at the
263              global level in the target interpreter; otherwise it is  invoked
264              at the current call frame and can access local variables in that
265              and outer call frames.  The --  flag  allows  the  hiddenCmdName
266              argument  to start with a “-” character, and is otherwise unnec‐
267              essary.  If both the -namespace and -global flags  are  present,
268              the  -namespace  flag  is ignored.  Note that the hidden command
269              will be executed (by default) in the current context stack frame
270              of  the path interpreter.  Hidden commands are explained in more
271              detail in HIDDEN COMMANDS, below.
272
273       interp issafe ?path?
274              Returns 1 if the interpreter identified by the specified path is
275              safe, 0 otherwise.
276
277       interp limit path limitType ?-option? ?value ...?
278              Sets  up,  manipulates  and  queries  the  configuration  of the
279              resource limit limitType for the interpreter  denoted  by  path.
280              If  no -option is specified, return the current configuration of
281              the limit.  If -option is the sole argument, return the value of
282              that  option.  Otherwise, a list of -option/value argument pairs
283              must supplied. See RESOURCE LIMITS below  for  a  more  detailed
284              explanation of what limits and options are supported.
285
286       interp marktrusted path
287              Marks  the  interpreter  identified by path as trusted. Does not
288              expose the hidden commands. This command  can  only  be  invoked
289              from  a  trusted  interpreter.  The command has no effect if the
290              interpreter identified by path is already trusted.
291
292       interp recursionlimit path ?newlimit?
293              Returns the maximum allowable nesting depth for the  interpreter
294              specified  by  path.   If newlimit is specified, the interpreter
295              recursion limit will  be  set  so  that  nesting  of  more  than
296              newlimit calls to Tcl_Eval and related procedures in that inter‐
297              preter will  return  an  error.   The  newlimit  value  is  also
298              returned.  The newlimit value must be a positive integer between
299              1 and the maximum value of a non-long integer on the platform.
300
301              The command sets the maximum size of the Tcl call stack only. It
302              cannot  by  itself  prevent stack overflows on the C stack being
303              used by the application. If your machine has a limit on the size
304              of  the C stack, you may get stack overflows before reaching the
305              limit set by the command. If this happens, see  if  there  is  a
306              mechanism  in your system for increasing the maximum size of the
307              C stack.
308
309       interp share srcPath channelId destPath
310              Causes the IO channel identified by channelId to  become  shared
311              between  the  interpreter  identified  by srcPath and the inter‐
312              preter identified by destPath. Both interpreters have  the  same
313              permissions  on the IO channel.  Both interpreters must close it
314              to close the underlying IO channel; IO channels accessible in an
315              interpreter  are  automatically  closed  when  an interpreter is
316              destroyed.
317
318       interp slaves ?path?
319              Returns a Tcl list of the names of all  the  slave  interpreters
320              associated  with  the interpreter identified by path. If path is
321              omitted, the invoking interpreter is used.
322
323       interp target path alias
324              Returns a Tcl list describing  the  target  interpreter  for  an
325              alias.  The  alias  is  specified  with  an interpreter path and
326              source command name, just as in interp alias above. The name  of
327              the target interpreter is returned as an interpreter path, rela‐
328              tive to the invoking interpreter.  If the target interpreter for
329              the  alias  is  the  invoking  interpreter then an empty list is
330              returned. If the target interpreter for the  alias  is  not  the
331              invoking  interpreter or one of its descendants then an error is
332              generated.  The target command does not have to  be  defined  at
333              the time of this invocation.
334
335       interp transfer srcPath channelId destPath
336              Causes  the  IO channel identified by channelId to become avail‐
337              able in the interpreter identified by destPath  and  unavailable
338              in the interpreter identified by srcPath.
339

