'\" '\" Copyright (c) 1995-1996 Sun Microsystems, Inc. '\" '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" '\" RCS: @(#) $Id: interp.n,v 1.9.2.2 2004/10/27 14:23:56 dkf Exp $ '\" .so man.macros .TH interp n 7.6 Tcl "Tcl Built-In Commands" .BS '\" Note: do not modify the .SH NAME line immediately below! .SH NAME interp \- Create and manipulate Tcl interpreters .SH SYNOPSIS \fBinterp \fIoption \fR?\fIarg arg ...\fR? .BE .SH DESCRIPTION .PP This command makes it possible to create one or more new Tcl interpreters that co-exist with the creating interpreter in the same application. The creating interpreter is called the \fImaster\fR and the new interpreter is called a \fIslave\fR. A master can create any number of slaves, and each slave can itself create additional slaves for which it is master, resulting in a hierarchy of interpreters. .PP Each interpreter is independent from the others: it has its own name space for commands, procedures, and global variables. A master interpreter may create connections between its slaves and itself using a mechanism called an \fIalias\fR. An \fIalias\fR is a command in a slave interpreter which, when invoked, causes a command to be invoked in its master interpreter or in another slave interpreter. The only other connections between interpreters are through environment variables (the \fBenv\fR variable), which are normally shared among all interpreters in the application. Note that the name space for files (such as the names returned by the \fBopen\fR command) is no longer shared between interpreters. Explicit commands are provided to share files and to transfer references to open files from one interpreter to another. .PP The \fBinterp\fR command also provides support for \fIsafe\fR interpreters. A safe interpreter is a slave whose functions have been greatly restricted, so that it is safe to execute untrusted scripts without fear of them damaging other interpreters or the application's environment. For example, all IO channel creation commands and subprocess creation commands are made inaccessible to safe interpreters. .VS 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 slave interpreter and its master. See \fBALIAS INVOCATION\fR, below, for more details on how the alias mechanism works. .VE .PP A qualified interpreter name is a proper Tcl lists containing a subset of its ancestors in the interpreter hierarchy, terminated by the string naming the interpreter in its immediate master. Interpreter names are relative to the interpreter in which they are used. For example, if \fBa\fR is a slave of the current interpreter and it has a slave \fBa1\fR, which in turn has a slave \fBa11\fR, the qualified name of \fBa11\fR in \fBa\fR is the list \fBa1 a11\fR. .PP The \fBinterp\fR command, described below, accepts qualified interpreter names as arguments; the interpreter in which the command is being evaluated can always be referred to as \fB{}\fR (the empty list or string). Note that it is impossible to refer to a master (ancestor) interpreter by name in a slave interpreter except through aliases. Also, there is no global name by which one can refer to the first interpreter created in an application. Both restrictions are motivated by safety concerns. .SH "THE INTERP COMMAND" .PP The \fBinterp\fR command is used to create, delete, and manipulate slave interpreters, and to share or transfer channels between interpreters. It can have any of several forms, depending on the \fIoption\fR argument: .TP \fBinterp\fR \fBalias\fR \fIsrcPath\fR \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 name of the source command in the slave is different from \fIsrcToken\fR). .TP \fBinterp\fR \fBalias\fR \fIsrcPath\fR \fIsrcToken\fR \fB{}\fR Deletes the alias for \fIsrcToken\fR in the slave 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\fR \fBalias\fR \fIsrcPath\fR \fIsrcCmd\fR \fItargetPath\fR \fItargetCmd \fR?\fIarg arg ...\fR? This command creates an alias between one slave and another (see the \fBalias\fR slave command below for creating aliases between a slave and its master). In this command, either of the slave interpreters may be anywhere in the hierarchy of interpreters under the interpreter invoking the command. \fISrcPath\fR and \fIsrcCmd\fR identify the source of the alias. \fISrcPath\fR is a Tcl list whose elements select a particular interpreter. For example, ``\fBa b\fR'' identifies an interpreter \fBb\fR, which is a slave of interpreter \fBa\fR, which is a slave of the invoking interpreter. An empty list specifies the interpreter invoking the command. \fIsrcCmd\fR gives the name of a new command, which will be created in the source interpreter. \fITargetPath\fR and \fItargetCmd\fR specify a target interpreter and command, and the \fIarg\fR arguments, if any, specify additional arguments to \fItargetCmd\fR which are prepended to any arguments specified in the invocation of \fIsrcCmd\fR. \fITargetCmd\fR may be undefined at the time of this call, or it may already exist; it is not created by this command. The alias arranges for the given target command to be invoked in the target interpreter whenever the given source command is invoked in the source interpreter. See \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. .TP \fBinterp\fR \fBaliases \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). .TP \fBinterp\fR \fBcreate \fR?