interp(n) Tcl Built-In Commands interp(n)______________________________________________________________________________NAMEinterp - Create and manipulate Tcl interpreters
SYNOPSISinterp subcommand ?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, and by resource limit exceeded callbacks. Note that the
name space for files (such as the names returned by the open command)
is no longer shared between interpreters. Explicit commands are pro‐
vided to share files and to transfer references to open files from one
interpreter to another.
The interp command also provides support for safe interpreters. A safe
interpreter is a 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 HIDDEN COMMANDS, 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 COMMAND
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 subcommand argument:
interp alias srcPath srcToken
Returns a Tcl list whose elements are the targetCmd and args
associated with the alias represented by srcToken (this is the
value returned when the alias was created; it is possible that
the name of the source command in the slave is different from
srcToken).
interp alias srcPath srcToken {}
Deletes the alias for srcToken in the slave interpreter identi‐
fied by srcPath. srcToken refers to the value returned when the
alias was created; if the source command has been renamed, the
renamed command will be deleted.
interp alias srcPath srcCmd targetPath targetCmd ?arg arg ...?
This command creates an alias between one 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
interpreter and command, and the arg arguments, if any, specify
additional arguments to targetCmd which are prepended to any
arguments 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. The
command returns a token that uniquely identifies the command
created srcCmd, even if the command is renamed afterwards. The
token may but does not have to be equal to srcCmd.
interp aliases ?path?
This command returns a Tcl list of the tokens of all the source
commands for aliases defined in the interpreter identified by
path. The tokens correspond to the values returned when the
aliases were created (which may not be the same as the current
names of the commands).
interp bgerror path ?cmdPrefix?
This command either gets or sets the current background excep‐
tion handler for the interpreter identified by path. If cmdPre‐
fix is absent, the current background exception handler is
returned, and if it is present, it is a list of words (of mini‐
mum length one) that describes what to set the interpreter's
background exception handler to. See the BACKGROUND EXCEPTION
HANDLING section for more details.
interp cancel ?-unwind? ?--? ?path? ?result?
Cancels the script being evaluated in the interpreter identified │
by path. Without the -unwind switch the evaluation stack for the │
interpreter is unwound until an enclosing catch command is found │
or there are no further invocations of the interpreter left on │
the call stack. With the -unwind switch the evaluation stack for │
the interpreter is unwound without regard to any intervening │
catch command until there are no further invocations of the │
interpreter left on the call stack. The -- switch can be used to │
mark the end of switches; it may be needed if path is an unusual │
value such as -safe. If result is present, it will be used as │
the error message string; otherwise, a default error message │
string will be used.
interp create ?-safe? ?--? ?path?
Creates a 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 debug path ?-frame ?bool??
Controls whether frame-level stack information is captured in
the slave interpreter identified by path. If no arguments are
given, option and current setting are returned. If -frame is
given, the debug setting is set to the given boolean if provided
and the current setting is returned. This only effects the out‐
put of info frame, in that exact frame-level information for
command invocation at the bytecode level is only captured with
this setting on.
For example, with code like
proc mycontrol {... script} {
...
uplevel 1 $script
...
}
proc dosomething {...} {
...
mycontrol {
somecode
}
}
the standard setting will provide a relative line number for the
command somecode and the relevant frame will be of type eval.
With frame-debug active on the other hand the tracking extends
so far that the system will be able to determine the file and
absolute line number of this command, and return a frame of type
source. This more exact information is paid for with slower exe‐
cution of all commands.
Note that once it is on, this flag cannot be switched back off:
such attempts are silently ignored. This is needed to maintain
the consistency of the underlying interpreter's state.
interp delete ?path ...?
Deletes zero or more interpreters given by the optional path
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 all return
options, such as -errorinfo and -errorcode information, if an
error occurs) is returned to the invoking interpreter. Note
that the script will be executed in the current context stack
frame of the path interpreter; this is so that the implementa‐
tions (in a master interpreter) of aliases in a slave inter‐
preter can execute scripts in the slave that find out informa‐
tion about the slave's current state and stack frame.
interp exists path
Returns 1 if a slave interpreter by the specified path exists in
this master, 0 otherwise. If path is omitted, the invoking
interpreter is used.
interp expose path hiddenName ?exposedCmdName?
Makes the hidden command hiddenName exposed, eventually bringing
it back under a new exposedCmdName name (this name is currently
accepted only if it is a valid global name space name without
any ::), in the interpreter denoted by path. If an exposed com‐
mand with the targeted name already exists, this command fails.
