path: root/doc/interp.n

'\"
'\" Copyright (c) 1995-1996 Sun Microsystems, Inc.
'\" Copyright (c) 2004 Donal K. Fellows
'\" Copyright (c) 2006-2008 Joe Mistachkin.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.TH interp n 8.6 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
interp \- Create and manipulate Tcl interpreters
.SH SYNOPSIS
\fBinterp \fIsubcommand \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 \fIparent\fR
and the new interpreter is called a \fIchild\fR.
A parent can create any number of children, and each child can
itself create additional children for which it is parent, 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 parent interpreter may create connections between its children and
itself using a mechanism called an \fIalias\fR.  An \fIalias\fR is
a command in a child interpreter which, when invoked, causes a
command to be invoked in its parent interpreter or in another child
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,
and by resource limit exceeded callbacks. 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 child 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 \fBSAFE INTERPRETERS\fR 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
\fBHIDDEN COMMANDS\fR, below.
The alias mechanism can be used for protected communication (analogous to a
kernel call) between a child interpreter and its parent.
See \fBALIAS INVOCATION\fR, below, for more details
on how the alias mechanism works.
.PP
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
.QW \fBa\fR
is a child of the current interpreter and it has a child
.QW \fBa1\fR ,
which in turn has a child
.QW \fBa11\fR ,
the qualified name of
.QW \fBa11\fR
in
.QW \fBa\fR
is the list
.QW "\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 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.
.SH "THE INTERP COMMAND"
.PP
The \fBinterp\fR command is used to create, delete, and manipulate
child interpreters, and to share or transfer
channels between interpreters.  It can have any of several forms, depending
on the \fIsubcommand\fR argument:
.\" METHOD: alias
.TP
\fBinterp alias\fI srcPath srcToken\fR
.
Returns a Tcl list whose elements are the \fItargetCmd\fR and
\fIarg\fRs associated with the alias represented by \fIsrcToken\fR
(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 \fIsrcToken\fR).
.TP
\fBinterp alias\fI srcPath srcToken\fR \fB{}\fR
.
Deletes the alias for \fIsrcToken\fR in the child interpreter identified by
\fIsrcPath\fR.
\fIsrcToken\fR refers to the value returned when the alias
was created;  if the source command has been renamed, the renamed
command will be deleted.
.TP
\fBinterp alias\fI srcPath srcCmd targetPath targetCmd \fR?\fIarg arg ...\fR?
.
This command creates an alias between one child and another (see the
\fBalias\fR child command below for creating aliases between a child
and its parent).  In this command, either of the child 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,
.QW "\fBa b\fR"
identifies an interpreter
.QW \fBb\fR ,
which is a child of interpreter
.QW \fBa\fR ,
which is a child 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 \fBALIAS INVOCATION\fR below for
more details.
The command returns a token that uniquely identifies the command created
\fIsrcCmd\fR, even if the command is renamed afterwards. The token may but
does not have to be equal to \fIsrcCmd\fR.
.\" METHOD: aliases
.TP
\fBinterp aliases \fR?\fIpath\fR?
.
This command returns a Tcl list of the tokens of all the source commands for
aliases defined in the interpreter identified by \fIpath\fR. 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).
.\" METHOD: bgerror
.TP
\fBinterp bgerror \fIpath\fR ?\fIcmdPrefix\fR?
.
This command either gets or sets the current background exception handler
for the interpreter identified by \fIpath\fR. If \fIcmdPrefix\fR 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
\fBBACKGROUND EXCEPTION HANDLING\fR section for more details.
.\" METHOD: cancel
.TP
\fBinterp cancel \fR?\fB\-unwind\fR? ?\fB\-\|\-\fR? ?\fIpath\fR? ?\fIresult\fR?
.
Cancels the script being evaluated in the interpreter identified by
\fIpath\fR. Without the \fB\-unwind\fR 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 \fB\-unwind\fR switch the evaluation stack for the
interpreter is unwound without regard to any intervening catch command
until there are no further invocations of the interpreter left on the
call stack. 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. If \fIresult\fR is present, it will be used as the
error message string; otherwise, a default error message string will be
used.
.\" METHOD: children
.TP
\fBinterp children\fR ?\fIpath\fR?
.
Returns a Tcl list of the names of all the child interpreters associated
with the interpreter identified by \fIpath\fR. If \fIpath\fR is omitted,
the invoking interpreter is used.
.\" METHOD: create
.TP
\fBinterp create \fR?\fB\-safe\fR? ?\fB\-\|\-\fR? ?\fIpath\fR?
.
