path: root/doc/interp.n

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'\" Copyright (c) 2004 Donal K. Fellows
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'\" RCS: @(#) $Id: interp.n,v 1.12 2004/05/29 18:50:33 jenglish Exp $
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.so man.macros
.TH interp n 7.6 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
interp \- Create and manipulate Tcl interpreters
.SH SYNOPSIS
\fBinterp \fIoption \fR?\fIarg arg ...\fR?
.BE

.SH DESCRIPTION
.PP
This command makes it possible to create one or more new Tcl 
interpreters that co-exist with the creating interpreter in the
same application.  The creating interpreter is called the \fImaster\fR
and the new interpreter is called a \fIslave\fR. 
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.
.PP
Each interpreter is independent from the others: it has its own name
space for commands, procedures, and global variables.
A master interpreter may create connections between its slaves and
itself using a mechanism called an \fIalias\fR.  An \fIalias\fR is
a command in a slave interpreter which, when invoked, causes a
command to be invoked in its master interpreter or in another slave
interpreter.  The only other connections between interpreters are
through environment variables (the \fBenv\fR variable), which are
normally shared among all interpreters in the application,
.VS 8.5
and by resource limit exceeded callbacks.
.VE
Note that the
name space for files (such as the names returned by the \fBopen\fR command)
is no longer shared between interpreters. Explicit commands are provided to
share files and to transfer references to open files from one interpreter
to another.
.PP
The \fBinterp\fR command also provides support for \fIsafe\fR
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 damaging 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 interpreter.
The dangerous functionality is not removed from the safe interpreter;
instead, it is \fIhidden\fR, 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 slave interpreter and its master. 
See ALIAS INVOCATION, below, for more details 
on how the alias mechanism works.
.PP
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 \fBa\fR is a slave of
the current interpreter and it has a slave \fBa1\fR, which in turn has a
slave \fBa11\fR, the qualified name of \fBa11\fR in \fBa\fR is the list
\fBa1 a11\fR.
.PP
The \fBinterp\fR command, described below, accepts qualified interpreter
names as arguments; the interpreter in which the command is being evaluated
can always be referred to as \fB{}\fR (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.
.SH "THE INTERP COMMAND"
.PP
The \fBinterp\fR 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 \fIoption\fR argument:
.TP
\fBinterp\fR \fBalias\fR \fIsrcPath\fR \fIsrcCmd\fR
Returns a Tcl list whose elements are the \fItargetCmd\fR and
\fIarg\fRs associated with the alias named \fIsrcCmd\fR
(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 \fIsrcCmd\fR if it was renamed).
.TP
\fBinterp\fR \fBalias\fR \fIsrcPath\fR \fIsrcCmd\fR \fB{}\fR
Deletes the alias for \fIsrcCmd\fR in the slave interpreter identified by
\fIsrcPath\fR.
\fIsrcCmd\fR refers to the name under which the alias
was created;  if the source command has been renamed, the renamed
command will be deleted.
.TP
\fBinterp\fR \fBalias\fR \fIsrcPath\fR \fIsrcCmd\fR \fItargetPath\fR \fItargetCmd \fR?\fIarg arg ...\fR?
This command creates an alias between one slave and another (see the
\fBalias\fR 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.
\fISrcPath\fR and \fIsrcCmd\fR identify the source of the alias.
\fISrcPath\fR is a Tcl list whose elements select a particular
interpreter.  For example, ``\fBa b\fR'' identifies an interpreter
\fBb\fR, which is a slave of interpreter \fBa\fR, which is a slave
of the invoking interpreter.  An empty list specifies the interpreter
invoking the command.  \fIsrcCmd\fR gives the name of a new
command, which will be created in the source interpreter.
\fITargetPath\fR and \fItargetCmd\fR specify a target interpreter
and command, and the \fIarg\fR arguments, if any, specify additional
arguments to \fItargetCmd\fR which are prepended to any arguments specified
in the invocation of \fIsrcCmd\fR.
\fITargetCmd\fR 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.
