'\"
'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" RCS: @(#) $Id: LinkVar.3,v 1.16 2007/12/13 15:22:31 dgp 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 "const char" *varName in
Name of global variable.
.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 \fBTCL_LINK_INT\fR,
.VS 8.5
\fBTCL_LINK_UINT\fR, \fBTCL_LINK_CHAR\fR, \fBTCL_LINK_UCHAR\fR,
\fBTCL_LINK_SHORT\fR, \fBTCL_LINK_USHORT\fR, \fBTCL_LINK_LONG\fR,
\fBTCL_LINK_ULONG\fR,
.VE 8.5
\fBTCL_LINK_WIDE_INT\fR,
.VS 8.5
\fBTCL_LINK_WIDE_UINT\fR, \fBTCL_LINK_FLOAT\fR,
.VE 8.5
\fBTCL_LINK_DOUBLE\fR, \fBTCL_LINK_BOOLEAN\fR, or
\fBTCL_LINK_STRING\fR, optionally OR'ed with \fBTCL_LINK_READ_ONLY\fR
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 \fBTCL_OK\fR;  if an error occurs
while setting up the link (e.g. because \fIvarName\fR is the
name of array) then \fBTCL_ERROR\fR is returned and the interpreter's result
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
\fBTCL_LINK_READ_ONLY\fR:
.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_GetIntFromObj\fR;  attempts to write
non-integer values into \fIvarName\fR will be rejected with
Tcl errors.
.VS 8.5
.TP
\fBTCL_LINK_UINT\fR
The C variable is of type \fBunsigned int\fR.
Any value written into the Tcl variable must have a proper unsigned
integer form acceptable to \fBTcl_GetWideIntFromObj\fR and in the
platform's defined range for the \fBunsigned int\fR type; attempts to
write non-integer values (or values outside the range) into
\fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_CHAR\fR
The C variable is of type \fBchar\fR.
Any value written into the Tcl variable must have a proper integer
form acceptable to \fBTcl_GetIntFromObj\fR and be in the range of the
\fBchar\fR datatype; attempts to write non-integer or out-of-range
values into \fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_UCHAR\fR
The C variable is of type \fBunsigned char\fR.
Any value written into the Tcl variable must have a proper unsigned
integer form acceptable to \fBTcl_GetIntFromObj\fR and in the
platform's defined range for the \fBunsigned char\fR type; attempts to
write non-integer values (or values outside the range) into
\fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_SHORT\fR
The C variable is of type \fBshort\fR.
Any value written into the Tcl variable must have a proper integer
form acceptable to \fBTcl_GetIntFromObj\fR and be in the range of the
\fBshort\fR datatype; attempts to write non-integer or out-of-range
values into \fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_USHORT\fR
The C variable is of type \fBunsigned short\fR.
Any value written into the Tcl variable must have a proper unsigned
integer form acceptable to \fBTcl_GetIntFromObj\fR and in the
platform's defined range for the \fBunsigned short\fR type; attempts to
write non-integer values (or values outside the range) into
\fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_LONG\fR
The C variable is of type \fBlong\fR.
Any value written into the Tcl variable must have a proper integer
form acceptable to \fBTcl_GetLongFromObj\fR; attempts to write
non-integer or out-of-range
values into \fIvarName\fR will be rejected with Tcl errors.
.TP
\fBTCL_LINK_ULONG\fR
The C variable is of type \fBunsigned long\fR.
Any value written into the Tcl variable must have a proper unsigned
integer form acceptable to \fBTcl_GetWideIntFromObj\fR and in the
platform's defined range for the \fBunsigned long\fR type; attempts to
write non-integer values (or values outside the range) into
\fIvarName\fR will be rejected with Tcl errors.
.VE 8.5
.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_GetDoubleFromObj\fR;  attempts to write
non-real values into \fIvarName\fR will be rejected with
Tcl errors.
.VS 8.5
.TP
\fBTCL_LINK_FLOAT\fR
The C variable is of type \fBfloat\fR.
Any value written into the Tcl variable must have a proper real
form acceptable to \fBTcl_GetDoubleFromObj\fR and must be within the
range acceptable for a \fBfloat\fR; attempts to
write non-real values (or values outside the range) into
\fIvarName\fR will be rejected with Tcl errors.
.VE 8.5
.TP
\fBTCL_LINK_WIDE_INT\fR
The C variable is of type \fBTcl_WideInt\fR (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 \fBTcl_GetWideIntFromObj\fR;  attempts to write
non-integer values into \fIvarName\fR will be rejected with
Tcl errors.
.VS 8.5
.TP
\fBTCL_LINK_WIDE_UINT\fR
The C variable is of type \fBTcl_WideUInt\fR (which is an unsigned
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 unsigned
integer form acceptable to \fBTcl_GetWideIntFromObj\fR (it will be
cast to unsigned);
.\" FIXME! Use bignums instead.
attempts to write non-integer values into \fIvarName\fR will be
rejected with Tcl errors.
.VE 8.5
.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
.QW 0 ;
otherwise it will read from Tcl as
.QW 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_GetBooleanFromObj\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.
If its value is not NULL then it must be a pointer to a string
allocated with \fBTcl_Alloc\fR or \fBckalloc\fR.
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
.QW NULL .
.PP
If the \fBTCL_LINK_READ_ONLY\fR 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