Creates a child interpreter identified by \fIpath\fR and a new command,
called a \fIchild command\fR. The name of the child command is the last
component of \fIpath\fR. The new child interpreter and the child command
are created in the interpreter identified by the path obtained by removing
the last component from \fIpath\fR. For example, if \fIpath\fR is \fBa b
c\fR then a new child interpreter and child command named \fBc\fR are
created in the interpreter identified by the path \fBa b\fR.
The child 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 child command. If the \fB\-safe\fR switch is specified
(or if the parent interpreter is a safe interpreter), 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 \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 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.
.\" METHOD: debug
.TP
\fBinterp debug \fIpath\fR ?\fB\-frame\fR ?\fIbool\fR??
.
Controls whether frame-level stack information is captured in the
child interpreter identified by \fIpath\fR.  If no arguments are
given, option and current setting are returned.  If \fB\-frame\fR
is given, the debug setting is set to the given boolean if provided
and the current setting is returned.
This only affects the output of \fBinfo frame\fR, in that exact
frame-level information for command invocation at the bytecode level
is only captured with this setting on.
.RS
.PP
For example, with code like
.PP
.CS
\fBproc\fR mycontrol {... script} {
  ...
  \fBuplevel\fR 1 $script
  ...
}

\fBproc\fR dosomething {...} {
  ...
  mycontrol {
    somecode
  }
}
.CE
.PP
the standard setting will provide a relative line number for the
command \fBsomecode\fR and the relevant frame will be of type
\fBeval\fR. 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
\fBsource\fR. This more exact information is paid for with slower
execution of all commands.
.PP
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.
.RE
.\" METHOD: delete
.TP
\fBinterp delete \fR?\fIpath ...\fR?
.
Deletes zero or more interpreters given by the optional \fIpath\fR
arguments, and for each interpreter, it also deletes its children. The
command also deletes the child command for each interpreter deleted.
For each \fIpath\fR argument, if no interpreter by that name
exists, the command raises an error.
.\" METHOD: eval
.TP
\fBinterp eval\fI path 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 child interpreter identified by \fIpath\fR. The result
of this evaluation (including all \fBreturn\fR options,
such as \fB\-errorinfo\fR and \fB\-errorcode\fR information, if an
error occurs) is returned to the invoking interpreter.
.RS
.PP
Note that the script will be executed in the current context stack frame of the
\fIpath\fR interpreter; this is so that the implementations (in a parent
interpreter) of aliases in a child interpreter can execute scripts in
the child that find out information about the child's current state
and stack frame.
.RE
.\" METHOD: exists
.TP
\fBinterp exists \fIpath\fR
.
Returns \fB1\fR if a child interpreter by the specified \fIpath\fR
exists in this parent, \fB0\fR otherwise. If \fIpath\fR is omitted, the
invoking interpreter is used.
.\" METHOD: expose
.TP
\fBinterp expose \fIpath hiddenName\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 \fBHIDDEN COMMANDS\fR, below.
.\" METHOD: hide
.TP
\fBinterp hide\fI path exposedCmdName\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 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 \fBHIDDEN COMMANDS\fR, below.
.\" METHOD: hidden
.TP
\fBinterp hidden\fI path\fR
.
Returns a list of the names of all hidden commands in the interpreter
identified by \fIpath\fR.
.\" METHOD: invokehidden
.TP
\fBinterp invokehidden\fI path\fR ?\fI\-option ...\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. Three \fI\-option\fRs are supported, all
of which start with \fB\-\fR: \fB\-namespace\fR (which takes a single
argument afterwards, \fInsName\fR), \fB\-global\fR, and \fB\-\|\-\fR.
If the \fB\-namespace\fR flag is present, the hidden command is invoked in
the namespace called \fInsName\fR in the target interpreter.
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.
The \fB\-\|\-\fR flag allows the \fIhiddenCmdName\fR argument to start with a
.QW \-
character, and is otherwise unnecessary.
If both the \fB\-namespace\fR and \fB\-global\fR flags are present, the
\fB\-namespace\fR flag is ignored.
.RS
.PP
Note that the hidden command will be executed (by default) in the
current context stack frame of the \fIpath\fR interpreter.
.PP
Hidden commands are explained in more detail in \fBHIDDEN COMMANDS\fR, below.
.RE
.\" METHOD: issafe
.TP
\fBinterp issafe\fR ?\fIpath\fR?
.
Returns \fB1\fR if the interpreter identified by the specified \fIpath\fR
is safe, \fB0\fR otherwise.
.\" METHOD: limit
.TP
\fBinterp limit\fI path limitType\fR ?\fI\-option\fR? ?\fIvalue ...\fR?
.