Hidden commands are explained in more detail in HIDDEN COMMANDS,
below.
interp hide path exposedCmdName ?hiddenCmdName?
Makes the exposed command exposedCmdName hidden, renaming it to
the hidden command hiddenCmdName, or keeping the same name if
hiddenCmdName is not given, in the interpreter denoted by path.
If a hidden command with the targeted name already exists, this
command fails. Currently both exposedCmdName and hiddenCmdName
can not contain namespace qualifiers, or an error is raised.
Commands to be hidden by interp hide are looked up in the global
namespace even if the current namespace is not the global one.
This prevents 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 HIDDEN COMMANDS, below.
interp hidden path
Returns a list of the names of all hidden commands in the inter‐
preter identified by path.
interp invokehidden path ?-option ...? hiddenCmdName ?arg ...?
Invokes the hidden command hiddenCmdName with the arguments sup‐
plied in the interpreter denoted by path. No substitutions or
evaluation are applied to the arguments. Three -options are sup‐
ported, all of which start with -: -namespace (which takes a
single argument afterwards, nsName), -global, and --. If the
-namespace flag is present, the hidden command is invoked in the
namespace called nsName in the target interpreter. If the
-global flag is present, the hidden command is invoked at the
global level in the target interpreter; otherwise it is invoked
at the current call frame and can access local variables in that
and outer call frames. The -- flag allows the hiddenCmdName
argument to start with a “-” character, and is otherwise unnec‐
essary. If both the -namespace and -global flags are present,
the -namespace flag is ignored. Note that the hidden command
will be executed (by default) in the current context stack frame
of the path interpreter. Hidden commands are explained in more
detail in HIDDEN COMMANDS, below.
interp issafe ?path?
Returns 1 if the interpreter identified by the specified path is
safe, 0 otherwise.
interp limit path limitType ?-option? ?value ...?
Sets up, manipulates and queries the configuration of the
resource limit limitType for the interpreter denoted by path.
If no -option is specified, return the current configuration of
the limit. If -option is the sole argument, return the value of
that option. Otherwise, a list of -option/value argument pairs
must supplied. See RESOURCE LIMITS below for a more detailed
explanation of what limits and options are supported.
interp marktrusted path
Marks the interpreter identified by path as trusted. Does not
expose the hidden commands. This command can only be invoked
from a trusted interpreter. The command has no effect if the
interpreter identified by path is already trusted.
interp recursionlimit path ?newlimit?
Returns the maximum allowable nesting depth for the interpreter
specified by path. If newlimit is specified, the interpreter
recursion limit will be set so that nesting of more than
newlimit calls to Tcl_Eval and related procedures in that inter‐
preter will return an error. The newlimit value is also
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 COMMAND
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 tokens of all the
aliases in slave. 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).
slave alias srcToken
Returns a Tcl list whose elements are the targetCmd and args
associated with the alias represented by srcToken (this is the
value returned when the alias was created; it is possible that
the actual source command in the slave is different from srcTo‐
ken).
slave alias srcToken {}
Deletes the alias for srcToken in the slave interpreter. srcTo‐
ken refers to the value returned when the alias was created; if
the source command has been renamed, the renamed command will be
deleted.
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. The command returns a token that
uniquely identifies the command created srcCmd, even if the com‐
mand is renamed afterwards. The token may but does not have to
be equal to srcCmd.
slave bgerror ?cmdPrefix?
This command either gets or sets the current background excep‐
tion handler for the slave interpreter. If cmdPrefix is absent,
the current background exception handler is returned, and if it
is present, it is a list of words (of minimum length one) that
describes what to set the interpreter's background exception
handler to. See the BACKGROUND EXCEPTION HANDLING section for
more details.
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 all return options, such as -errorinfo and -errorcode
information, if an error occurs) is returned to the invoking
interpreter. Note that the script will be executed in the cur‐
rent context stack frame of slave; this is so that the implemen‐
tations (in a master interpreter) of aliases in a slave inter‐
preter can execute scripts in the slave that find out informa‐
tion about the slave's current state and stack frame.
slave expose hiddenName ?exposedCmdName?
This command exposes the hidden command hiddenName, eventually
bringing it back under a new exposedCmdName name (this name is
currently accepted only if it is a valid global name space name
without any ::), in slave. If an exposed command with the tar‐
geted name already exists, this command fails. For more details
on hidden commands, see HIDDEN COMMANDS, below.
slave hide exposedCmdName ?hiddenCmdName?
