interp(3tcl) Tcl Built-In Commands interp(3tcl)
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NAME
interp - Create and manipulate Tcl interpreters
SYNOPSIS
interp subcommand ?arg arg ...?
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DESCRIPTION
This command makes it possible to create one or more new Tcl inter-
preters that co-exist with the creating interpreter in the same appli-
cation. The creating interpreter is called the parent and the new in-
terpreter is called a child. A parent can create any number of chil-
dren, and each child can itself create additional children for which it
is parent, resulting in a hierarchy of interpreters.
Each interpreter is independent from the others: it has its own name
space for commands, procedures, and global variables. A parent inter-
preter may create connections between its children and itself using a
mechanism called an alias. An alias is a command in a child inter-
preter which, when invoked, causes a command to be invoked in its par-
ent interpreter or in another child 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 child 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 in-
terpreter; instead, it is hidden, so that only trusted interpreters can
obtain access to it. For a detailed explanation of hidden commands, see
HIDDEN COMMANDS, below. The alias mechanism can be used for protected
communication (analogous to a kernel call) between a child interpreter
and its parent. See ALIAS INVOCATION, below, for more details on how
the alias mechanism works.
A qualified interpreter name is a proper Tcl list containing a subset
of its ancestors in the interpreter hierarchy, terminated by the string
naming the interpreter in its immediate parent. Interpreter names are
relative to the interpreter in which they are used. For example, if “a”
is a child of the current interpreter and it has a child “a1”, which in
turn has a child “a11”, the qualified name of “a11” in “a” is the list
“a1 a11”.
The interp command, described below, accepts qualified interpreter
names as arguments; the interpreter in which the command is being eval-
uated can always be referred to as {} (the empty list or string). Note
that it is impossible to refer to a parent (ancestor) interpreter by
name in a child interpreter except through aliases. Also, there is no
global name by which one can refer to the first interpreter created in
an application. Both restrictions are motivated by safety concerns.
THE INTERP COMMAND
The interp command is used to create, delete, and manipulate child in-
terpreters, 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 as-
sociated with the alias represented by srcToken (this is the
value returned when the alias was created; it is possible that
the name of the source command in the child is different from
srcToken).
interp alias srcPath srcToken {}
Deletes the alias for srcToken in the child interpreter 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 child and another (see
the alias child command below for creating aliases between a
child and its parent). In this command, either of the child in-
terpreters may be anywhere in the hierarchy of interpreters un-
der the interpreter invoking the command. SrcPath and srcCmd
identify the source of the alias. SrcPath is a Tcl list whose
elements select a particular interpreter. For example, “a b”
identifies an interpreter “b”, which is a child of interpreter
“a”, which is a child of the invoking interpreter. An empty
list specifies the interpreter invoking the command. srcCmd
gives the name of a new command, which will be created in the
source interpreter. TargetPath and targetCmd specify a target
interpreter and command, and the arg arguments, if any, specify
additional arguments to targetCmd which are prepended to any ar-
guments 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 in-
terpreter. 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 excep-
tion handler for the interpreter identified by path. If cmdPre-
fix is absent, the current background exception handler is re-
turned, and if it is present, it is a list of words (of minimum
length one) that describes what to set the interpreter's back-
ground exception handler to. See the BACKGROUND EXCEPTION HAN-
DLING section for more details.
interp cancel ?-unwind? ?--? ?path? ?result?
Cancels the script being evaluated in the interpreter identified │
by path. Without the -unwind switch the evaluation stack for the │
interpreter is unwound until an enclosing catch command is found │
or there are no further invocations of the interpreter left on │
the call stack. With the -unwind switch the evaluation stack for │
the interpreter is unwound without regard to any intervening │
catch command until there are no further invocations of the in- │
terpreter left on the call stack. The -- switch can be used to │
mark the end of switches; it may be needed if path is an unusual │
value such as -safe. If result is present, it will be used as │
the error message string; otherwise, a default error message │
string will be used.
interp create ?-safe? ?--? ?path?