.TP
\fBinterp\fR \fBaliases \fR?\fIpath\fR?
This command returns a Tcl list of the names of all the source commands for
aliases defined in the interpreter identified by \fIpath\fR.
.TP
\fBinterp\fR \fBcreate \fR?\fB\-safe\fR? ?\fB\-\|\-\fR? ?\fIpath\fR?
Creates a slave interpreter identified by \fIpath\fR and a new command,
called a \fIslave command\fR. The name of the slave command is the last
component of \fIpath\fR. The new slave interpreter and the slave command
are created in the interpreter identified by the path obtained by removing
the last component from \fIpath\fR. For example, if \fIpath is \fBa b
c\fR then a new slave interpreter and slave command named \fBc\fR are
created in the interpreter identified by the path \fBa b\fR.
The slave command may be used to manipulate the new interpreter as
described below. If \fIpath\fR is omitted, Tcl creates a unique name of the
form \fBinterp\fIx\fR, where \fIx\fR is an integer, and uses it for the
interpreter and the slave command. If the \fB\-safe\fR 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 variables. The \fB\-\|\-\fR switch can be used to
mark the end of switches;  it may be needed if \fIpath\fR is an unusual
value such as \fB\-safe\fR. 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.
.TP
\fBinterp\fR \fBdelete \fR?\fIpath ...?\fR
Deletes zero or more interpreters given by the optional \fIpath\fR
arguments, and for each interpreter, it also deletes its slaves. The
command also deletes the slave command for each interpreter deleted.
For each \fIpath\fR argument, if no interpreter by that name
exists, the command raises an error.
.TP
\fBinterp\fR \fBeval\fR \fIpath arg \fR?\fIarg ...\fR?
This command concatenates all of the \fIarg\fR arguments in the same
fashion as the \fBconcat\fR command, then evaluates the resulting string as
a Tcl script in the slave interpreter identified by \fIpath\fR. The result
of this evaluation (including error information such as the \fBerrorInfo\fR
and \fBerrorCode\fR variables, if an error occurs) is returned to the
invoking interpreter.
.TP
\fBinterp exists \fIpath\fR
Returns  \fB1\fR if a slave interpreter by the specified \fIpath\fR
exists in this master, \fB0\fR otherwise. If \fIpath\fR is omitted, the
invoking interpreter is used.
.TP
\fBinterp expose \fIpath\fR \fIhiddenName\fR ?\fIexposedCmdName\fR?
Makes the hidden command \fIhiddenName\fR exposed, eventually bringing
it back under a new \fIexposedCmdName\fR name (this name is currently
accepted only if it is a valid global name space name without any ::),
in the interpreter
denoted by \fIpath\fR.
If an exposed command with the targeted name already exists, this command
fails.
Hidden commands are explained in more detail in HIDDEN COMMANDS, below.
.TP
\fBinterp\fR \fBhide\fR \fIpath\fR \fIexposedCmdName\fR ?\fIhiddenCmdName\fR?
Makes the exposed command \fIexposedCmdName\fR hidden, renaming
it to the hidden command \fIhiddenCmdName\fR, or keeping the same name if
\fIhiddenCmdName\fR is not given, in the interpreter denoted 
by \fIpath\fR.
If a hidden command with the targeted name already exists, this command
fails.
Currently both \fIexposedCmdName\fR and \fIhiddenCmdName\fR can 
not contain namespace qualifiers, or an error is raised.
Commands to be hidden by \fBinterp hide\fR 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.
Hidden commands are explained in more detail in HIDDEN COMMANDS, below.
.TP
\fBinterp\fR \fBhidden\fR \fIpath\fR
Returns a list of the names of all hidden commands in the interpreter
identified by \fIpath\fR.
.TP
\fBinterp\fR \fBinvokehidden\fR \fIpath\fR ?\fB-global\fR? \fIhiddenCmdName\fR ?\fIarg ...\fR?
Invokes the hidden command \fIhiddenCmdName\fR with the arguments supplied
in the interpreter denoted by \fIpath\fR. No substitutions or evaluation
are applied to the arguments.