Sets up, manipulates and queries the configuration of the resource
limit \fIlimitType\fR for the interpreter denoted by \fIpath\fR.  If
no \fI\-option\fR is specified, return the current configuration of the
limit.  If \fI\-option\fR is the sole argument, return the value of that
option.  Otherwise, a list of \fI\-option\fR/\fIvalue\fR argument pairs
must supplied. See \fBRESOURCE LIMITS\fR below for a more detailed
explanation of what limits and options are supported.
.\" METHOD: marktrusted
.TP
\fBinterp marktrusted\fI path\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.
.\" METHOD: recursionlimit
.TP
\fBinterp recursionlimit\fI path\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.
.RS
.PP
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.
.RE
.\" METHOD: share
.TP
\fBinterp share\fI srcPath channel destPath\fR
.
Causes the IO channel identified by \fIchannel\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.
.\" METHOD: target
.TP
\fBinterp target\fI path 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.
.\" METHOD: transfer
.TP
\fBinterp transfer\fI srcPath channel destPath\fR
.
Causes the IO channel identified by \fIchannel\fR to become available in
the interpreter identified by \fIdestPath\fR and unavailable in the
interpreter identified by \fIsrcPath\fR.
.SH "CHILD COMMAND"
.PP
For each child interpreter created with the \fBinterp\fR 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:
.PP
.CS
\fIchild command \fR?\fIarg arg ...\fR?
.CE
.PP
\fIChild\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:
.\" METHOD: aliases
.TP
\fIchild \fBaliases\fR
.
Returns a Tcl list whose elements are the tokens of all the
aliases in \fIchild\fR.  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).
.\" METHOD: alias
.TP
\fIchild \fBalias \fIsrcToken\fR
.
Returns a Tcl list whose elements are the \fItargetCmd\fR and
\fIarg\fRs associated with the alias represented by \fIsrcToken\fR
(this is the value returned when the alias was
created; it is possible that the actual source command in the
child is different from \fIsrcToken\fR).
.TP
\fIchild \fBalias \fIsrcToken \fB{}\fR
.
Deletes the alias for \fIsrcToken\fR in the child interpreter.
\fIsrcToken\fR refers to the value returned when the alias
was created;  if the source command has been renamed, the renamed
command will be deleted.
.TP
\fIchild \fBalias \fIsrcCmd targetCmd \fR?\fIarg ..\fR?
.
Creates an alias such that whenever \fIsrcCmd\fR is invoked
in \fIchild\fR, \fItargetCmd\fR is invoked in the parent.
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 \fBALIAS INVOCATION\fR below for details.
The command returns a token that uniquely identifies the command created
\fIsrcCmd\fR, even if the command is renamed afterwards. The token may but
does not have to be equal to \fIsrcCmd\fR.
.\" METHOD: bgerror
.TP
\fIchild \fBbgerror\fR ?\fIcmdPrefix\fR?
.
This command either gets or sets the current background exception handler
for the \fIchild\fR interpreter. If \fIcmdPrefix\fR 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
\fBBACKGROUND EXCEPTION HANDLING\fR section for more details.
.\" METHOD: eval
.TP
\fIchild \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 \fIchild\fR.
The result of this evaluation (including all \fBreturn\fR options,
such as \fB\-errorinfo\fR and \fB\-errorcode\fR information, if an
error occurs) is returned to the invoking interpreter.
.RS
.PP
Note that the script will be executed in the current context stack frame
of \fIchild\fR; this is so that the implementations (in a parent
interpreter) of aliases in a child interpreter can execute scripts in
the child that find out information about the child's current state
and stack frame.
.RE
.\" METHOD: expose
.TP
\fIchild \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 \fIchild\fR.
If an exposed command with the targeted name already exists, this command
fails.
For more details on hidden commands, see \fBHIDDEN COMMANDS\fR, below.
.\" METHOD: hide
.TP
\fIchild \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 \fIchild\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 children 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 \fBHIDDEN COMMANDS\fR, below.
.\" METHOD: hidden
.TP
\fIchild \fBhidden\fR
.
Returns a list of the names of all hidden commands in \fIchild\fR.
.\" METHOD: invokehidden
.TP
\fIchild \fBinvokehidden\fR ?\fI\-option ...\fR? \fIhiddenName \fR?\fIarg ..\fR?
.
This command invokes the hidden command \fIhiddenName\fR with the
supplied arguments, in \fIchild\fR. No substitutions or evaluations are
applied to the arguments. Three \fI\-option\fRs are supported, all
of which start with \fB\-\fR: \fB\-namespace\fR (which takes a single
argument afterwards, \fInsName\fR), \fB\-global\fR, and \fB\-\|\-\fR.