This command hides the exposed command exposedCmdName, renaming
it to the hidden command hiddenCmdName, or keeping the same name
if the argument is not given, in the slave interpreter. If a
hidden command with the targeted name already exists, this com‐
mand fails. Currently both exposedCmdName and hiddenCmdName can
not contain namespace qualifiers, or an error is raised. Com‐
mands to be hidden are looked up in the global namespace even if
the current namespace is not the global one. This prevents
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.
slave hidden
Returns a list of the names of all hidden commands in slave.
slave invokehidden ?-option ...? 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. Three -options are supported, all of
which start with -: -namespace (which takes a single argument
afterwards, nsName), -global, and --. If the -namespace flag is
given, the hidden command is invoked in the specified namespace
in the slave. 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. The -- flag allows the hiddenCmd‐
Name argument to start with a “-” character, and is otherwise
unnecessary. If both the -namespace and -global flags are
given, the -namespace flag is ignored. Note that the hidden
command will be executed (by default) in the current context
stack frame of slave. For more details on hidden commands, see
HIDDEN COMMANDS, below.
slave issafe
Returns 1 if the slave interpreter is safe, 0 otherwise.
slave limit limitType ?-option? ?value ...?
Sets up, manipulates and queries the configuration of the
resource limit limitType for the slave interpreter. If no
-option is specified, return the current configuration of the
limit. If -option is the sole argument, return the value of
that option. Otherwise, a list of -option/value argument pairs
must supplied. See RESOURCE LIMITS below for a more detailed
explanation of what limits and options are supported.
slave marktrusted
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
Tcl_Eval() 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 apply array
binary break catch chan clock close con‐
cat continue dict eof error eval
expr fblocked fcopy fileevent
flush for foreach format
gets global if incr
info interp join lappend lassign lin‐
dex linsert list llength lrange lrepeat lreplace
lsearch lset lsort namespace pack‐
age pid proc puts read regexp reg‐
sub rename return scan seek set
split string subst switch
tell time trace unset
update uplevel upvar variable vwait while The fol‐
lowing commands are hidden by interp create when it creates a safe
interpreter: cd encoding exec exit fconfig‐
ure file glob load
open pwd socket source unload These commands can be
recreated later as Tcl procedures or aliases, or re-exposed by interp
expose.
The following commands from Tcl's library of support procedures are not
present in a safe interpreter:
auto_exec_ok auto_import auto_load auto_load_index auto_qual‐
ify unknown Note in particular that safe interpreters have no
default unknown command, so Tcl's default autoloading facilities are
not available. Autoload access to Tcl's commands that are normally
autoloaded: auto_mkindex auto_mkindex_old
auto_reset history parray pkg_mkIndex
::pkg::create ::safe::interpAddToAccessPath ::safe::interpCre‐
ate ::safe::interpConfigure ::safe::interpDelete ::safe::interpFindI‐
nAccessPath ::safe::interpInit ::safe::setLogCmd tcl_endOf‐
Word tcl_findLibrary tcl_startOfNextWord tcl_startOfPrevious‐
Word tcl_wordBreakAfter tcl_wordBreakBefore can only be provided by
explicit definition of an unknown command in the safe interpreter.
This will involve exposing the source command. This is most easily
accomplished by creating the safe interpreter with Tcl's Safe-Tcl mech‐
anism. 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 PGPPASS. Making this variable available to untrusted
code executing in a safe interpreter would incur a security risk.
If extensions are loaded into a safe interpreter, they may also
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.
HIDDEN COMMANDS
Safe interpreters greatly restrict the functionality available to Tcl
programs executing within them. Allowing the untrusted Tcl program to
have direct access to this functionality is unsafe, because it can be
used for a variety of attacks on the environment. However, there are
times when there is a legitimate need to use the dangerous functional‐
ity in the context of the safe interpreter. For example, sometimes a
program must be sourced into the interpreter. Another example is Tk,
where windows are bound to the hierarchy of windows for a specific
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.
RESOURCE LIMITS
Every interpreter has two kinds of resource limits that may be imposed
by any master interpreter upon its slaves. Command limits (of type com‐
mand) restrict the total number of Tcl commands that may be executed by
an interpreter (as can be inspected via the info cmdcount command), and
time limits (of type time) place a limit by which execution within the
interpreter must complete. Note that time limits are expressed as abso‐
lute times (as in clock seconds) and not relative times (as in after)
because they may be modified after creation.