If the \fB-global\fR 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 HIDDEN COMMANDS, below.
.TP
\fBinterp\fR \fBlimit\fR \fIpath\fR \fIlimitType\fR ?\fIoption\fR? ?\fIvalue\fR \fI...\fR?
.VS 8.5
Sets up, manipulates and queries the configuration of the resource
limit \fIlimitType\fR for the interpreter denoted by \fIpath\fR.  If
no \fIoption\fR is specified, return the current configuration of the
limit.  If \fIoption\fR is the sole argument, return the value of that
option.  Otherwise, a list of \fIoption\fR/\fIvalue\fR argument pairs
must supplied. See RESOURCE LIMITS below for a more detailed
explanation of what limits and options are supported.
.VE 8.5
.TP
\fBinterp issafe\fR ?\fIpath\fR?
Returns \fB1\fR if the interpreter identified by the specified \fIpath\fR
is safe, \fB0\fR otherwise.
.TP
\fBinterp marktrusted\fR \fIpath\fR
Marks the interpreter identified by \fIpath\fR 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 \fIpath\fR is
already trusted.
.TP
\fBinterp\fR \fBrecursionlimit\fR \fIpath\fR ?\fInewlimit\fR?
Returns the maximum allowable nesting depth for the interpreter
specified by \fIpath\fR.  If \fInewlimit\fR is specified,
the interpreter recursion limit will be set so that nesting
of more than \fInewlimit\fR calls to \fBTcl_Eval()\fR
and related procedures in that interpreter will return an error.
The \fInewlimit\fR value is also returned.
The \fInewlimit\fR value must be a positive integer between 1 and the
maximum value of a non-long integer on the platform.  
.sp
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. 
.TP
\fBinterp\fR \fBshare\fR \fIsrcPath channelId destPath\fR
Causes the IO channel identified by \fIchannelId\fR to become shared
between the interpreter identified by \fIsrcPath\fR and the interpreter
identified by \fIdestPath\fR. 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.
.TP
\fBinterp\fR \fBslaves\fR ?\fIpath\fR?
Returns a Tcl list of the names of all the slave interpreters associated
with the interpreter identified by \fIpath\fR. If \fIpath\fR is omitted,
the invoking interpreter is used.
.TP
\fBinterp\fR \fBtarget\fR \fIpath alias\fR
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 \fBinterp alias\fR above. The name of the target interpreter is
returned as an interpreter path, relative 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.
.TP
\fBinterp\fR \fBtransfer\fR \fIsrcPath channelId destPath\fR
Causes the IO channel identified by \fIchannelId\fR to become available in
the interpreter identified by \fIdestPath\fR and unavailable in the
interpreter identified by \fIsrcPath\fR.
.SH "SLAVE COMMAND"
.PP
For each slave interpreter created with the \fBinterp\fR 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:
.CS
\fIslave command \fR?\fIarg arg ...\fR?
.CE
\fISlave\fR is the name of the interpreter, and \fIcommand\fR
and the \fIarg\fRs determine the exact behavior of the command.
The valid forms of this command are:
.TP
\fIslave \fBaliases\fR
Returns a Tcl list whose elements are the names of all the
aliases in \fIslave\fR.  The names returned are the \fIsrcCmd\fR
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).
.TP
\fIslave \fBalias \fIsrcCmd\fR
Returns a Tcl list whose elements are the \fItargetCmd\fR and
\fIarg\fRs associated with the alias named \fIsrcCmd\fR
(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 \fIsrcCmd\fR if it was renamed).
.TP
\fIslave \fBalias \fIsrcCmd \fB{}\fR
Deletes the alias for \fIsrcCmd\fR in the slave interpreter.
\fIsrcCmd\fR refers to the name under which the alias
was created;  if the source command has been renamed, the renamed
command will be deleted.
.TP
\fIslave \fBalias \fIsrcCmd targetCmd \fR?\fIarg ..\fR?