\fB\-safe\fR? ?\fB\-\|\-\fR? ?\fIpath\fR? Creates a slave interpreter identified by \fIpath\fR and a new command, called a \fIslave command\fR. The name of the slave command is the last component of \fIpath\fR. The new slave interpreter and the slave command are created in the interpreter identified by the path obtained by removing the last component from \fIpath\fR. For example, if \fIpath is \fBa b c\fR then a new slave interpreter and slave command named \fBc\fR are created in the interpreter identified by the path \fBa b\fR. The slave command may be used to manipulate the new interpreter as described below. If \fIpath\fR is omitted, Tcl creates a unique name of the form \fBinterp\fIx\fR, where \fIx\fR is an integer, and uses it for the interpreter and the slave command. If the \fB\-safe\fR switch is specified (or if the master interpreter is a safe interpreter), the new slave interpreter will be created as a safe interpreter with limited functionality; otherwise the slave will include the full set of Tcl built-in commands and variables. The \fB\-\|\-\fR switch can be used to mark the end of switches; it may be needed if \fIpath\fR is an unusual value such as \fB\-safe\fR. The result of the command is the name of the new interpreter. The name of a slave interpreter must be unique among all the slaves for its master; an error occurs if a slave interpreter by the given name already exists in this master. The initial recursion limit of the slave interpreter is set to the current recursion limit of its parent interpreter. .TP \fBinterp\fR \fBdelete \fR?\fIpath ...?\fR Deletes zero or more interpreters given by the optional \fIpath\fR arguments, and for each interpreter, it also deletes its slaves. The command also deletes the slave command for each interpreter deleted. For each \fIpath\fR argument, if no interpreter by that name exists, the command raises an error. .TP \fBinterp\fR \fBeval\fR \fIpath arg \fR?\fIarg ...\fR? This command concatenates all of the \fIarg\fR arguments in the same fashion as the \fBconcat\fR command, then evaluates the resulting string as a Tcl script in the slave interpreter identified by \fIpath\fR. The result of this evaluation (including error information such as the \fBerrorInfo\fR and \fBerrorCode\fR variables, if an error occurs) is returned to the invoking interpreter. 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 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. .TP \fBinterp exists \fIpath\fR Returns \fB1\fR if a slave interpreter by the specified \fIpath\fR exists in this master, \fB0\fR otherwise. If \fIpath\fR is omitted, the invoking interpreter is used. .VS "" BR .TP \fBinterp expose \fIpath\fR \fIhiddenName\fR ?\fIexposedCmdName\fR? Makes the hidden command \fIhiddenName\fR exposed, eventually bringing it back under a new \fIexposedCmdName\fR name (this name is currently accepted only if it is a valid global name space name without any ::), in the interpreter denoted by \fIpath\fR. If an exposed command with the targeted name already exists, this command fails. Hidden commands are explained in more detail in \fBHIDDEN COMMANDS\fR, below. .TP \fBinterp\fR \fBhide\fR \fIpath\fR \fIexposedCmdName\fR ?\fIhiddenCmdName\fR? Makes the exposed command \fIexposedCmdName\fR hidden, renaming it to the hidden command \fIhiddenCmdName\fR, or keeping the same name if \fIhiddenCmdName\fR is not given, in the interpreter denoted by \fIpath\fR. If a hidden command with the targeted name already exists, this command fails. Currently both \fIexposedCmdName\fR and \fIhiddenCmdName\fR can not contain namespace qualifiers, or an error is raised. Commands to be hidden by \fBinterp hide\fR are looked up in the global namespace even if the current namespace is not the global one. This prevents slaves from fooling a master interpreter into hiding the wrong command, by making the current namespace be different from the global one. Hidden commands are explained in more detail in \fBHIDDEN COMMANDS\fR, below. .TP \fBinterp\fR \fBhidden\fR \fIpath\fR Returns a list of the names of all hidden commands in the interpreter identified by \fIpath\fR. .TP \fBinterp\fR \fBinvokehidden\fR \fIpath\fR ?