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.  Whenever 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_GetIntFromObj;  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_GetDoubleFromObj;  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