Creates an alias such that whenever \fIsrcCmd\fR is invoked
in \fIslave\fR, \fItargetCmd\fR is invoked in the master.
The \fIarg\fR arguments will be passed to \fItargetCmd\fR as additional
arguments, prepended before any arguments passed in the invocation of
\fIsrcCmd\fR.
See ALIAS INVOCATION below for details.
.TP
\fIslave \fBeval \fIarg \fR?\fIarg ..\fR?
This command concatenates all of the \fIarg\fR arguments in
the same fashion as the \fBconcat\fR command, then evaluates
the resulting string as a Tcl script in \fIslave\fR.
The result of this evaluation (including error information
such as the \fBerrorInfo\fR and \fBerrorCode\fR variables, if an
error occurs) is returned to the invoking interpreter.
.TP
\fIslave \fBexpose \fIhiddenName \fR?\fIexposedCmdName\fR?
This command exposes the hidden command \fIhiddenName\fR, eventually bringing
it back under a new \fIexposedCmdName\fR name (this name is currently
accepted only if it is a valid global name space name without any ::),
in \fIslave\fR.
If an exposed command with the targeted name already exists, this command
fails.
For more details on hidden commands, see HIDDEN COMMANDS, below.
.TP
\fIslave \fBhide \fIexposedCmdName\fR ?\fIhiddenCmdName\fR?
This command hides the exposed command \fIexposedCmdName\fR, renaming it to 
the hidden command \fIhiddenCmdName\fR, or keeping the same name if the
argument is not given, in the \fIslave\fR interpreter.
If a hidden command with the targeted name already exists, this command
fails.
Currently both \fIexposedCmdName\fR and \fIhiddenCmdName\fR 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 HIDDEN COMMANDS, below.
.TP
\fIslave \fBhidden\fR
Returns a list of the names of all hidden commands in \fIslave\fR.
.TP
\fIslave \fBinvokehidden\fR ?\fB-global\fR \fIhiddenName \fR?\fIarg ..\fR?
This command invokes the hidden command \fIhiddenName\fR with the
supplied arguments, in \fIslave\fR. No substitutions or evaluations are
applied to the arguments.
If the \fB-global\fR 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 HIDDEN COMMANDS, below.
.TP
\fIslave \fBissafe\fR
Returns  \fB1\fR if the slave interpreter is safe, \fB0\fR otherwise.
.TP
\fIslave \fBlimit\fR \fIlimitType\fR ?\fIoption\fR? ?\fIvalue\fR \fI...\fR?
.VS 8.5
Sets up, manipulates and queries the configuration of the resource
limit \fIlimitType\fR for the slave interpreter.  If no \fIoption\fR
is specified, return the current configuration of the limit.  If
\fIoption\fR is the sole argument, return the value of that option.
Otherwise, a list of \fIoption\fR/\fIvalue\fR argument pairs must
supplied. See RESOURCE LIMITS below for a more detailed explanation of
what limits and options are supported.
.VE 8.5
.TP
\fIslave \fBmarktrusted\fR
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.
.TP
\fIslave\fR \fBrecursionlimit\fR ?\fInewlimit\fR?
Returns the maximum allowable nesting depth for the \fIslave\fR interpreter.
If \fInewlimit\fR is specified, the recursion limit in \fIslave\fR will be
set so that nesting of more than \fInewlimit\fR calls to \fBTcl_Eval()\fR
and related procedures in \fIslave\fR will return an error.
The \fInewlimit\fR value is also returned.
The \fInewlimit\fR value must be a positive integer between 1 and the
maximum value of a non-long integer on the platform.  
.sp
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. 
.SH "SAFE INTERPRETERS"
.PP
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
\fBexec\fR 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 subprocess invocation might be allowed for a carefully selected and
fixed set of programs.
.PP
A safe interpreter is created by specifying the \fB\-safe\fR switch
to the \fBinterp create\fR command.  Furthermore, any slave created
by a safe interpreter will also be safe.
