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Tcl_LinkVar(3)		    Tcl Library Procedures		Tcl_LinkVar(3)

______________________________________________________________________________

NAME
       Tcl_LinkVar,  Tcl_UnlinkVar, Tcl_UpdateLinkedVar - link Tcl variable to
       C variable

SYNOPSIS
       #include <tcl.h>

       int
       Tcl_LinkVar(interp, varName, addr, type)

       Tcl_UnlinkVar(interp, varName)

       Tcl_UpdateLinkedVar(interp, varName)

ARGUMENTS
       Tcl_Interp   *interp    (in)	 Interpreter  that  contains  varName.
					 Also  used  by	 Tcl_LinkVar to return
					 error messages.

       char	    *varName   (in)	 Name of global variable.  Must be  in
					 writable  memory: Tcl may make tempo‐
					 rary modifications to it while	 pars‐
					 ing the variable name.

       char	    *addr      (in)	 Address  of  C variable that is to be
					 linked to varName.

       int	    type       (in)	 Type of C variable.  Must be  one  of
					 TCL_LINK_INT,	      TCL_LINK_DOUBLE,
					 TCL_LINK_BOOLEAN, or TCL_LINK_STRING,
					 optionally	    OR'ed	  with
					 TCL_LINK_READ_ONLY to make Tcl	 vari‐
					 able read-only.
_________________________________________________________________

DESCRIPTION
       Tcl_LinkVar uses variable traces to keep the Tcl variable named by var‐
       Name in sync with the C variable at the address given by	 addr.	 When‐
       ever  the  Tcl  variable	 is  read  the value of the C variable will be
       returned, and whenever the Tcl variable is written the C variable  will
       be  updated  to	have  the  same	 value.	  Tcl_LinkVar normally returns
       TCL_OK;	if an error occurs while setting up  the  link	(e.g.  because
       varName	 is  the  name	of  array)  then  TCL_ERROR  is	 returned  and
       interp->result contains an error message.

       The type argument specifies the type of the C variable, and  must  have
       one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:

       TCL_LINK_INT
	      The  C  variable is of type int.	Any value written into the Tcl
	      variable	must  have  a  proper  integer	form   acceptable   to
	      Tcl_GetInt;   attempts  to write non-integer values into varName
	      will be rejected with Tcl errors.

       TCL_LINK_DOUBLE
	      The C variable is of type double.	 Any value  written  into  the
	      Tcl variable must have a proper real form acceptable to Tcl_Get‐
	      Double;  attempts to write non-real values into varName will  be
	      rejected with Tcl errors.

       TCL_LINK_BOOLEAN
	      The  C  variable	is  of type int.  If its value is zero then it
	      will read from Tcl as ``0''; otherwise it will read from Tcl  as
	      ``1''.  Whenever varName is modified, the C variable will be set
	      to a 0 or 1 value.  Any value written into the Tcl variable must
	      have   a	proper	boolean	 form  acceptable  to  Tcl_GetBoolean;
	      attempts to  write  non-boolean  values  into  varName  will  be
	      rejected with Tcl errors.

       TCL_LINK_STRING
	      The C variable is of type char *.	 If its value is not null then │
	      it must be a pointer  to	a  string  allocated  with  Tcl_Alloc.
	      Whenever	the Tcl variable is modified the current C string will
	      be freed and new memory will be allocated to hold a copy of  the
	      variable's new value.  If the C variable contains a null pointer
	      then the Tcl variable will read as ``NULL''.

       If the TCL_LINK_READ_ONLY flag is present in  type  then	 the  variable
       will  be	 read-only  from Tcl, so that its value can only be changed by
       modifying the C variable.  Attempts to write the variable from Tcl will
       be rejected with errors.

       Tcl_UnlinkVar removes the link previously set up for the variable given
       by varName.  If there does not exist a link for varName then the proce‐
       dure has no effect.

       Tcl_UpdateLinkedVar  may be invoked after the C variable has changed to
       force the Tcl variable to be updated immediately.  In many  cases  this
       procedure  is  not  needed,  since any attempt to read the Tcl variable
       will return the latest value of the C variable.	However,  if  a	 trace
       has  been  set  on the Tcl variable (such as a Tk widget that wishes to
       display the value of the variable), the trace will not trigger when the
       C variable has changed.	Tcl_UpdateLinkedVar ensures that any traces on
       the Tcl variable are invoked.

KEYWORDS
       boolean, integer, link, read-only, real, string, traces, variable

Tcl				      7.5			Tcl_LinkVar(3)
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