Creates a child interpreter identified by path and a new com-
mand, called a child command. The name of the child command is
the last component of path. The new child interpreter and the
child command are created in the interpreter identified by the
path obtained by removing the last component from path. For ex-
ample, if path is a b c then a new child interpreter and child
command named c are created in the interpreter identified by the
path a b. The child command may be used to manipulate the new
interpreter as described below. If path is omitted, Tcl creates
a unique name of the form interpx, where x is an integer, and
uses it for the interpreter and the child command. If the -safe
switch is specified (or if the parent interpreter is a safe in-
terpreter), the new child interpreter will be created as a safe
interpreter with limited functionality; otherwise the child will
include the full set of Tcl built-in commands and variables. The
-- switch can be used to mark the end of switches; it may be
needed if path is an unusual value such as -safe. The result of
the command is the name of the new interpreter. The name of a
child interpreter must be unique among all the children for its
parent; an error occurs if a child interpreter by the given
name already exists in this parent. The initial recursion limit
of the child interpreter is set to the current recursion limit
of its parent interpreter.
interp debug path ?-frame ?bool??
Controls whether frame-level stack information is captured in
the child interpreter identified by path. If no arguments are
given, option and current setting are returned. If -frame is
given, the debug setting is set to the given boolean if provided
and the current setting is returned. This only affects the out-
put of info frame, in that exact frame-level information for
command invocation at the bytecode level is only captured with
this setting on.
For example, with code like
proc mycontrol {... script} {
...
uplevel 1 $script
...
}
proc dosomething {...} {
...
mycontrol {
somecode
}
}
the standard setting will provide a relative line number for the
command somecode and the relevant frame will be of type eval.
With frame-debug active on the other hand the tracking extends
so far that the system will be able to determine the file and
absolute line number of this command, and return a frame of type
source. This more exact information is paid for with slower exe-
cution of all commands.
Note that once it is on, this flag cannot be switched back off:
such attempts are silently ignored. This is needed to maintain
the consistency of the underlying interpreter's state.
interp delete ?path ...?
Deletes zero or more interpreters given by the optional path ar-
guments, and for each interpreter, it also deletes its children.
The command also deletes the child command for each interpreter
deleted. For each path argument, if no interpreter by that name
exists, the command raises an error.
interp eval path arg ?arg ...?
This command concatenates all of the arg arguments in the same
fashion as the concat command, then evaluates the resulting
string as a Tcl script in the child interpreter identified by
path. The result of this evaluation (including all return op-
tions, such as -errorinfo and -errorcode information, if an er-
ror occurs) is returned to the invoking interpreter. Note that
the script will be executed in the current context stack frame
of the path interpreter; this is so that the implementations (in
a parent interpreter) of aliases in a child interpreter can exe-
cute scripts in the child that find out information about the
child's current state and stack frame.
interp exists path
Returns 1 if a child interpreter by the specified path exists in
this parent, 0 otherwise. If path is omitted, the invoking in-
terpreter 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 children from fooling a parent interpreter into
hiding the wrong command, by making the current namespace be
different from the global one. Hidden commands are explained in
more detail in HIDDEN COMMANDS, below.
interp hidden path
Returns a list of the names of all hidden commands in the 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 ar-
gument to start with a “-” character, and is otherwise unneces-
sary. 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 de-
tail in HIDDEN COMMANDS, below.
interp issafe ?path?
Returns 1 if the interpreter identified by the specified path is
safe, 0 otherwise.
interp limit path limitType ?-option? ?value ...?
Sets up, manipulates and queries the configuration of the re-
source 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 ex-
planation of what limits and options are supported.
interp marktrusted path
Marks the interpreter identified by path as trusted. Does not
expose the hidden commands. This command can only be invoked
from a trusted interpreter. The command has no effect if the
interpreter identified by path is already trusted.
interp recursionlimit path ?newlimit?
Returns the maximum allowable nesting depth for the interpreter
specified by path. If newlimit is specified, the interpreter
recursion limit will be set so that nesting of more than
newlimit calls to Tcl_Eval and related procedures in that inter-
preter will return an error. The newlimit value is also re-
turned. 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 de-
stroyed.
interp slaves ?path?