SLAVE COMMAND

341       For  each  slave interpreter created with the interp command, a new Tcl
342       command is created in the master interpreter with the same name as  the
343       new  interpreter. This command may be used to invoke various operations
344       on the interpreter.  It has the following general form:
345
346              slave command ?arg arg ...?
347
348       Slave is the name of the interpreter, and command and the  args  deter‐
349       mine  the  exact behavior of the command.  The valid forms of this com‐
350       mand are:
351
352       slave aliases
353              Returns a Tcl list whose elements are  the  tokens  of  all  the
354              aliases  in slave.  The tokens correspond to the values returned
355              when the aliases were created (which may not be the same as  the
356              current names of the commands).
357
358       slave alias srcToken
359              Returns  a  Tcl  list  whose elements are the targetCmd and args
360              associated with the alias represented by srcToken (this  is  the
361              value  returned  when the alias was created; it is possible that
362              the actual source command in the slave is different from  srcTo‐
363              ken).
364
365       slave alias srcToken {}
366              Deletes the alias for srcToken in the slave interpreter.  srcTo‐
367              ken refers to the value returned when the alias was created;  if
368              the source command has been renamed, the renamed command will be
369              deleted.
370
371       slave alias srcCmd targetCmd ?arg ..?
372              Creates an alias such that whenever srcCmd is invoked in  slave,
373              targetCmd  is  invoked in the master.  The arg arguments will be
374              passed to targetCmd as additional  arguments,  prepended  before
375              any  arguments  passed  in  the invocation of srcCmd.  See ALIAS
376              INVOCATION below for details.  The command returns a token  that
377              uniquely identifies the command created srcCmd, even if the com‐
378              mand is renamed afterwards. The token may but does not  have  to
379              be equal to srcCmd.
380
381       slave bgerror ?cmdPrefix?
382              This  command  either gets or sets the current background excep‐
383              tion handler for the slave interpreter. If cmdPrefix is  absent,
384              the  current background exception handler is returned, and if it
385              is present, it is a list of words (of minimum length  one)  that
386              describes  what  to  set  the interpreter's background exception
387              handler to. See the BACKGROUND EXCEPTION  HANDLING  section  for
388              more details.
389
390       slave eval arg ?arg ..?
391              This  command  concatenates all of the arg arguments in the same
392              fashion as the concat  command,  then  evaluates  the  resulting
393              string  as a Tcl script in slave.  The result of this evaluation
394              (including all return options, such as -errorinfo and -errorcode
395              information,  if  an  error  occurs) is returned to the invoking
396              interpreter.  Note that the script will be executed in the  cur‐
397              rent context stack frame of slave; this is so that the implemen‐
398              tations (in a master interpreter) of aliases in a  slave  inter‐
399              preter  can  execute scripts in the slave that find out informa‐
400              tion about the slave's current state and stack frame.
401
402       slave expose hiddenName ?exposedCmdName?
403              This command exposes the hidden command  hiddenName,  eventually
404              bringing  it  back under a new exposedCmdName name (this name is
405              currently accepted only if it is a valid global name space  name
406              without  any ::), in slave.  If an exposed command with the tar‐
407              geted name already exists, this command fails.  For more details
408              on hidden commands, see HIDDEN COMMANDS, below.
409
410       slave hide exposedCmdName ?hiddenCmdName?
411              This  command hides the exposed command exposedCmdName, renaming
412              it to the hidden command hiddenCmdName, or keeping the same name
413              if  the  argument  is not given, in the slave interpreter.  If a
414              hidden command with the targeted name already exists, this  com‐
415              mand fails.  Currently both exposedCmdName and hiddenCmdName can
416              not contain namespace qualifiers, or an error is  raised.   Com‐
417              mands to be hidden are looked up in the global namespace even if
418              the current namespace is  not  the  global  one.  This  prevents
419              slaves  from  fooling a master interpreter into hiding the wrong
420              command, by making the current namespace be different  from  the
421              global  one.   For  more  details on hidden commands, see HIDDEN
422              COMMANDS, below.
423
424       slave hidden
425              Returns a list of the names of all hidden commands in slave.
426
427       slave invokehidden ?-option ...? hiddenName ?arg ..?
428              This command invokes the hidden command hiddenName with the sup‐
429              plied  arguments,  in slave. No substitutions or evaluations are
430              applied to the arguments. Three -options are supported,  all  of
431              which  start  with  -: -namespace (which takes a single argument
432              afterwards, nsName), -global, and --.  If the -namespace flag is
433              given,  the hidden command is invoked in the specified namespace
434              in the slave.  If the -global flag  is  given,  the  command  is
435              invoked  at  the  global  level  in  the  slave; otherwise it is
436              invoked at the current call frame and can access local variables
437              in that or outer call frames.  The -- flag allows the hiddenCmd‐
438              Name argument to start with a “-” character,  and  is  otherwise
439              unnecessary.   If  both  the  -namespace  and  -global flags are
440              given, the -namespace flag is ignored.   Note  that  the  hidden
441              command  will  be  executed  (by default) in the current context
442              stack frame of slave.  For more details on hidden commands,  see
443              HIDDEN COMMANDS, below.
444
445       slave issafe
446              Returns  1 if the slave interpreter is safe, 0 otherwise.
447
448       slave limit limitType ?-option? ?value ...?
449              Sets  up,  manipulates  and  queries  the  configuration  of the
450              resource limit limitType  for  the  slave  interpreter.   If  no
451              -option  is  specified,  return the current configuration of the
452              limit.  If -option is the sole argument,  return  the  value  of
453              that  option.  Otherwise, a list of -option/value argument pairs
454              must supplied. See RESOURCE LIMITS below  for  a  more  detailed
455              explanation of what limits and options are supported.
456
457       slave marktrusted
458              Marks the slave interpreter as trusted. Can only be invoked by a
459              trusted interpreter. This command does  not  expose  any  hidden
460              commands  in the slave interpreter. The command has no effect if
461              the slave is already trusted.
462
463       slave recursionlimit ?newlimit?
464              Returns the maximum allowable nesting depth for the slave inter‐
465              preter.   If newlimit is specified, the recursion limit in slave
466              will be set so that nesting  of  more  than  newlimit  calls  to
467              Tcl_Eval() and related procedures in slave will return an error.
468              The newlimit value is also returned.  The newlimit value must be
469              a positive integer between 1 and the maximum value of a non-long
470              integer on the platform.
471
472              The command sets the maximum size of the Tcl call stack only. It
473              cannot  by  itself  prevent stack overflows on the C stack being
474              used by the application. If your machine has a limit on the size
475              of  the C stack, you may get stack overflows before reaching the
476              limit set by the command. If this happens, see  if  there  is  a
477              mechanism  in your system for increasing the maximum size of the
478              C stack.
479

