Tcl_CreateInterp, Tcl_DeleteInterp, Tcl_InterpDeleted - create and delete Tcl command interpreters
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
Clients are only allowed to access a few of the fields of
Tcl_Interp structures; see the Tcl_Interp
and Tcl_CreateCommand man pages for details.
The new interpreter is initialized with no defined variables and only
the built-in Tcl commands. To bind in additional commands, call
- Tcl_Interp *interp (in)
Token for interpreter to be destroyed.
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 interp->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_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
- 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.
Do not call Tcl_DeleteInterp on an interpreter for which
Tcl_InterpDeleted returns nonzero.
- 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 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; 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
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.
command, create, delete, interpreter
Copyright © 1989-1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995, 1996 Roger E. Critchlow Jr.