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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: @(#) trace.n 1.12 96/08/26 13:00:18
+'\" 
+.so man.macros
+.TH trace n "" Tcl "Tcl Built-In Commands"
+.BS
+'\" Note:  do not modify the .SH NAME line immediately below!
+.SH NAME
+trace \- Monitor variable accesses
+.SH SYNOPSIS
+\fBtrace \fIoption\fR ?\fIarg arg ...\fR?
+.BE
+
+.SH DESCRIPTION
+.PP
+This command causes Tcl commands to be executed whenever certain operations are
+invoked.  At present, only variable tracing is implemented. The
+legal \fIoption\fR's (which may be abbreviated) are:
+.TP
+\fBtrace variable \fIname ops command\fR
+Arrange for \fIcommand\fR to be executed whenever variable \fIname\fR
+is accessed in one of the ways given by \fIops\fR.  \fIName\fR may
+refer to a normal variable, an element of an array, or to an array
+as a whole (i.e. \fIname\fR may be just the name of an array, with no
+parenthesized index).  If \fIname\fR refers to a whole array, then
+\fIcommand\fR is invoked whenever any element of the array is
+manipulated.
+.RS
+.PP
+\fIOps\fR indicates which operations are of interest, and consists of
+one or more of the following letters:
+.TP
+\fBr\fR
+Invoke \fIcommand\fR whenever the variable is read.
+.TP
+\fBw\fR
+Invoke \fIcommand\fR whenever the variable is written.
+.TP
+\fBu\fR
+Invoke \fIcommand\fR whenever the variable is unset.  Variables
+can be unset explicitly with the \fBunset\fR command, or
+implicitly when procedures return (all of their local variables
+are unset).  Variables are also unset when interpreters are
+deleted, but traces will not be invoked because there is no
+interpreter in which to execute them.
+.PP
+When the trace triggers, three arguments are appended to
+\fIcommand\fR so that the actual command is as follows:
+.CS
+\fIcommand name1 name2 op\fR
+.CE
+\fIName1\fR and \fIname2\fR give the name(s) for the variable
+being accessed:  if the variable is a scalar then \fIname1\fR
+gives the variable's name and \fIname2\fR is an empty string;
+if the variable is an array element then \fIname1\fR gives the
+name of the array and name2 gives the index into the array;
+if an entire array is being deleted and the trace was registered
+on the overall array, rather than a single element, then \fIname1\fR
+gives the array name and \fIname2\fR is an empty string.
+\fIName1\fR and \fIname2\fR are not necessarily the same as the
+name used in the \fBtrace variable\fR command:  the \fBupvar\fR
+command allows a procedure to reference a variable under a
+different name.
+\fIOp\fR indicates what operation is being performed on the
+variable, and is one of \fBr\fR, \fBw\fR, or \fBu\fR as
+defined above.
+.PP
+\fICommand\fR executes in the same context as the code that invoked
+the traced operation:  if the variable was accessed as part of a
+Tcl procedure, then \fIcommand\fR will have access to the same
+local variables as code in the procedure.  This context may be
+different than the context in which the trace was created.
+If \fIcommand\fR invokes a procedure (which it normally does) then
+the procedure will have to use \fBupvar\fR or \fBuplevel\fR if it
+wishes to access the traced variable.
+Note also that \fIname1\fR may not necessarily be the same as the name
+used to set the trace on the variable;  differences can occur if
+the access is made through a variable defined with the \fBupvar\fR
+command.
+.PP
+For read and write traces, \fIcommand\fR can modify
+the variable to affect the result of the traced operation.
+If \fIcommand\fR modifies the value of a variable during a
+read or write trace, then the new value will be returned as the
+result of the traced operation.
+The return value from  \fIcommand\fR is ignored except that
+if it returns an error of any sort then the traced operation
+also returns an error with
+the same error message returned by the trace command
+(this mechanism can be used to implement read-only variables, for
+example).
+For write traces, \fIcommand\fR is invoked after the variable's
+value has been changed; it can write a new value into the variable
+to override the original value specified in the write operation.
+To implement read-only variables, \fIcommand\fR will have to restore
+the old value of the variable.
+.PP
+While \fIcommand\fR is executing during a read or write trace, traces
+on the variable are temporarily disabled.
+This means that reads and writes invoked by
+\fIcommand\fR will occur directly, without invoking \fIcommand\fR
+(or any other traces) again.
+However, if \fIcommand\fR unsets the variable then unset traces
+will be invoked.
+.PP
+When an unset trace is invoked, the variable has already been
+deleted:  it will appear to be undefined with no traces.
+If an unset occurs because of a procedure return, then the
+trace will be invoked in the variable context of the procedure
+being returned to:  the stack frame of the returning procedure
+will no longer exist.
+Traces are not disabled during unset traces, so if an unset trace
+command creates a new trace and accesses the variable, the
+trace will be invoked.
+Any errors in unset traces are ignored.
+.PP
+If there are multiple traces on a variable they are invoked
+in order of creation, most-recent first.
+If one trace returns an error, then no further traces are
+invoked for the variable.
+If an array element has a trace set, and there is also a trace
+set on the array as a whole, the trace on the overall array
+is invoked before the one on the element.
+.PP
+Once created, the trace remains in effect either until the
+trace is removed with the \fBtrace vdelete\fR command described
+below, until the variable is unset, or until the interpreter
+is deleted.
+Unsetting an element of array will remove any traces on that
+element, but will not remove traces on the overall array.
+.PP
+This command returns an empty string.
+.RE
+.TP
+\fBtrace vdelete \fIname ops command\fR
+If there is a trace set on variable \fIname\fR with the
+operations and command given by \fIops\fR and \fIcommand\fR,
+then the trace is removed, so that \fIcommand\fR will never
+again be invoked.
+Returns an empty string.
+.TP
+\fBtrace vinfo \fIname\fR
+Returns a list containing one element for each trace
+currently set on variable \fIname\fR.
+Each element of the list is itself a list containing two
+elements, which are the \fIops\fR and \fIcommand\fR associated
+with the trace.
+If \fIname\fR doesn't exist or doesn't have any traces set, then
+the result of the command will be an empty string.
+
+.SH KEYWORDS
+read, variable, write, trace, unset