\fB-global\fR? \fIhiddenCmdName\fR ?\fIarg ...\fR? Invokes the hidden command \fIhiddenCmdName\fR with the arguments supplied in the interpreter denoted by \fIpath\fR. No substitutions or evaluation are applied to the arguments. If the \fB-global\fR flag is present, the hidden command is invoked at the global level in the target interpreter; otherwise it is invoked at the current call frame and can access local variables in that and outer call frames. Hidden commands are explained in more detail in \fBHIDDEN COMMANDS\fR, below. .VE .TP \fBinterp issafe\fR ?\fIpath\fR? Returns \fB1\fR if the interpreter identified by the specified \fIpath\fR is safe, \fB0\fR otherwise. .VS "" BR .TP \fBinterp marktrusted\fR \fIpath\fR Marks the interpreter identified by \fIpath\fR as trusted. Does not expose the hidden commands. This command can only be invoked from a trusted interpreter. The command has no effect if the interpreter identified by \fIpath\fR is already trusted. .VE .TP \fBinterp\fR \fBrecursionlimit\fR \fIpath\fR ?\fInewlimit\fR? Returns the maximum allowable nesting depth for the interpreter specified by \fIpath\fR. If \fInewlimit\fR is specified, the interpreter recursion limit will be set so that nesting of more than \fInewlimit\fR calls to \fBTcl_Eval()\fR and related procedures in that interpreter will return an error. The \fInewlimit\fR value is also returned. The \fInewlimit\fR value must be a positive integer between 1 and the maximum value of a non-long integer on the platform. .sp The command sets the maximum size of the Tcl call stack only. It cannot by itself prevent stack overflows on the C stack being used by the application. If your machine has a limit on the size of the C stack, you may get stack overflows before reaching the limit set by the command. If this happens, see if there is a mechanism in your system for increasing the maximum size of the C stack. .TP \fBinterp\fR \fBshare\fR \fIsrcPath channelId destPath\fR Causes the IO channel identified by \fIchannelId\fR to become shared between the interpreter identified by \fIsrcPath\fR and the interpreter identified by \fIdestPath\fR. Both interpreters have the same permissions on the IO channel. Both interpreters must close it to close the underlying IO channel; IO channels accessible in an interpreter are automatically closed when an interpreter is destroyed. .TP \fBinterp\fR \fBslaves\fR ?\fIpath\fR? Returns a Tcl list of the names of all the slave interpreters associated with the interpreter identified by \fIpath\fR. If \fIpath\fR is omitted, the invoking interpreter is used. .TP \fBinterp\fR \fBtarget\fR \fIpath alias\fR Returns a Tcl list describing the target interpreter for an alias. The alias is specified with an interpreter path and source command name, just as in \fBinterp alias\fR above. The name of the target interpreter is returned as an interpreter path, relative to the invoking interpreter. If the target interpreter for the alias is the invoking interpreter then an empty list is returned. If the target interpreter for the alias is not the invoking interpreter or one of its descendants then an error is generated. The target command does not have to be defined at the time of this invocation. .TP \fBinterp\fR \fBtransfer\fR \fIsrcPath channelId destPath\fR Causes the IO channel identified by \fIchannelId\fR to become available in the interpreter identified by \fIdestPath\fR and unavailable in the interpreter identified by \fIsrcPath\fR. .SH "SLAVE COMMAND" .PP For each slave interpreter created with the \fBinterp\fR command, a new Tcl command is created in the master interpreter with the same name as the new interpreter. This command may be used to invoke various operations on the interpreter. It has the following general form: .CS \fIslave command \fR?\fIarg arg ...\fR? .CE \fISlave\fR is the name of the interpreter, and \fIcommand\fR and the \fIarg\fRs determine the exact behavior of the command. The valid forms of this command are: .TP \fIslave \fBaliases\fR Returns a Tcl list whose elements are the tokens of all the aliases in \fIslave\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). .TP \fIslave \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 slave is different from \fIsrcToken\fR). .TP \fIslave \fBalias \fIsrcToken \fB{}\fR Deletes the alias for \fIsrcToken\fR in the slave 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 \fIslave \fBalias \fIsrcCmd targetCmd \fR?\fIarg ..\fR? Creates an alias such that whenever \fIsrcCmd\fR is invoked in \fIslave\fR, \fItargetCmd\fR is invoked in the master. The \fIarg\fR arguments will be passed to \fItargetCmd\fR as additional arguments, prepended before any arguments passed in the invocation of \fIsrcCmd\fR. See \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. .TP \fIslave \fBeval \fIarg \fR?