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(3tcl).  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
       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 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  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.

SEE ALSO

       Tcl_Preserve(3tcl), Tcl_Release(3tcl)

KEYWORDS

       command, create, delete, interpreter