Returns a Tcl list of the names of all the child interpreters
associated with the interpreter identified by path. If path is
omitted, the invoking interpreter is used. │
interp children ?path? │
Synonym for . interp slaves ?path?
interp target path alias
Returns a Tcl list describing the target interpreter for an
alias. The alias is specified with an interpreter path and
source command name, just as in interp alias above. The name of
the target interpreter is returned as an interpreter path, rela-
tive to the invoking interpreter. If the target interpreter for
the alias is the invoking interpreter then an empty list is re-
turned. If the target interpreter for the alias is not the in-
voking 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.
child COMMAND
For each child interpreter created with the interp command, a new Tcl
command is created in the parent interpreter with the same name as the
new interpreter. This command may be used to invoke various operations
on the interpreter. It has the following general form:
child command ?arg arg ...?
child is the name of the interpreter, and command and the args deter-
mine the exact behavior of the command. The valid forms of this com-
mand are:
child aliases
Returns a Tcl list whose elements are the tokens of all the
aliases in child. The tokens correspond to the values returned
when the aliases were created (which may not be the same as the
current names of the commands).
child alias srcToken
Returns a Tcl list whose elements are the targetCmd and args as-
sociated with the alias represented by srcToken (this is the
value returned when the alias was created; it is possible that
the actual source command in the child is different from srcTo-
ken).
child alias srcToken {}
Deletes the alias for srcToken in the child 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.
child alias srcCmd targetCmd ?arg ..?
Creates an alias such that whenever srcCmd is invoked in child,
targetCmd is invoked in the parent. The arg arguments will be
passed to targetCmd as additional arguments, prepended before
any arguments passed in the invocation of srcCmd. See ALIAS IN-
VOCATION 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.
child bgerror ?cmdPrefix?
This command either gets or sets the current background excep-
tion handler for the child interpreter. If cmdPrefix is absent,
the current background exception handler is returned, and if it
is present, it is a list of words (of minimum length one) that
describes what to set the interpreter's background exception
handler to. See the BACKGROUND EXCEPTION HANDLING section for
more details.
child eval arg ?arg ..?
This command concatenates all of the arg arguments in the same
fashion as the concat command, then evaluates the resulting
string as a Tcl script in child. The result of this evaluation
(including all return options, such as -errorinfo and -errorcode
information, if an error occurs) is returned to the invoking in-
terpreter. Note that the script will be executed in the current
context stack frame of child; this is so that the implementa-
tions (in a parent interpreter) of aliases in a child inter-
preter can execute scripts in the child that find out informa-
tion about the child's current state and stack frame.
child expose hiddenName ?exposedCmdName?
This command exposes the hidden command hiddenName, eventually
bringing it back under a new exposedCmdName name (this name is
currently accepted only if it is a valid global name space name
without any ::), in child. If an exposed command with the tar-
geted name already exists, this command fails. For more details
on hidden commands, see HIDDEN COMMANDS, below.
child hide exposedCmdName ?hiddenCmdName?
This command hides the exposed command exposedCmdName, renaming
it to the hidden command hiddenCmdName, or keeping the same name
if the argument is not given, in the child interpreter. If a
hidden command with the targeted name already exists, this 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 chil-
dren from fooling a parent interpreter into hiding the wrong
command, by making the current namespace be different from the
global one. For more details on hidden commands, see HIDDEN
COMMANDS, below.
child hidden
Returns a list of the names of all hidden commands in child.
child invokehidden ?-option ...? hiddenName ?arg ..?
This command invokes the hidden command hiddenName with the sup-
plied arguments, in child. No substitutions or evaluations are
applied to the arguments. Three -options are supported, all of
which start with -: -namespace (which takes a single argument
afterwards, nsName), -global, and --. If the -namespace flag is
given, the hidden command is invoked in the specified namespace
in the child. If the -global flag is given, the command is in-
voked at the global level in the child; otherwise it is invoked
at the current call frame and can access local variables in that
or outer call frames. The -- flag allows the hiddenCmdName ar-
gument to start with a “-” character, and is otherwise unneces-
sary. If both the -namespace and -global flags are given, the
-namespace flag is ignored. Note that the hidden command will
be executed (by default) in the current context stack frame of
child. For more details on hidden commands, see HIDDEN COM-
MANDS, below.
child issafe
Returns 1 if the child interpreter is safe, 0 otherwise.
child limit limitType ?-option? ?value ...?
Sets up, manipulates and queries the configuration of the re-
source limit limitType for the child interpreter. If no -option
is specified, return the current configuration of the limit. If
-option is the sole argument, return the value of that option.
Otherwise, a list of -option/value argument pairs must supplied.
See RESOURCE LIMITS below for a more detailed explanation of
what limits and options are supported.
child marktrusted
Marks the child interpreter as trusted. Can only be invoked by a
trusted interpreter. This command does not expose any hidden
commands in the child interpreter. The command has no effect if
the child is already trusted.
child recursionlimit ?newlimit?