\fIarg ..\fR? This command concatenates all of the \fIarg\fR arguments in the same fashion as the \fBconcat\fR command, then evaluates the resulting string as a Tcl script in \fIslave\fR. The result of this evaluation (including error information such as the \fBerrorInfo\fR and \fBerrorCode\fR variables, if an error occurs) is returned to the invoking interpreter. Note that the script will be executed in the current context stack frame of \fIslave\fR; 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. .VS "" BR .TP \fIslave \fBexpose \fIhiddenName \fR?\fIexposedCmdName\fR? This command exposes the hidden command \fIhiddenName\fR, eventually bringing it back under a new \fIexposedCmdName\fR name (this name is currently accepted only if it is a valid global name space name without any ::), in \fIslave\fR. If an exposed command with the targeted name already exists, this command fails. For more details on hidden commands, see \fBHIDDEN COMMANDS\fR, below. .TP \fIslave \fBhide \fIexposedCmdName\fR ?\fIhiddenCmdName\fR? This command hides the exposed command \fIexposedCmdName\fR, renaming it to the hidden command \fIhiddenCmdName\fR, or keeping the same name if the argument is not given, in the \fIslave\fR interpreter. If a hidden command with the targeted name already exists, this command fails. Currently both \fIexposedCmdName\fR and \fIhiddenCmdName\fR can not contain namespace qualifiers, or an error is raised. Commands to be hidden are looked up in the global namespace even if the current namespace is not the global one. This prevents slaves from fooling a master interpreter into hiding the wrong command, by making the current namespace be different from the global one. For more details on hidden commands, see \fBHIDDEN COMMANDS\fR, below. .TP \fIslave \fBhidden\fR Returns a list of the names of all hidden commands in \fIslave\fR. .TP \fIslave \fBinvokehidden\fR ?\fB-global\fR \fIhiddenName \fR?\fIarg ..\fR? This command invokes the hidden command \fIhiddenName\fR with the supplied arguments, in \fIslave\fR. No substitutions or evaluations are applied to the arguments. If the \fB-global\fR flag is given, the command is invoked at the global level in the slave; otherwise it is invoked at the current call frame and can access local variables in that or outer call frames. For more details on hidden commands, see \fBHIDDEN COMMANDS\fR, below. .VE .TP \fIslave \fBissafe\fR Returns \fB1\fR if the slave interpreter is safe, \fB0\fR otherwise. .VS "" BR .TP \fIslave \fBmarktrusted\fR Marks the slave interpreter as trusted. Can only be invoked by a trusted interpreter. This command does not expose any hidden commands in the slave interpreter. The command has no effect if the slave is already trusted. .VE .TP \fIslave\fR \fBrecursionlimit\fR ?\fInewlimit\fR? Returns the maximum allowable nesting depth for the \fIslave\fR interpreter. If \fInewlimit\fR is specified, the recursion limit in \fIslave\fR will be set so that nesting of more than \fInewlimit\fR calls to \fBTcl_Eval()\fR and related procedures in \fIslave\fR will return an error. The \fInewlimit\fR value is also returned. The \fInewlimit\fR value must be a positive integer between 1 and the maximum value of a non-long integer on the platform. .sp The command sets the maximum size of the Tcl call stack only. It cannot by itself prevent stack overflows on the C stack being used by the application. If your machine has a limit on the size of the C stack, you may get stack overflows before reaching the limit set by the command. If this happens, see if there is a mechanism in your system for increasing the maximum size of the C stack. .SH "SAFE INTERPRETERS" .PP A safe interpreter is one with restricted functionality, so that is safe to execute an arbitrary script from your worst enemy without fear of that script damaging the enclosing application or the rest of your computing environment. In order to make an interpreter safe, certain commands and variables are removed from the interpreter. For example, commands to create files on disk are removed, and the \fBexec\fR command is removed, since it could be used to cause damage through subprocesses. Limited access to these facilities can be provided, by creating aliases to the master interpreter which check their arguments carefully and provide restricted access to a safe subset of facilities. For example, file creation might be allowed in a particular subdirectory and subprocess invocation might be allowed for a carefully selected and fixed set of programs. .PP A safe interpreter is created by specifying the \fB\-safe\fR switch to the \fBinterp create\fR