If the \fB\-namespace\fR flag is given, the hidden command is invoked in
the specified namespace in the child.
If the \fB\-global\fR flag is given, the command is invoked 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 \fB\-\|\-\fR flag allows the \fIhiddenCmdName\fR argument to start with a
.QW \-
character, and is otherwise unnecessary.
If both the \fB\-namespace\fR and \fB\-global\fR flags are given, the
\fB\-namespace\fR flag is ignored.
.RS
.PP
Note that the hidden command will be executed (by default) in the
current context stack frame of \fIchild\fR.
.PP
For more details on hidden commands, see \fBHIDDEN COMMANDS\fR, below.
.RE
.\" METHOD: issafe
.TP
\fIchild \fBissafe\fR
.
Returns \fB1\fR if the child interpreter is safe, \fB0\fR otherwise.
.\" METHOD: limit
.TP
\fIchild \fBlimit\fI limitType\fR ?\fI\-option\fR? ?\fIvalue ...\fR?
.
Sets up, manipulates and queries the configuration of the resource
limit \fIlimitType\fR for the child interpreter.  If no \fI\-option\fR
is specified, return the current configuration of the limit.  If
\fI\-option\fR is the sole argument, return the value of that option.
Otherwise, a list of \fI\-option\fR/\fIvalue\fR argument pairs must
supplied. See \fBRESOURCE LIMITS\fR below for a more detailed explanation of
what limits and options are supported.
.\" METHOD: marktrusted
.TP
\fIchild \fBmarktrusted\fR
.
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.
.\" METHOD: recursionlimit
.TP
\fIchild \fBrecursionlimit\fR ?\fInewlimit\fR?
.
Returns the maximum allowable nesting depth for the \fIchild\fR interpreter.
If \fInewlimit\fR is specified, the recursion limit in \fIchild\fR will be
set so that nesting of more than \fInewlimit\fR calls to \fBTcl_Eval()\fR
and related procedures in \fIchild\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.
.RS
.PP
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.
.RE
.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 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 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 child 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\fR	\fBappend\fR	\fBapply\fR	\fBarray\fR
\fBbinary\fR	\fBbreak\fR	\fBcatch\fR	\fBchan\fR
\fBclock\fR	\fBclose\fR	\fBconcat\fR	\fBcontinue\fR
\fBdict\fR	\fBeof\fR	\fBerror\fR	\fBeval\fR
\fBexpr\fR	\fBfblocked\fR	\fBfcopy\fR	\fBfileevent\fR
\fBflush\fR	\fBfor\fR	\fBforeach\fR	\fBformat\fR
\fBgets\fR	\fBglobal\fR	\fBif\fR	\fBincr\fR
\fBinfo\fR	\fBinterp\fR	\fBjoin\fR	\fBlappend\fR
\fBlassign\fR	\fBledit\fR	\fBlindex\fR	\fBlinsert\fR
\fBlist\fR	\fBllength\fR	\fBlrange\fR	\fBlrepeat\fR
\fBlreplace\fR	\fBlsearch\fR	\fBlseq\fR      \fBlset\fR
\fBlsort\fR     \fBnamespace\fR	\fBpackage\fR	\fBpid\fR
\fBproc\fR      \fBputs\fR	\fBread\fR	\fBregexp\fR
\fBregsub\fR	\fBrename\fR	\fBreturn\fR	\fBscan\fR
\fBseek\fR	\fBset\fR	\fBsplit\fR	\fBstring\fR
\fBsubst\fR	\fBswitch\fR	\fBtell\fR	\fBtime\fR
\fBtrace\fR	\fBunset\fR	\fBupdate\fR	\fBuplevel\fR
\fBupvar\fR	\fBvariable\fR	\fBvwait\fR	\fBwhile\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\fR	\fBencoding\fR	\fBexec\fR	\fBexit\fR
\fBfconfigure\fR	\fBfile\fR	\fBglob\fR	\fBload\fR
\fBopen\fR	\fBpwd\fR	\fBsocket\fR	\fBsource\fR
\fBunload\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\fR	\fBauto_import\fR	\fBauto_load\fR
\fBauto_load_index\fR	\fBauto_qualify\fR	\fBunknown\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
\fBauto_mkindex\fR	\fBauto_mkindex_old\fR
\fBauto_reset\fR	\fBhistory\fR
\fBparray\fR	\fBpkg_mkIndex\fR
\fB::pkg::create\fR	\fB::safe::interpAddToAccessPath\fR
\fB::safe::interpCreate\fR	\fB::safe::interpConfigure\fR
\fB::safe::interpDelete\fR	\fB::safe::interpFindInAccessPath\fR
\fB::safe::interpInit\fR	\fB::safe::setLogCmd\fR
\fBtcl_endOfWord\fR	\fBtcl_findLibrary\fR
\fBtcl_startOfNextWord\fR	\fBtcl_startOfPreviousWord\fR
\fBtcl_wordBreakAfter\fR	\fBtcl_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 in an untrusted script which is being executed in a
safe interpreter even if the target of the alias is not a safe
interpreter.  The most important thing in guaranteeing safety is to
ensure that information passed from the child to the parent is
never evaluated or substituted 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.
