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Tcl_CreateInterp(3) Tcl Library Procedures Tcl_CreateInterp(3)
Tcl_CreateInterp, Tcl_DeleteInterp, Tcl_InterpActive, Tcl_InterpDeleted
- create and delete Tcl command interpreters
Tcl_Interp *interp (in) Token for interpreter to be destroyed
Tcl_CreateInterp creates a new interpreter structure and returns a
token for it. The token is required in calls to most other Tcl
procedures, such as Tcl_CreateCommand, Tcl_Eval, and Tcl_DeleteInterp.
The token returned by Tcl_CreateInterp may only be passed to Tcl
routines called from the same thread as the original Tcl_CreateInterp
call. It is not safe for multiple threads to pass the same token to
Tcl's routines. The new interpreter is initialized with the built-in
Tcl commands and with standard variables like tcl_platform and env. To
bind in additional commands, call Tcl_CreateCommand, and to create
additional variables, call Tcl_SetVar.
Tcl_DeleteInterp marks an interpreter as deleted; the interpreter will
eventually be deleted when all calls to Tcl_Preserve for it have been
matched by calls to Tcl_Release. At that time, all of the resources
associated with it, including variables, procedures, and application-
specific command bindings, will be deleted. After Tcl_DeleteInterp
returns any attempt to use Tcl_Eval on the interpreter will fail and
return TCL_ERROR. After the call to Tcl_DeleteInterp it is safe to
examine the interpreter's result, query or set the values of variables,
define, undefine or retrieve procedures, and examine the runtime
evaluation stack. See below, in the section INTERPRETERS AND MEMORY
MANAGEMENT for details.
Tcl_InterpDeleted returns nonzero if Tcl_DeleteInterp was called with
interp as its argument; this indicates that the interpreter will
eventually be deleted, when the last call to Tcl_Preserve for it is
matched by a call to Tcl_Release. If nonzero is returned, further calls
to Tcl_Eval in this interpreter will return TCL_ERROR.
Tcl_InterpDeleted is useful in deletion callbacks to distinguish
between when only the memory the callback is responsible for is being
deleted and when the whole interpreter is being deleted. In the former
case the callback may recreate the data being deleted, but this would
lead to an infinite loop if the interpreter were being deleted.
Tcl_InterpActive is useful for determining whether there is any |
execution of scripts ongoing in an interpreter, which is a useful piece |
of information when Tcl is embedded in a garbage-collected environment |
and it becomes necessary to determine whether the interpreter is a |
candidate for deletion. The function returns a true value if the |
interpreter has at least one active execution running inside it, and a |
false value otherwise.
INTERPRETERS AND MEMORY MANAGEMENT
Tcl_DeleteInterp can be called at any time on an interpreter that may
be used by nested evaluations and C code in various extensions. Tcl
implements a simple mechanism that allows callers to use interpreters
without worrying about the interpreter being deleted in a nested call,
and without requiring special code to protect the interpreter, in most
cases. This mechanism ensures that nested uses of an interpreter can
safely continue using it even after Tcl_DeleteInterp is called.
The mechanism relies on matching up calls to Tcl_Preserve with calls to
Tcl_Release. If Tcl_DeleteInterp has been called, only when the last
call to Tcl_Preserve is matched by a call to Tcl_Release, will the
interpreter be freed. See the manual entry for Tcl_Preserve for a
description of these functions.
The rules for when the user of an interpreter must call Tcl_Preserve
and Tcl_Release are simple:
Interpreters Passed As Arguments
Functions that are passed an interpreter as an argument can
safely use the interpreter without any special protection. Thus,
when you write an extension consisting of new Tcl commands, no
special code is needed to protect interpreters received as
arguments. This covers the majority of all uses.
Interpreter Creation And Deletion
When a new interpreter is created and used in a call to
Tcl_Eval, Tcl_VarEval, Tcl_GlobalEval, Tcl_SetVar, or
Tcl_GetVar, a pair of calls to Tcl_Preserve and Tcl_Release
should be wrapped around all uses of the interpreter. Remember
that it is unsafe to use the interpreter once Tcl_Release has
been called. To ensure that the interpreter is properly deleted
when it is no longer needed, call Tcl_InterpDeleted to test if
some other code already called Tcl_DeleteInterp; if not, call
Tcl_DeleteInterp before calling Tcl_Release in your own code.
Retrieving An Interpreter From A Data Structure
When an interpreter is retrieved from a data structure (e.g. the
client data of a callback) for use in one of the evaluation
functions (Tcl_Eval, Tcl_VarEval, Tcl_GlobalEval, Tcl_EvalObjv,
etc.) or variable access functions (Tcl_SetVar, Tcl_GetVar,
Tcl_SetVar2Ex, etc.), a pair of calls to Tcl_Preserve and
Tcl_Release should be wrapped around all uses of the
interpreter; it is unsafe to reuse the interpreter once
Tcl_Release has been called. If an interpreter is stored inside
a callback data structure, an appropriate deletion cleanup
mechanism should be set up by the code that creates the data
structure so that the interpreter is removed from the data
structure (e.g. by setting the field to NULL) when the
interpreter is deleted. Otherwise, you may be using an
interpreter that has been freed and whose memory may already
have been reused.
All uses of interpreters in Tcl and Tk have already been protected.
Extension writers should ensure that their code also properly protects
any additional interpreters used, as described above.
Note that the protection mechanisms do not work well with conventional |
garbage collection systems. When in such a managed environment, |
Tcl_InterpActive should be used to determine when an interpreter is a |
candidate for deletion due to inactivity.
command, create, delete, interpreter
Tcl 7.5 Tcl_CreateInterp(3)