SAFE INTERPRETERS

481       A safe interpreter is one with restricted  functionality,  so  that  is
482       safe  to execute an arbitrary script from your worst enemy without fear
483       of that script damaging the enclosing application or the rest  of  your
484       computing  environment.   In order to make an interpreter safe, certain
485       commands and variables are removed from the interpreter.  For  example,
486       commands  to  create files on disk are removed, and the exec command is
487       removed, since it could be used to cause damage  through  subprocesses.
488       Limited access to these facilities can be provided, by creating aliases
489       to the master interpreter which check  their  arguments  carefully  and
490       provide restricted access to a safe subset of facilities.  For example,
491       file creation might be allowed in a particular subdirectory and subpro‐
492       cess invocation might be allowed for a carefully selected and fixed set
493       of programs.
494
495       A safe interpreter is created by specifying the  -safe  switch  to  the
496       interp create command.  Furthermore, any slave created by a safe inter‐
497       preter will also be safe.
498
499       A safe interpreter is created with exactly the following set of  built-
500       in commands:
501
502              after       append      apply       array
503              binary      break       catch       chan
504              clock       close       concat      continue
505              dict        eof         error       eval
506              expr        fblocked    fcopy       fileevent
507              flush       for         foreach     format
508              gets        global      if          incr
509              info        interp      join        lappend
510              lassign     lindex      linsert     list
511              llength     lrange      lrepeat     lreplace
512              lsearch     lset        lsort       namespace
513              package     pid         proc        puts
514              read        regexp      regsub      rename
515              return      scan        seek        set
516              split       string      subst       switch
517              tell        time        trace       unset
518              update      uplevel     upvar       variable
519              vwait       while
520
521       The  following  commands  are hidden by interp create when it creates a
522       safe interpreter:
523
524              cd          encoding    exec        exit
525              fconfigure  file        glob        load
526              open        pwd         socket      source
527              unload
528
529       These commands can be recreated later as Tcl procedures or aliases,  or
530       re-exposed by interp expose.
531
532       The following commands from Tcl's library of support procedures are not
533       present in a safe interpreter:
534
535              auto_exec_ok    auto_import     auto_load
536              auto_load_index auto_qualify    unknown
537
538       Note in particular that safe interpreters have no default unknown  com‐
539       mand,  so  Tcl's  default  autoloading  facilities  are  not available.
540       Autoload access to Tcl's commands that are normally autoloaded:
541
542              auto_mkindex         auto_mkindex_old
543              auto_reset           history
544              parray               pkg_mkIndex
545              ::pkg::create        ::safe::interpAddToAccessPath
546              ::safe::interpCreate ::safe::interpConfigure
547              ::safe::interpDelete ::safe::interpFindInAccessPath
548              ::safe::interpInit   ::safe::setLogCmd
549              tcl_endOfWord        tcl_findLibrary
550              tcl_startOfNextWord  tcl_startOfPreviousWord
551              tcl_wordBreakAfter   tcl_wordBreakBefore
552
553       can only be provided by explicit definition of an  unknown  command  in
554       the  safe  interpreter.  This will involve exposing the source command.
555       This is most easily accomplished by creating the safe interpreter  with
556       Tcl's  Safe-Tcl  mechanism.  Safe-Tcl provides safe versions of source,
557       load, and other Tcl commands needed to support autoloading of  commands
558       and the loading of packages.
559
560       In  addition, the env variable is not present in a safe interpreter, so
561       it cannot share environment variables with other interpreters. The  env
562       variable  poses  a  security  risk,  because  users can store sensitive
563       information in an environment variable. For  example,  the  PGP  manual
564       recommends storing the PGP private key protection password in the envi‐
565       ronment variable PGPPASS. Making this variable available  to  untrusted
566       code executing in a safe interpreter would incur a security risk.
567
568       If  extensions  are  loaded  into  a  safe  interpreter,  they may also
569       restrict their own functionality to eliminate unsafe  commands.  For  a
570       discussion  of  management  of  extensions  for  safety  see the manual
571       entries for Safe-Tcl and the load Tcl command.
572
573       A safe interpreter may not alter the  recursion  limit  of  any  inter‐
574       preter, including itself.
575