.PP
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 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
.QW "\fIsrcCmd arg1 arg2 ... argN\fR" ,
the new set of words will be
.QW "\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 child interpreter could execute arbitrary
code in the parent.  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 child interpreter can be invoked in the body of
procedures called in the parent during alias invocation. For example, an
alias for \fBsource\fR 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 \fBsource\fR command in the child
interpreter to actually source in the contents of the file. Note that two
commands named \fBsource\fR exist in the child interpreter: the alias, and
the hidden command.
.PP
Because a parent 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 child interpreters could cause a trusted parent
interpreter to execute dangerous commands on their behalf. See the section
on \fBALIAS INVOCATION\fR 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 them 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 children from fooling a parent interpreter into hiding the wrong
command, by making the current namespace be different from the global one.
.SH "RESOURCE LIMITS"
.PP
Every interpreter has two kinds of resource limits that may be imposed by any
parent interpreter upon its children. 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. Note that time limits are expressed as
\fIabsolute\fR times (as in \fBclock seconds\fR) and not relative times (as in
\fBafter\fR) because they may be modified after creation.
.PP
When a limit is exceeded for an interpreter, first any handler callbacks
defined by parent 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.
.SS "LIMIT OPTIONS"
.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.
.\" OPTION: -command
.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 exception, it
is reported through the background exception mechanism (see
\fBBACKGROUND EXCEPTION HANDLING\fR).
.RS
.PP
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.
.RE
.\" OPTION: -granularity
.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.
.\" OPTION: -milliseconds
.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 invocation.)
.\" OPTION: -seconds
.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.
.\" OPTION: -value
.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.
.PP
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 \fBinterp\fR command in the child and
then use aliases and the \fBinterp invokehidden\fR subcommand to provide such
access as it chooses to the \fBinterp\fR command to the limited parent as
necessary.
.SH "BACKGROUND EXCEPTION HANDLING"
.PP
When an exception happens in a situation where it cannot be reported directly up
the stack (e.g. when processing events in an \fBupdate\fR or \fBvwait\fR call)
the exception is instead reported through the background exception handling
mechanism. Every interpreter has a background exception handler registered;
the default exception
handler arranges for the \fBbgerror\fR 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.
.PP
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 options at the time of
the exception.  These are the same values that \fBcatch\fR can capture
when it controls script evaluation in a non-background situation.
The resulting list will then be executed
in the interpreter's global namespace without further substitutions being
performed.
.SH CREDITS
The safe interpreter mechanism is based on the Safe-Tcl prototype implemented
by Nathaniel Borenstein and Marshall Rose.
.SH EXAMPLES
.PP
Creating and using an alias for a command in the current interpreter:
.PP
.CS
\fBinterp alias\fR {} getIndex {} lsearch {alpha beta gamma delta}
set idx [getIndex delta]
.CE
.PP
Executing an arbitrary command in a safe interpreter where every
invocation of \fBlappend\fR is logged:
.PP
.CS
set i [\fBinterp create\fR -safe]
\fBinterp hide\fR $i lappend
\fBinterp alias\fR $i lappend {} loggedLappend $i
proc loggedLappend {i args} {
    puts "logged invocation of lappend $args"
    \fBinterp invokehidden\fR $i lappend {*}$args
}
\fBinterp eval\fR $i $someUntrustedScript
.CE
.PP
Setting a resource limit on an interpreter so that an infinite loop
terminates.
.PP
.CS
set i [\fBinterp create\fR]
\fBinterp limit\fR $i command -value 1000
\fBinterp eval\fR $i {
    set x 0
    while {1} {
        puts "Counting up... [incr x]"
    }
}
.CE
.SH "SEE ALSO"
bgerror(n), load(n), safe(n),
Tcl_CreateChild(3), Tcl_Eval(3), Tcl_BackgroundException(3)
.SH KEYWORDS
alias, parent interpreter, safe interpreter, child interpreter
'\"Local Variables:
'\"mode: nroff
'\"End: