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'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\" RCS: @(#) $Id: LinkVar.3,v 1.1.1.1 2002-01-16 10:25:24 markom Exp $
'\" 
.so man.macros
.TH Tcl_LinkVar 3 7.5 Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_LinkVar, Tcl_UnlinkVar, Tcl_UpdateLinkedVar \- link Tcl variable to C variable
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
int
\fBTcl_LinkVar\fR(\fIinterp, varName, addr, type\fR)
.sp
\fBTcl_UnlinkVar\fR(\fIinterp, varName\fR)
.sp
\fBTcl_UpdateLinkedVar\fR(\fIinterp, varName\fR)
.SH ARGUMENTS
.AS Tcl_Interp writable
.AP Tcl_Interp *interp in
Interpreter that contains \fIvarName\fR.
Also used by \fBTcl_LinkVar\fR to return error messages.
.AP char *varName in
Name of global variable.  Must be in writable memory: Tcl may make
temporary modifications to it while parsing the variable name.
.AP char *addr in
Address of C variable that is to be linked to \fIvarName\fR.
.AP 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 variable read-only.
.BE

.SH DESCRIPTION
.PP
\fBTcl_LinkVar\fR uses variable traces to keep the Tcl variable
named by \fIvarName\fR in sync with the C variable at the address
given by \fIaddr\fR.
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.
\fBTcl_LinkVar\fR normally returns TCL_OK;  if an error occurs
while setting up the link (e.g. because \fIvarName\fR is the
name of array) then TCL_ERROR is returned and \fIinterp->result\fR
contains an error message.
.PP
The \fItype\fR argument specifies the type of the C variable,
and must have one of the following values, optionally OR'ed with
TCL_LINK_READ_ONLY:
.TP
\fBTCL_LINK_INT\fR
The C variable is of type \fBint\fR.
Any value written into the Tcl variable must have a proper integer
form acceptable to \fBTcl_GetInt\fR;  attempts to write
non-integer values into \fIvarName\fR will be rejected with
Tcl errors.
.TP
\fBTCL_LINK_DOUBLE\fR
The C variable is of type \fBdouble\fR.
Any value written into the Tcl variable must have a proper real
form acceptable to \fBTcl_GetDouble\fR;  attempts to write
non-real values into \fIvarName\fR will be rejected with
Tcl errors.
.TP
\fBTCL_LINK_BOOLEAN\fR
The C variable is of type \fBint\fR.
If its value is zero then it will read from Tcl as ``0'';
otherwise it will read from Tcl as ``1''.
Whenever \fIvarName\fR 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 \fBTcl_GetBoolean\fR;  attempts to write
non-boolean values into \fIvarName\fR will be rejected with
Tcl errors.
.TP
\fBTCL_LINK_STRING\fR
The C variable is of type \fBchar *\fR.
.VS
If its value is not null then it must be a pointer to a string
allocated with \fBTcl_Alloc\fR.
.VE
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''.
.PP
If the TCL_LINK_READ_ONLY flag is present in \fItype\fR 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.
.PP
\fBTcl_UnlinkVar\fR removes the link previously set up for the
variable given by \fIvarName\fR.  If there does not exist a link
for \fIvarName\fR then the procedure has no effect.
.PP
\fBTcl_UpdateLinkedVar\fR 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.
\fBTcl_UpdateLinkedVar\fR ensures that any traces on the Tcl
variable are invoked.

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