ALIAS INVOCATION

577       The  alias mechanism has been carefully designed so that it can be used
578       safely when an untrusted script is executing in a safe  slave  and  the
579       target  of  the alias is a trusted master.  The most important thing in
580       guaranteeing safety is to ensure that information passed from the slave
581       to the master is never evaluated or substituted in the master;  if this
582       were to occur, it would enable an evil script in the  slave  to  invoke
583       arbitrary functions in the master, which would compromise security.
584
585       When  the  source for an alias is invoked in the slave interpreter, the
586       usual Tcl substitutions are performed when parsing that command.  These
587       substitutions  are  carried  out in the source interpreter just as they
588       would be for any other command invoked in that interpreter.   The  com‐
589       mand  procedure  for  the source command takes its arguments and merges
590       them with the targetCmd and args for the alias to create a new array of
591       arguments.   If  the  words of srcCmd were “srcCmd arg1 arg2 ... argN”,
592       the new set of words will be “targetCmd arg arg ... arg arg1  arg2  ...
593       argN”,  where targetCmd and args are the values supplied when the alias
594       was created.  TargetCmd is then used to locate a command  procedure  in
595       the  target interpreter, and that command procedure is invoked with the
596       new set of arguments.  An error occurs if there  is  no  command  named
597       targetCmd  in  the target interpreter.  No additional substitutions are
598       performed on the  words:   the  target  command  procedure  is  invoked
599       directly,  without  going  through the normal Tcl evaluation mechanism.
600       Substitutions are thus performed on each word exactly  once:  targetCmd
601       and  args  were  substituted  when parsing the command that created the
602       alias, and arg1 - argN are substituted when the alias's source  command
603       is parsed in the source interpreter.
604
605       When  writing  the  targetCmds  for aliases in safe interpreters, it is
606       very important that the arguments to that command never be evaluated or
607       substituted,  since  this would provide an escape mechanism whereby the
608       slave interpreter could execute arbitrary code in the master.  This  in
609       turn would compromise the security of the system.
610