command. Furthermore, any slave created by a safe interpreter will also be safe. .PP A safe interpreter is created with exactly the following set of built-in commands: .DS .ta 1.2i 2.4i 3.6i \fBafter append array binary break case catch clock close concat continue eof error eval expr fblocked fcopy fileevent flush for foreach format gets global if incr info interp join lappend lindex linsert list llength lrange lreplace lsearch lsort namespace package pid proc puts read regexp regsub rename return scan seek set split string subst switch tell time trace unset update uplevel upvar variable vwait while\fR .DE .VS "" BR The following commands are hidden by \fBinterp create\fR when it creates a safe interpreter: .DS .ta 1.2i 2.4i 3.6i \fBcd encoding exec exit fconfigure file glob load open pwd socket source\fR .DE These commands can be recreated later as Tcl procedures or aliases, or re-exposed by \fBinterp expose\fR. .PP The following commands from Tcl's library of support procedures are not present in a safe interpreter: .DS .ta 1.6i 3.2i \fBauto_exec_ok auto_import auto_load auto_load_index auto_qualify unknown\fR .DE Note in particular that safe interpreters have no default \fBunknown\fR command, so Tcl's default autoloading facilities are not available. Autoload access to Tcl's commands that are normally autoloaded: .DS .ta 2.1i \fB auto_mkindex auto_mkindex_old auto_reset history parray pkg_mkIndex ::pkg::create ::safe::interpAddToAccessPath ::safe::interpCreate ::safe::interpConfigure ::safe::interpDelete ::safe::interpFindInAccessPath ::safe::interpInit ::safe::setLogCmd tcl_endOfWord tcl_findLibrary tcl_startOfNextWord tcl_startOfPreviousWord tcl_wordBreakAfter tcl_wordBreakBefore\fR .DE can only be provided by explicit definition of an \fBunknown\fR command in the safe interpreter. This will involve exposing the \fBsource\fR command. This is most easily accomplished by creating the safe interpreter with Tcl's \fBSafe\-Tcl\fR mechanism. \fBSafe\-Tcl\fR provides safe versions of \fBsource\fR, \fBload\fR, and other Tcl commands needed to support autoloading of commands and the loading of packages. .VE .PP In addition, the \fBenv\fR variable is not present in a safe interpreter, so it cannot share environment variables with other interpreters. The \fBenv\fR variable poses a security risk, because users can store sensitive information in an environment variable. For example, the PGP manual recommends storing the PGP private key protection password in the environment variable \fIPGPPASS\fR. Making this variable available to untrusted code executing in a safe interpreter would incur a security risk. .PP If extensions are loaded into a safe interpreter, they may also restrict their own functionality to eliminate unsafe commands. For a discussion of management of extensions for safety see the manual entries for \fBSafe\-Tcl\fR and the \fBload\fR Tcl command. .PP A safe interpreter may not alter the recursion limit of any interpreter, including itself. .SH "ALIAS INVOCATION" .PP The alias mechanism has been carefully designed so that it can be used safely when an untrusted script is executing in a safe slave and the target of the alias is a trusted master. The most important thing in guaranteeing safety is to ensure that information passed from the slave to the master is never evaluated or substituted in the master; if this were to occur, it would enable an evil script in the slave to invoke arbitrary functions in the master, which would compromise security. .PP When the source for an alias is invoked in the slave interpreter, the usual Tcl substitutions are performed when parsing that command. These substitutions are carried out in the source interpreter just as they would be for any other command invoked in that interpreter. The command procedure for the source command takes its arguments and merges them with the \fItargetCmd\fR and \fIarg\fRs for the alias to create a new array of arguments. If the words of \fIsrcCmd\fR were ``\fIsrcCmd arg1 arg2 ... argN\fR'', the new set of words will be ``\fItargetCmd arg arg ... arg arg1 arg2 ... argN\fR'', where \fItargetCmd\fR and \fIarg\fRs are the values supplied when the alias was created. \fITargetCmd\fR is then used to locate a command procedure in the target interpreter, and that command procedure is invoked with the new set of arguments. An error occurs if there is no command named \fItargetCmd\fR in the target interpreter. No additional substitutions are performed on the words: the target command procedure is invoked directly, without going through the normal Tcl evaluation mechanism. Substitutions are thus performed on each word exactly once: \fItargetCmd\fR and \fIargs\fR were substituted when parsing the command that created