When a limit is exceeded for an interpreter, first any handler call‐
backs 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 gen‐
erated at that point and normal processing of errors within the inter‐
preter (by the catch command) is disabled, so the error propagates out‐
wards (building a stack-trace as it goes) to the point where the lim‐
ited interpreter was invoked (e.g. by interp eval) where it becomes the
responsibility of the calling code to catch and handle.
LIMIT OPTIONS
Every limit has a number of options associated with it, some of which
are common across all kinds of limits, and others of which are particu‐
lar to the kind of limit.
-command
This option (common for all limit types) specifies (if non-
empty) a Tcl script to be executed in the global namespace of
the interpreter reading and writing the option when the particu‐
lar limit in the limited interpreter is exceeded. The callback
may modify the limit on the interpreter if it wishes the limited
interpreter to continue executing. If the callback generates an
exception, it is reported through the background exception mech‐
anism (see BACKGROUND EXCEPTION HANDLING). Note that the call‐
backs defined by one interpreter are completely isolated from
the callbacks defined by another, and that the order in which
those callbacks are called is undefined.
-granularity
This option (common for all limit types) specifies how fre‐
quently (out of the points when the Tcl interpreter is in a con‐
sistent state where limit checking is possible) that the limit
is actually checked. This allows the tuning of how frequently a
limit is checked, and hence how often the limit-checking over‐
head (which may be substantial in the case of time limits) is
incurred.
-milliseconds
This option specifies the number of milliseconds after the
moment defined in the -seconds option that the time limit will
fire. It should only ever be specified in conjunction with the
-seconds option (whether it was set previously or is being set
this invocation.)
-seconds
This option specifies the number of seconds after the epoch (see
clock seconds) that the time limit for the interpreter will be
triggered. The limit will be triggered at the start of the sec‐
ond unless specified at a sub-second level using the -millisec‐
onds option. This option may be the empty string, which indi‐
cates that a time limit is not set for the interpreter.
-value This option specifies the number of commands that the inter‐
preter may execute before triggering the command limit. This
option may be the empty string, which indicates that a command
limit is not set for the interpreter.
Where an interpreter with a resource limit set on it creates a slave
interpreter, that slave interpreter will have resource limits imposed
on it that are at least as restrictive as the limits on the creating
master interpreter. If the master interpreter of the limited master
wishes to relax these conditions, it should hide the interp command in
the child and then use aliases and the interp invokehidden subcommand
to provide such access as it chooses to the interp command to the lim‐
ited master as necessary.
BACKGROUND EXCEPTION HANDLING
When an exception happens in a situation where it cannot be reported
directly up the stack (e.g. when processing events in an update or
vwait call) the exception is instead reported through the background
exception handling mechanism. Every interpreter has a background
exception handler registered; the default exception handler arranges
for the bgerror command in the interpreter's global namespace to be
called, but other exception handlers may be installed and process back‐
ground exceptions in substantially different ways.
A background exception handler consists of a non-empty list of words to
which will be appended two further words at invocation time. The first
word will be the interpreter result at time of the exception, typically
an error message, and the second will be the dictionary of return
options at the time of the exception. These are the same values that
catch can capture when it controls script evaluation in a non-back‐
ground situation. The resulting list will then be executed in the
interpreter's global namespace without further substitutions being per‐
formed.
CREDITS
The safe interpreter mechanism is based on the Safe-Tcl prototype
implemented by Nathaniel Borenstein and Marshall Rose.
EXAMPLES
Creating and using an alias for a command in the current interpreter:
interp alias {} getIndex {} lsearch {alpha beta gamma delta}
set idx [getIndex delta]
Executing an arbitrary command in a safe interpreter where every invo‐
cation of lappend is logged:
set i [interp create -safe]
interp hide $i lappend
interp alias $i lappend {} loggedLappend $i
proc loggedLappend {i args} {
puts "logged invocation of lappend $args"
interp invokehidden $i lappend {*}$args
}
interp eval $i $someUntrustedScript
Setting a resource limit on an interpreter so that an infinite loop
terminates.
set i [interp create]
interp limit $i command -value 1000
interp eval $i {
set x 0
while {1} {
puts "Counting up... [incr x]"
}
}
SEE ALSObgerror(n), load(n), safe(n), Tcl_CreateSlave(3), Tcl_Eval(3),
Tcl_BackgroundException(3)KEYWORDS
alias, master interpreter, safe interpreter, slave interpreter
Tcl 8.6 interp(n)