Returns the maximum allowable nesting depth for the child inter-
preter. If newlimit is specified, the recursion limit in child
will be set so that nesting of more than newlimit calls to
Tcl_Eval() and related procedures in child will return an error.
The newlimit value is also returned. The newlimit value must be
a positive integer between 1 and the maximum value of a non-long
integer on the platform.
The command sets the maximum size of the Tcl call stack only. It
cannot by itself prevent stack overflows on the C stack being
used by the application. If your machine has a limit on the size
of the C stack, you may get stack overflows before reaching the
limit set by the command. If this happens, see if there is a
mechanism in your system for increasing the maximum size of the
C stack.
SAFE INTERPRETERS
A safe interpreter is one with restricted functionality, so that is
safe to execute an arbitrary script from your worst enemy without fear
of that script damaging the enclosing application or the rest of your
computing environment. In order to make an interpreter safe, certain
commands and variables are removed from the interpreter. For example,
commands to create files on disk are removed, and the exec command is
removed, since it could be used to cause damage through subprocesses.
Limited access to these facilities can be provided, by creating aliases
to the parent interpreter which check their arguments carefully and
provide restricted access to a safe subset of facilities. For example,
file creation might be allowed in a particular subdirectory and 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 in-
terp create command. Furthermore, any child 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 concat continue
dict eof error eval
expr fblocked fcopy fileevent
flush for foreach format
gets global if incr
info interp join lappend
lassign lindex linsert list
llength lrange lrepeat lreplace
lsearch lset lsort namespace
package pid proc puts
read regexp regsub rename
return scan seek set
split string subst switch
tell time trace unset
update uplevel upvar variable
vwait while
The following commands are hidden by interp create when it creates a
safe interpreter:
cd encoding exec exit
fconfigure file glob load
open pwd socket source
unload
These commands can be recreated later as Tcl procedures or aliases, or
re-exposed by interp expose.
The following commands from Tcl's library of support procedures are not
present in a safe interpreter:
auto_exec_ok auto_import auto_load
auto_load_index auto_qualify unknown
Note in particular that safe interpreters have no default unknown com-
mand, so Tcl's default autoloading facilities are not available. Au-
toload access to Tcl's commands that are normally autoloaded:
auto_mkindex auto_mkindex_old
auto_reset history
parray pkg_mkIndex
::pkg::create ::safe::interpAddToAccessPath
::safe::interpCreate ::safe::interpConfigure
::safe::interpDelete ::safe::interpFindInAccessPath
::safe::interpInit ::safe::setLogCmd
tcl_endOfWord tcl_findLibrary
tcl_startOfNextWord tcl_startOfPreviousWord
tcl_wordBreakAfter tcl_wordBreakBefore
can only be provided by explicit definition of an unknown command in
the safe interpreter. This will involve exposing the source command.
This is most easily accomplished by creating the safe interpreter with
Tcl's Safe-Tcl mechanism. Safe-Tcl provides safe versions of source,
load, and other Tcl commands needed to support autoloading of commands
and the loading of packages.
In addition, the env variable is not present in a safe interpreter, so
it cannot share environment variables with other interpreters. The env
variable poses a security risk, because users can store sensitive in-
formation in an environment variable. For example, the PGP manual rec-
ommends storing the PGP private key protection password in the environ-
ment 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 re-
strict their own functionality to eliminate unsafe commands. For a dis-
cussion 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 in an untrusted script which is being executed in a safe inter-
preter even if the target of the alias is not a safe interpreter. The
most important thing in guaranteeing safety is to ensure that informa-
tion passed from the child to the parent is never evaluated or substi-
tuted in the parent; if this were to occur, it would enable an evil
script in the child to invoke arbitrary functions in the parent, which
would compromise security.
When the source for an alias is invoked in the child interpreter, the
usual Tcl substitutions are performed when parsing that command. These
substitutions are carried out in the source interpreter just as they
would be for any other command invoked in that interpreter. The 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 di-
rectly, without going through the normal Tcl evaluation mechanism.
Substitutions are thus performed on each word exactly once: targetCmd
and args were substituted when parsing the command that created the
alias, and arg1 - argN are substituted when the alias's source command
is parsed in the source interpreter.