the alias, and \fIarg1 - argN\fR are substituted when the alias's source command is parsed in the source interpreter. .PP When writing the \fItargetCmd\fRs for aliases in safe interpreters, it is very important that the arguments to that command never be evaluated or substituted, since this would provide an escape mechanism whereby the slave interpreter could execute arbitrary code in the master. This in turn would compromise the security of the system. .VS .SH "HIDDEN COMMANDS" .PP Safe interpreters greatly restrict the functionality available to Tcl programs executing within them. Allowing the untrusted Tcl program to have direct access to this functionality is unsafe, because it can be used for a variety of attacks on the environment. However, there are times when there is a legitimate need to use the dangerous functionality in the context of the safe interpreter. For example, sometimes a program must be \fBsource\fRd into the interpreter. Another example is Tk, where windows are bound to the hierarchy of windows for a specific interpreter; some potentially dangerous functions, e.g. window management, must be performed on these windows within the interpreter context. .PP The \fBinterp\fR command provides a solution to this problem in the form of \fIhidden commands\fR. Instead of removing the dangerous commands entirely from a safe interpreter, these commands are hidden so they become unavailable to Tcl scripts executing in the interpreter. However, such hidden commands can be invoked by any trusted ancestor of the safe interpreter, in the context of the safe interpreter, using \fBinterp invoke\fR. Hidden commands and exposed commands reside in separate name spaces. It is possible to define a hidden command and an exposed command by the same name within one interpreter. .PP Hidden commands in a slave interpreter can be invoked in the body of procedures called in the master during alias invocation. For example, an alias for \fBsource\fR could be created in a slave interpreter. When it is invoked in the slave interpreter, a procedure is called in the master interpreter to check that the operation is allowable (e.g. it asks to source a file that the slave interpreter is allowed to access). The procedure then it invokes the hidden \fBsource\fR command in the slave interpreter to actually source in the contents of the file. Note that two commands named \fBsource\fR exist in the slave interpreter: the alias, and the hidden command. .PP Because a master interpreter may invoke a hidden command as part of handling an alias invocation, great care must be taken to avoid evaluating any arguments passed in through the alias invocation. Otherwise, malicious slave interpreters could cause a trusted master interpreter to execute dangerous commands on their behalf. See the section on \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 safe slaves from gaining access to hidden functionality in themselves or their descendants. .PP The set of hidden commands in an interpreter can be manipulated by a trusted interpreter using \fBinterp expose\fR and \fBinterp hide\fR. The \fBinterp expose\fR command moves a hidden command to the set of exposed commands in the interpreter identified by \fIpath\fR, potentially renaming the command in the process. If an exposed command by the targeted name already exists, the operation fails. Similarly, \fBinterp hide\fR moves an exposed command to the set of hidden commands in that interpreter. Safe interpreters are not allowed to move commands between the set of hidden and exposed commands, in either themselves or their descendants. .PP Currently, the names of hidden commands cannot contain namespace qualifiers, and you must first rename a command in a namespace to the global namespace before you can hide it. Commands to be hidden by \fBinterp hide\fR are looked up in the global namespace even if the current namespace is not the global one. This prevents slaves from fooling a master interpreter into hiding the wrong command, by making the current namespace be different from the global one. .VE .SH CREDITS .PP This mechanism is based on the Safe-Tcl prototype implemented by Nathaniel Borenstein and Marshall Rose. .SH EXAMPLES Creating and using an alias for a command in the current interpreter: .CS \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 invokation of \fBlappend\fR is logged: .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 invokation of lappend $args" # Be extremely careful about command construction eval [linsert $args 0 \\ \fBinterp invokehidden\fR $i lappend] } \fBinterp eval\fR $i $someUntrustedScript .CE .SH "SEE ALSO" load(n), safe(n), Tcl_CreateSlave(3) .SH KEYWORDS alias, master interpreter, safe interpreter, slave interpreter