.PP
A safe interpreter is created with exactly the following set of
built-in commands:
.DS
.ta 1.2i 2.4i 3.6i
\fBafter	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\fR
.DE
The following commands are hidden by \fBinterp create\fR when it
creates a safe interpreter:
.DS
.ta 1.2i 2.4i 3.6i
\fBcd	encoding	exec	exit
fconfigure	file	glob	load
open	pwd	socket	source\fR
.DE
These commands can be recreated later as Tcl procedures or aliases, or
re-exposed by \fBinterp expose\fR.
.PP
The following commands from Tcl's library of support procedures are
not present in a safe interpreter:
.DS
.ta 1.6i 3.2i
\fBauto_exec_ok	auto_import	auto_load
auto_load_index	auto_qualify	unknown\fR
.DE
Note in particular that safe interpreters have no default \fBunknown\fR
command, so Tcl's default autoloading facilities are not available.  
Autoload access to Tcl's commands that are normally autoloaded:
.DS
.ta 2.1i
\fB
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\fR
.DE
can only be provided by explicit definition of an \fBunknown\fR command
in the safe interpreter.  This will involve exposing the \fBsource\fR
command.  This is most easily accomplished by creating the safe interpreter
with Tcl's \fBSafe\-Tcl\fR mechanism.  \fBSafe\-Tcl\fR provides safe
versions of \fBsource\fR, \fBload\fR, and other Tcl commands needed
to support autoloading of commands and the loading of packages.
.PP
In addition, the \fBenv\fR variable is not present in a safe interpreter,
so it cannot share environment variables with other interpreters. The
\fBenv\fR 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 environment variable \fIPGPPASS\fR. Making this variable available
to untrusted code executing in a safe interpreter would incur a
security risk.
.PP
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
\fBSafe\-Tcl\fR and the \fBload\fR Tcl command.
.PP
A safe interpreter may not alter the recursion limit of any interpreter,
including itself.
.SH "ALIAS INVOCATION"
.PP
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.
.PP
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 command procedure for the source command takes its arguments
and merges them with the \fItargetCmd\fR and \fIarg\fRs for the
alias to create a new array of arguments.  If the words
of \fIsrcCmd\fR were ``\fIsrcCmd arg1 arg2 ... argN\fR'',
the new set of words will be
``\fItargetCmd arg arg ... arg arg1 arg2 ... argN\fR'',
where \fItargetCmd\fR and \fIarg\fRs are the values supplied when the
alias was created.  \fITargetCmd\fR 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 \fItargetCmd\fR 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:
\fItargetCmd\fR and \fIargs\fR were substituted when parsing the command
that created the alias, and \fIarg1 - argN\fR are substituted when
the alias's source command is parsed in the source interpreter.
.PP
When writing the \fItargetCmd\fRs 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.
.SH "HIDDEN COMMANDS"
.PP
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 functionality in the context of the safe interpreter. For
example, sometimes a program must be \fBsource\fRd 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 management, must be performed on these windows within the
interpreter context.
.PP
The \fBinterp\fR command provides a solution to this problem in the form of
\fIhidden commands\fR. 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 \fBinterp
invoke\fR. 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.
.PP
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 \fBsource\fR 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 \fBsource\fR command in the slave
interpreter to actually source in the contents of the file. Note that two
commands named \fBsource\fR exist in the slave interpreter: the alias, and
the hidden command.
.PP
Because a master interpreter may invoke a hidden command as part of
handling an alias invocation, great care must be taken to avoid evaluating
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 \fBinterp invokehidden\fR.
.PP
Safe interpreters are not allowed to invoke hidden commands in themselves
or in their descendants. This prevents safe slaves from gaining access to
hidden functionality in themselves or their descendants.
.PP
The set of hidden commands in an interpreter can be manipulated by a trusted
interpreter using \fBinterp expose\fR and \fBinterp hide\fR. The \fBinterp
expose\fR command moves a hidden command to the
set of exposed commands in the interpreter identified by \fIpath\fR,
potentially renaming the command in the process. If an exposed command by
the targeted name already exists, the operation fails. Similarly,
\fBinterp hide\fR 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 commands, in either themselves or
their descendants.