HIDDEN COMMANDS

612       Safe  interpreters  greatly restrict the functionality available to Tcl
613       programs executing within them.  Allowing the untrusted Tcl program  to
614       have  direct  access to this functionality is unsafe, because it can be
615       used for a variety of attacks on the environment.  However,  there  are
616       times  when there is a legitimate need to use the dangerous functional‐
617       ity in the context of the safe interpreter. For  example,  sometimes  a
618       program  must  be sourced into the interpreter.  Another example is Tk,
619       where windows are bound to the hierarchy  of  windows  for  a  specific
620       interpreter; some potentially dangerous functions, e.g.  window manage‐
621       ment, must be performed on these windows within  the  interpreter  con‐
622       text.
623
624       The  interp  command provides a solution to this problem in the form of
625       hidden commands. Instead of removing the  dangerous  commands  entirely
626       from  a  safe  interpreter,  these  commands  are hidden so they become
627       unavailable to Tcl scripts executing in the interpreter. However,  such
628       hidden  commands  can  be  invoked  by any trusted ancestor of the safe
629       interpreter, in the context  of  the  safe  interpreter,  using  interp
630       invoke.  Hidden  commands  and exposed commands reside in separate name
631       spaces. It is possible to define a hidden command and an  exposed  com‐
632       mand by the same name within one interpreter.
633
634       Hidden  commands  in  a slave interpreter can be invoked in the body of
635       procedures called in the master during alias invocation.  For  example,
636       an alias for source could be created in a slave interpreter. When it is
637       invoked in the slave interpreter, a procedure is called in  the  master
638       interpreter  to  check that the operation is allowable (e.g. it asks to
639       source a file that the slave interpreter is  allowed  to  access).  The
640       procedure then it invokes the hidden source command in the slave inter‐
641       preter to actually source in the contents of the file.  Note  that  two
642       commands  named  source  exist in the slave interpreter: the alias, and
643       the hidden command.
644
645       Because a master interpreter may invoke a hidden  command  as  part  of
646       handling  an alias invocation, great care must be taken to avoid evalu‐
647       ating any arguments passed in through the alias invocation.  Otherwise,
648       malicious  slave  interpreters could cause a trusted master interpreter
649       to execute dangerous commands on their behalf. See the section on ALIAS
650       INVOCATION for a more complete discussion of this topic.  To help avoid
651       this problem, no substitutions or evaluations are applied to  arguments
652       of interp invokehidden.
653
654       Safe  interpreters  are  not allowed to invoke hidden commands in them‐
655       selves or in their descendants. This prevents safe slaves from  gaining
656       access to hidden functionality in themselves or their descendants.
657
658       The  set  of  hidden commands in an interpreter can be manipulated by a
659       trusted interpreter using interp expose and  interp  hide.  The  interp
660       expose command moves a hidden command to the set of exposed commands in
661       the interpreter identified by path, potentially renaming the command in
662       the process. If an exposed command by the targeted name already exists,
663       the operation fails. Similarly, interp hide moves an exposed command to
664       the  set  of hidden commands in that interpreter. Safe interpreters are
665       not allowed to move commands between the set of hidden and exposed com‐
666       mands, in either themselves or their descendants.
667
668       Currently, the names of hidden commands cannot contain namespace quali‐
669       fiers, and you must first rename a command in a namespace to the global
670       namespace before you can hide it.  Commands to be hidden by interp hide
671       are looked up in the global namespace even if the current namespace  is
672       not  the  global one. This prevents slaves from fooling a master inter‐
673       preter into hiding the wrong command, by making the  current  namespace
674       be different from the global one.
675

