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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 procedures, 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 the built-in Tcl commands and with the variables
documented in tclvars(n). 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 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 argu-
ment; 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, fur-
ther 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 inter-
preter 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_Inter-
pDeleted 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 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; 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.
SEE ALSO
Tcl_Preserve(3), Tcl_Release(3)
KEYWORDS
command, create, delete, interpreter
Tcl 7.5 Tcl_CreateInterp(3)
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