.PP
Currently, the names of hidden commands cannot contain namespace
qualifiers, and you must first rename a command in a namespace to the
global namespace before you can hide it.
Commands to be hidden by \fBinterp hide\fR 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.
.SH "RESOURCE LIMITS"
.VS 8.5
.PP
Every interpreter has two kinds of resource limits that may be imposed
by any master interpreter upon its slaves.  Command limits (of type
\fBcommand\fR) restrict the total number of Tcl commands that may be
executed by an interpreter (as can be inspected via the \fBinfo
cmdcount\fR command), and time limits (of type \fBtime\fR) place a
limit by which execution within the interpreter must complete.
.PP
When a limit is exceeded for an interpreter, first any handler
callbacks defined by master interpreters are called.  If those
callbacks increase or remove the limit, execution within the
(previously) limited interpreter continues.  If the limit is still in
force, an error is generated at that point and normal processing of
errors within the interpreter (by the \fBcatch\fR command) is
disabled, so the error propagates outwards (building a stack-trace as
it goes) to the point where the limited interpreter was invoked
(e.g. by \fBinterp eval\fR) where it becomes the responsibility of the
calling code to catch and handle.
.PP
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
particular to the kind of limit.
.TP
\fB\-command\fR
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 particular limit in the
limited interpreter is exceeded.  The callback may modify the limit on
the interpreter if it wishes the limited interpreter to continue
executing.  If the callback generates an error, it is reported through
the background error mechansism (see \fBbgerror\fR).  Note that the
callbacks defined by one interpreter are completely isolated from the
callbacks defined by another, and that the order in which those
callbacks are called is undefined.
.TP
\fB\-granularity\fR
This option (common for all limit types) specifies how frequently (out
of the points when the Tcl interpreter is in a consistent 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 overhead (which may be substantial in the
case of time limits) is incurred.
.TP
\fB\-milliseconds\fR
This option specifies the number of milliseconds after the moment
defined in the \fB\-seconds\fR option that the time limit will fire.
It should only ever be specified in conjunction with the
\fB\-seconds\fR option (whether it was set previously or is being set
this invokation.)
.TP
\fB\-seconds\fR
This option specifies the number of seconds after the epoch (see
\fBclock seconds\fR) that the time limit for the interpreter will be
triggered. The limit will be triggered at the start of the second
unless specified at a sub-second level using the \fB\-milliseconds\fR
option.  This option may be the empty string, which indicates that a
time limit is not set for the interpreter.
.TP
\fB\-value\fR
This option specifies the number of commands that the interpreter may
execute before triggering the command limit.  This option may be the
empty string, which indicates that a command limit is not set for the
interpreter.
.VE 8.5
.SH CREDITS
This mechanism is based on the Safe-Tcl prototype implemented
by Nathaniel Borenstein and Marshall Rose.
.SH EXAMPLES
Creating and using an alias for a command in the current interpreter:
.CS
interp alias {} getIndex {} lsearch {alpha beta gamma delta}
set idx [getIndex delta]
.CE

.VS 8.5
Executing an arbitrary command in a safe interpreter where every
invokation of \fBlappend\fR is logged:
.CS
set i [interp create -safe]
$i hide lappend
$i alias lappend loggedLappend $i
proc loggedLappend {i args} {
   puts "logged invokation of lappend $args"
   $i invokehidden lappend {expand}$args
}
$i eval $someUntrustedScript
.CE

Setting a resource limit on an interpreter so that an infinite loop
terminates.
.CS
set i [interp create]
$i limit command -value 1000
$i eval {
   set x 0
   while {1} {
      puts "Counting up... [incr x]"
   }
}
.CE
.VE 8.5

.SH "SEE ALSO"
load(n), safe(n), Tcl_CreateSlave(3)

.SH KEYWORDS
alias, master interpreter, safe interpreter, slave interpreter