interp(3tcl)
NAME
interp - Create and manipulate Tcl interpreters
SYNOPSIS
interp subcommand ?arg arg ...? ______________________________________________________________________________
DESCRIPTION
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 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
interpreter may create connections between its slaves and itself using
a mechanism called an alias. An alias 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
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
provided 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
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 hidden, so that only trusted
interpreters can obtain access to it. For a detailed explanation of
hidden commands, see HIDDEN COMMANDS, below. The alias mechanism can
be used for protected communication (analogous to a kernel call)
between a 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
evaluated 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
identified 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 error
handler for the interpreter identified by path. If cmdPrefix is
absent, the current background error 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 error
to. See the BACKGROUND ERROR HANDLING section for more details.
interp create ?-safe? ?--? ?path?
Creates a slave interpreter identified by path and a new
command, 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
variables. The -- switch can be used to mark the end of
switches; it may be needed if path is an unusual value such as
-safe. The result of the command is the name of the new
interpreter. The name of a 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
output 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
execution of all commands.
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
implementations (in a master interpreter) of aliases in a slave
interpreter can execute scripts in the slave that find out
information 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
command 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
hiding the wrong command, by making the current namespace be
different from the global one. Hidden commands are explained in
more detail in HIDDEN COMMANDS, below.
interp hidden path
Returns a list of the names of all hidden commands in the
interpreter identified by path.
interp invokehidden path ?-option ...? hiddenCmdName ?arg ...?
Invokes the hidden command hiddenCmdName with the arguments
supplied in the interpreter denoted by path. No substitutions or
evaluation 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 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
unnecessary. 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 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 issafe ?path?
Returns 1 if the interpreter identified by the specified path is
safe, 0 otherwise.
interp marktrusted path
Marks the interpreter identified by path as trusted. Does not
expose the hidden commands. This command can only be invoked
from a trusted interpreter. The command has no effect if the
interpreter identified by path is already trusted.
interp recursionlimit path ?newlimit?
Returns the maximum allowable nesting depth for the interpreter
specified by path. If newlimit is specified, the interpreter
recursion limit will be set so that nesting of more than
newlimit calls to Tcl_Eval() and related procedures in that
interpreter will return an error. The newlimit value is also
returned. The newlimit value must be a positive integer between
1 and the maximum value of a non-long integer on the platform.
The command sets the maximum size of the Tcl call stack only. It
cannot by itself prevent stack overflows on the C stack being
used by the application. If your machine has a limit on the size
of the C stack, you may get stack overflows before reaching the
limit set by the command. If this happens, see if there is a
mechanism in your system for increasing the maximum size of the
C stack.
interp share srcPath channelId destPath
Causes the IO channel identified by channelId to become shared
between the interpreter identified by srcPath and the
interpreter 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,
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.
interp transfer srcPath channelId destPath
Causes the IO channel identified by channelId to become
available 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
determine the exact behavior of the command. The valid forms of this
command 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
srcToken).
slave alias srcToken {}
Deletes the alias for srcToken in the slave interpreter.
srcToken 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
command 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 error
handler for the slave interpreter. If cmdPrefix is absent, the
current background error 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 error to. See
the BACKGROUND ERROR 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
current context stack frame of slave; this is so that the
implementations (in a master interpreter) of aliases in a slave
interpreter can execute scripts in the slave that find out
information 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
targeted 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
command fails. Currently both exposedCmdName and hiddenCmdName
can not contain namespace qualifiers, or an error is raised.
Commands to be hidden are looked up in the global namespace even
if the current namespace is not the global one. This prevents
slaves from fooling a master interpreter into hiding the wrong
command, by making the current namespace be different from the
global one. For more details on hidden commands, see 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
supplied 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
hiddenCmdName 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
interpreter. 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
subprocess 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
interpreter will also be safe.
A safe interpreter is created with exactly the following set of built-
in commands:
after append apply array
binary break catch chan
clock close concat continue
dict eof error eval
expr fblocked fcopy fileevent
flush for foreach format
gets global if incr
info interp join lappend
lassign lindex linsert list
llength lrange lrepeat lreplace
lsearch lset lsort namespace
package pid proc puts
read regexp regsub rename
return scan seek set
split string subst switch
tell time trace unset
update uplevel upvar variable
vwait while
The following commands are hidden by interp create when it creates a
safe interpreter:
cd encoding exec exit
fconfigure file glob load
open pwd socket source
unload
These commands can be recreated later as Tcl procedures or aliases, or
re-exposed by interp expose.
The following commands from Tcl's library of support procedures are not
present in a safe interpreter:
auto_exec_ok auto_import auto_load
auto_load_index auto_qualify unknown
Note in particular that safe interpreters have no default unknown
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::interpCreate ::safe::interpConfigure
::safe::interpDelete ::safe::interpFindInAccessPath
::safe::interpInit ::safe::setLogCmd
tcl_endOfWord tcl_findLibrary
tcl_startOfNextWord tcl_startOfPreviousWord
tcl_wordBreakAfter tcl_wordBreakBefore
can only be provided by explicit definition of an unknown command in
the safe interpreter. This will involve exposing the source command.
This is most easily accomplished by creating the safe interpreter with
Tcl's Safe-Tcl mechanism. Safe-Tcl provides safe versions of source,
load, and other Tcl commands needed to support autoloading of commands
and the loading of packages.
In addition, the env variable is not present in a safe interpreter, so
it cannot share environment variables with other interpreters. The env
variable poses a security risk, because users can store sensitive
information in an environment variable. For example, the PGP manual
recommends storing the PGP private key protection password in the
environment 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
interpreter, 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 command 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 functionality 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 management, must be performed on these windows within the interpreter context. 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 command 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 interpreter 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 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 interp invokehidden. 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. 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 commands, in either themselves or their descendants. 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 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 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
command) restrict the total number of Tcl commands that may be executed
by an interpreter (as can be inspected via the info cmdcount command),
and time limits (of type time) place a limit by which execution within
the interpreter must complete. Note that time limits are expressed as
absolute times (as in clock seconds) and not relative times (as in
after) because they may be modified after creation.
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 catch 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 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
particular 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
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
mechanism (see BACKGROUND ERROR HANDLING). 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.
-granularity
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.
-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
second unless specified at a sub-second level using the
-milliseconds option. This option may be the empty string, which
indicates that a time limit is not set for the interpreter.
-value 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.
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
limited master as necessary.
BACKGROUND ERROR HANDLING
When an error 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 error is instead reported through the background error
handling mechanism. Every interpreter has a background error handler
registered; the default error handler arranges for the bgerror command
in the interpreter's global namespace to be called, but other error
handlers may be installed and process background errors in
substantially different ways.
A background error 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 error message string, and the second will a dictionary
of return options (this is also the sort of information that can be
obtained by trapping a normal error using catch of course.) The
resulting list will then be executed in the interpreter's global
namespace without further substitutions being performed.
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
invocation of lappend is logged:
set i [interp create -safe]
interp hide $i lappend
interp alias $i lappend {} loggedLappend $i
proc loggedLappend {i args} {
puts "logged invocation of lappend $args"
interp invokehidden $i lappend {*}$args
}
interp eval $i $someUntrustedScript
Setting a resource limit on an interpreter so that an infinite loop
terminates.
set i [interp create]
interp limit $i command -value 1000
interp eval $i {
set x 0
while {1} {
puts "Counting up... [incr x]"
}
}
SEE ALSO
bgerror(3tcl), load(3tcl), safe(3tcl), Tcl_CreateSlave(3tcl)
KEYWORDS
alias, master interpreter, safe interpreter, slave interpreter
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