interp(n) Tcl Built-In Commands interp(n)
NAME
interp - Create and manipulate Tcl interpreters
SYNOPSIS
interp option ?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. Note that the name space for files (such as the names
returned by the open command) is no longer shared between interpreters.
Explicit commands are provided to share files and to transfer refer-
ences 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 HIDEN COMANDS, 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 COMAND
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 option argument:
interp alias srcPath srcCmd
Returns a Tcl list whose elements are the targetCmd and args
associated with the alias named srcCmd (all of these are the
values specified when the alias was created; it is possible that
the actual source command in the slave is different from srcCmd
if it was renamed).
interp alias srcPath srcCmd {{}}
Deletes the alias for srcCmd in the slave interpreter identified
by srcPath. srcCmd refers to the name under which 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.
interp aliases ?path?
This command returns a Tcl list of the names of all the source
commands for aliases defined in the interpreter identified by
path.
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 error information
such as the errorInfo and errorCode variables, if an error
occurs) is returned to the invoking interpreter.
interp exists path
Returns 1 if a slave interpreter by the specified path exists
in this master, 00 otherwise. If path is omitted, the invoking
interpreter is used.
interp expose path hiddenName ?exposedCmd-
Name?
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 HIDEN COMANDS,
below.
interp hide path exposedCmdName ?hiddenCmd-
Name?
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 HIDEN COMANDS, below.
interp hidden
path
Returns a list of the names of all hidden commands in the inter-
preter identified by path.
interp invokehidden path ?-global? 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. 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. Hidden commands are explained in more detail in HIDEN
COMANDS, below.
interp issafe ?path?
Returns 1 if the interpreter identified by the specified path is
safe, 00 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 TclEval() 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 COMAND
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 names of all the
aliases in slave. The names returned are the srcCmd values used
when the aliases were created (which may not be the same as the
current names of the commands, if they have been renamed).
slave alias srcCmd
Returns a Tcl list whose elements are the targetCmd and args
associated with the alias named srcCmd (all of these are the
values specified when the alias was created; it is possible that
the actual source command in the slave is different from srcCmd
if it was renamed).
slave alias srcCmd {{}}
Deletes the alias for srcCmd in the slave interpreter. srcCmd
refers to the name under which 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.
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 error information such as the errorInfo and errorCode
variables, if an error occurs) is returned to the invoking
interpreter.
slave expose hiddenName ?exposedCmd-
Name?
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 HIDEN COMANDS, below.
slave hide exposedCmdName ?hiddenCmd-
Name?
This command hides the exposed command exposedCmdName, renaming
it to the hidden command hiddenCmdName, or keeping the same name
if the the argument is not given, in the slave interpreter. If
a hidden command with the targeted name already exists, this
command fails. Currently both exposedCmdName and hiddenCmdName
can not contain namespace qualifiers, or an error is raised.
Commands to be hidden are looked up in the global namespace even
if the current namespace is not the global one. This prevents
slaves from fooling a master interpreter into hiding the wrong
command, by making the current namespace be different from the
global one. For more details on hidden commands, see HIDEN
COMANDS, below.
slave hid-
den
Returns a list of the names of all hidden commands in slave.
slave invokehidden ?-global 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. 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. For more details on
hidden commands, see HIDEN COMANDS, below.
slave issafe
Returns 1 if the slave interpreter is safe, 00 otherwise.
slave mark-
trusted
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
TclEval() 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 array binary
break case catch clock
close concat continue eof
error eval expr fblocked
fcopy fileevent flush for
foreach format gets global
if incr info interp
join lappend lindex linsert
list llength lrange lreplace
lsearch 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
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:
autoexecok autoimport autoload
autoloadindex autoqualify unknown
Note in particular that safe interpreters have no default unknown com-
mand, so Tcl's default autoloading facilities are not available.
Autoload access to Tcl's commands that are normally autoloaded:
automkindex automkindexold
autoreset history
parray pkgmkIndex
::::pkg::::create ::::safe::::interpAddToAccessPath
::::safe::::interpCreate ::::safe::::interpConfigure
::::safe::::interpDelete ::::safe::::interpFindInAccessPath
::::safe::::interpInit ::::safe::::setLogCmd
tclendOfWord tclfindLibrary
tclstartOfNextWord tclstartOfPreviousWord
tclwordBreakAfter tclwordBreakBefore
can only be provided by explicit definition of an unknown command in
the safe interpreter. This will involve exposing the source command.
This is most easily accomplished by creating the safe interpreter with
Tcl's Safe-Tcl mechanism. Safe-Tcl provides safe versions of source,
load, and other Tcl commands needed to support autoloading of commands
and the loading of packages.
In addition, the env variable is not present in a safe interpreter, so
it cannot share environment variables with other interpreters. The env
variable poses a security risk, because users can store sensitive
information in an environment variable. For example, the PGP manual
recommends storing the PGP private key protection password in the envi-
ronment variable PGPAS. 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.
HIDEN COMANDS
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.
CREDITS
This mechanism is based on the Safe-Tcl prototype implemented by
Nathaniel Borenstein and Marshall Rose.
SEE ALSO
load(n), safe(n), TclCreateSlave(3)
KEYWORDS
alias, master interpreter, safe interpreter, slave interpreter
Tcl 7.6 interp(n)
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