When writing the targetCmds for aliases in safe interpreters, it is
very important that the arguments to that command never be evaluated or
substituted, since this would provide an escape mechanism whereby the
child interpreter could execute arbitrary code in the parent. This in
turn would compromise the security of the system.
HIDDEN COMMANDS
Safe interpreters greatly restrict the functionality available to Tcl
programs executing within them. Allowing the untrusted Tcl program to
have direct access to this functionality is unsafe, because it can be
used for a variety of attacks on the environment. However, there are
times when there is a legitimate need to use the dangerous 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 in-
terpreter; 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 un-
available to Tcl scripts executing in the interpreter. However, such
hidden commands can be invoked by any trusted ancestor of the safe in-
terpreter, in the context of the safe interpreter, using interp invoke.
Hidden commands and exposed commands reside in separate name spaces. It
is possible to define a hidden command and an exposed command by the
same name within one interpreter.
Hidden commands in a child interpreter can be invoked in the body of
procedures called in the parent during alias invocation. For example,
an alias for source could be created in a child interpreter. When it is
invoked in the child interpreter, a procedure is called in the parent
interpreter to check that the operation is allowable (e.g. it asks to
source a file that the child interpreter is allowed to access). The
procedure then it invokes the hidden source command in the child inter-
preter to actually source in the contents of the file. Note that two
commands named source exist in the child interpreter: the alias, and
the hidden command.
Because a parent interpreter may invoke a hidden command as part of
handling an alias invocation, great care must be taken to avoid evalu-
ating any arguments passed in through the alias invocation. Otherwise,
malicious child interpreters could cause a trusted parent interpreter
to execute dangerous commands on their behalf. See the section on ALIAS
INVOCATION for a more complete discussion of this topic. To help avoid
this problem, no substitutions or evaluations are applied to arguments
of interp invokehidden.
Safe interpreters are not allowed to invoke hidden commands in them-
selves or in their descendants. This prevents them from gaining access
to hidden functionality in themselves or their descendants.
The set of hidden commands in an interpreter can be manipulated by a
trusted interpreter using interp expose and interp hide. The interp ex-
pose 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 children from fooling a parent 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 parent interpreter upon its children. Command limits (of type
command) restrict the total number of Tcl commands that may be executed
by an interpreter (as can be inspected via the info cmdcount command),
and time limits (of type time) place a limit by which execution within
the interpreter must complete. Note that time limits are expressed as
absolute times (as in clock seconds) and not relative times (as in af-
ter) because they may be modified after creation.
When a limit is exceeded for an interpreter, first any handler call-
backs defined by parent interpreters are called. If those callbacks in-
crease 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
exception, it is reported through the background exception mech-
anism (see BACKGROUND EXCEPTION HANDLING). Note that the call-
backs 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 mo-
ment 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 op-
tion may be the empty string, which indicates that a command
limit is not set for the interpreter.
Where an interpreter with a resource limit set on it creates a child
interpreter, that child interpreter will have resource limits imposed
on it that are at least as restrictive as the limits on the creating
parent interpreter. If the parent interpreter of the limited parent
wishes to relax these conditions, it should hide the interp command in
the child and then use aliases and the interp invokehidden subcommand
to provide such access as it chooses to the interp command to the lim-
ited parent as necessary.
BACKGROUND EXCEPTION HANDLING
When an exception happens in a situation where it cannot be reported
directly up the stack (e.g. when processing events in an update or
vwait call) the exception is instead reported through the background
exception handling mechanism. Every interpreter has a background ex-
ception handler registered; the default exception handler arranges for
the bgerror command in the interpreter's global namespace to be called,
but other exception handlers may be installed and process background
exceptions in substantially different ways.
A background exception handler consists of a non-empty list of words to
which will be appended two further words at invocation time. The first
word will be the interpreter result at time of the exception, typically
an error message, and the second will be the dictionary of return op-
tions at the time of the exception. These are the same values that
catch can capture when it controls script evaluation in a non-back-
ground situation. The resulting list will then be executed in the in-
terpreter's global namespace without further substitutions being per-
formed.
CREDITS
The safe interpreter mechanism is based on the Safe-Tcl prototype im-
plemented 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(3tcl), load(3tcl), safe(3tcl), Tcl_CreateChild(3tcl),
Tcl_Eval(3tcl), Tcl_BackgroundException(3tcl)
KEYWORDS
alias, parent interpreter, safe interpreter, child interpreter
Tcl 8.6 interp(3tcl)
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