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. 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 removed from safe interpreters. See SAFE INTERPRETERS below for more information on what features are present in a safe interpreter. The alias mechanism can be used for protected communication (analogous to a kernel call) between a slave interpreter and its master.
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 is used to create, delete, and manipulate slave interpreters. It can have any of several forms, depending on the option argument:
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:
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.
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:
append array break case catch clock close concat continue eof error eval expr fblocked flush for foreach format gets global history if incr info interp join lappend lindex list llength lrange lreplace pid proc puts read regexp regsub rename return scan set seek split string switch tell traceAll commands not on this list are removed from the interpreter by the interp create command. Of course, the missing commands can be recreated later as Tcl procedures or aliases.
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. The management of extensions for safety will be explained in the manual entries for the package and load Tcl commands.
This mechanism is based on the Safe-Tcl prototype implemented by Nathaniel Borenstein and Marshall Rose.