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.

       CONST char   *varName   (in)	 Name of global variable.

       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_WIDE_INT, TCL_LINK_BOOLEAN, |
					 or TCL_LINK_STRING, optionally OR'ed with TCL_LINK_READ_ONLY to make Tcl variable read-only.
_________________________________________________________________

DESCRIPTION

       Tcl_LinkVar uses variable traces to keep the Tcl variable named by varName 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 the interpreter's 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_GetIntFro-
	      mObj;  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_GetDouble-
	      FromObj;	attempts to write non-real values into varName will be rejected with Tcl errors.

       TCL_LINK_WIDE_INT
	      The C variable is of type Tcl_WideInt (which is an integer type at least 64-bits wide on all platforms that can  support	it.)   Any |
	      value  written  into  the  Tcl variable must have a proper integer form acceptable to Tcl_GetWideIntFromObj;  attempts to write non- |
	      integer 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_GetBooleanFromObj;	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 or |
	      ckalloc.	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
       procedure 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)