CrtInterp
Tcl_CreateInterp(3) Tcl Library Procedures Tcl_CreateInterp(3)
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NAME
Tcl_CreateInterp, Tcl_DeleteInterp, Tcl_InterpDeleted - create and
delete Tcl command interpreters
SYNOPSIS
#include <tcl.h>
Tcl_Interp *
Tcl_CreateInterp()
Tcl_DeleteInterp(interp)
int
Tcl_InterpDeleted(interp)
ARGUMENTS
Tcl_Interp *interp (in) Token for interpreter to be destroyed.
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DESCRIPTION
Tcl_CreateInterp creates a new interpreter structure and returns a
token for it. The token is required in calls to most other Tcl proce-
dures, such as Tcl_CreateCommand, Tcl_Eval, and Tcl_DeleteInterp.
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_CreateCommand.
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 evalu-
ation stack. See below, in the section INTERPRETERS AND MEMORY MANAGE-
MENT for details.
Tcl_InterpDeleted returns nonzero if Tcl_DeleteInterp was called with
interp as its argument; this indicates that the interpreter will even-
tually 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.
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 argu-
ments. 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_Get-
Var, 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 mech-
anism should be set up by the code that creates the data struc-
ture 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.
SEE ALSO
Tcl_Preserve(3), Tcl_Release(3)
KEYWORDS
command, create, delete, interpreter
Tcl 7.5 Tcl_CreateInterp(3)
