Tcl_CreateInterp(3) Tcl Library Procedures Tcl_CreateInterp(3)
Tcl_CreateInterp, Tcl_DeleteInterp, 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 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 the built-in Tcl com-
mands 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 applica-
tion-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 vari-
ables, 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.
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 inter-
preter 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 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 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.
command, create, delete, interpreter
Tcl 7.5 Tcl_CreateInterp(3)