RESOURCE LIMITS

677       Every  interpreter has two kinds of resource limits that may be imposed
678       by any master interpreter upon its slaves. Command limits (of type com‐
679       mand) restrict the total number of Tcl commands that may be executed by
680       an interpreter (as can be inspected via the info cmdcount command), and
681       time  limits (of type time) place a limit by which execution within the
682       interpreter must complete. Note that time limits are expressed as abso‐
683       lute  times  (as in clock seconds) and not relative times (as in after)
684       because they may be modified after creation.
685
686       When a limit is exceeded for an interpreter, first  any  handler  call‐
687       backs  defined  by  master  interpreters are called. If those callbacks
688       increase or remove the limit, execution within the (previously) limited
689       interpreter continues. If the limit is still in force, an error is gen‐
690       erated at that point and normal processing of errors within the  inter‐
691       preter (by the catch command) is disabled, so the error propagates out‐
692       wards (building a stack-trace as it goes) to the point where  the  lim‐
693       ited interpreter was invoked (e.g. by interp eval) where it becomes the
694       responsibility of the calling code to catch and handle.
695
696   LIMIT OPTIONS
697       Every limit has a number of options associated with it, some  of  which
698       are common across all kinds of limits, and others of which are particu‐
699       lar to the kind of limit.
700
701       -command
702              This option (common for all  limit  types)  specifies  (if  non-
703              empty)  a  Tcl  script to be executed in the global namespace of
704              the interpreter reading and writing the option when the particu‐
705              lar  limit in the limited interpreter is exceeded.  The callback
706              may modify the limit on the interpreter if it wishes the limited
707              interpreter  to continue executing. If the callback generates an
708              exception, it is reported through the background exception mech‐
709              anism  (see BACKGROUND EXCEPTION HANDLING).  Note that the call‐
710              backs defined by one interpreter are  completely  isolated  from
711              the  callbacks  defined  by another, and that the order in which
712              those callbacks are called is undefined.
713
714       -granularity
715              This option (common for all  limit  types)  specifies  how  fre‐
716              quently (out of the points when the Tcl interpreter is in a con‐
717              sistent state where limit checking is possible) that  the  limit
718              is  actually checked. This allows the tuning of how frequently a
719              limit is checked, and hence how often the  limit-checking  over‐
720              head  (which  may  be substantial in the case of time limits) is
721              incurred.
722
723       -milliseconds
724              This option specifies  the  number  of  milliseconds  after  the
725              moment  defined  in the -seconds option that the time limit will
726              fire. It should only ever be specified in conjunction  with  the
727              -seconds  option  (whether it was set previously or is being set
728              this invocation.)
729
730       -seconds
731              This option specifies the number of seconds after the epoch (see
732              clock  seconds)  that the time limit for the interpreter will be
733              triggered. The limit will be triggered at the start of the  sec‐
734              ond  unless specified at a sub-second level using the -millisec‐
735              onds option. This option may be the empty  string,  which  indi‐
736              cates that a time limit is not set for the interpreter.
737
738       -value This  option  specifies  the  number of commands that the inter‐
739              preter may execute before triggering  the  command  limit.  This
740              option  may  be the empty string, which indicates that a command
741              limit is not set for the interpreter.
742
743       Where an interpreter with a resource limit set on it  creates  a  slave
744       interpreter,  that  slave interpreter will have resource limits imposed
745       on it that are at least as restrictive as the limits  on  the  creating
746       master  interpreter.  If  the  master interpreter of the limited master
747       wishes to relax these conditions, it should hide the interp command  in
748       the  child  and then use aliases and the interp invokehidden subcommand
749       to provide such access as it chooses to the interp command to the  lim‐
750       ited master as necessary.
751

BACKGROUND EXCEPTION HANDLING

753       When  an  exception  happens in a situation where it cannot be reported
754       directly up the stack (e.g. when processing  events  in  an  update  or
755       vwait  call)  the  exception is instead reported through the background
756       exception handling  mechanism.   Every  interpreter  has  a  background
757       exception  handler  registered;  the default exception handler arranges
758       for the bgerror command in the interpreter's  global  namespace  to  be
759       called, but other exception handlers may be installed and process back‐
760       ground exceptions in substantially different ways.
761
762       A background exception handler consists of a non-empty list of words to
763       which  will be appended two further words at invocation time. The first
764       word will be the interpreter result at time of the exception, typically
765       an  error  message,  and  the  second  will be the dictionary of return
766       options at the time of the exception.  These are the same  values  that
767       catch  can  capture  when  it controls script evaluation in a non-back‐
768       ground situation.  The resulting list will  then  be  executed  in  the
769       interpreter's global namespace without further substitutions being per‐
770       formed.
771

CREDITS

773       The safe interpreter mechanism  is  based  on  the  Safe-Tcl  prototype
774       implemented by Nathaniel Borenstein and Marshall Rose.
775

EXAMPLES

777       Creating and using an alias for a command in the current interpreter:
778
779              interp alias {} getIndex {} lsearch {alpha beta gamma delta}
780              set idx [getIndex delta]
781
782       Executing  an arbitrary command in a safe interpreter where every invo‐
783       cation of lappend is logged:
784
785              set i [interp create -safe]
786              interp hide $i lappend
787              interp alias $i lappend {} loggedLappend $i
788              proc loggedLappend {i args} {
789                  puts "logged invocation of lappend $args"
790                  interp invokehidden $i lappend {*}$args
791              }
792              interp eval $i $someUntrustedScript
793
794       Setting a resource limit on an interpreter so  that  an  infinite  loop
795       terminates.
796
797              set i [interp create]
798              interp limit $i command -value 1000
799              interp eval $i {
800                  set x 0
801                  while {1} {
802                      puts "Counting up... [incr x]"
803                  }
804              }
805

SEE ALSO

807       bgerror(n),    load(n),   safe(n),   Tcl_CreateSlave(3),   Tcl_Eval(3),
808       Tcl_BackgroundException(3)
809

KEYWORDS

811       alias, master interpreter, safe interpreter, slave interpreter
812
813
814
815Tcl                                   8.6                            interp(n)
Impressum