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+'\"
+'\" 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.
+'\" 
+'\" SCCS: @(#) LinkVar.3 1.15 96/09/05 17:16:57
+'\" 
+.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