trace.n

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.TP
\fBarray\fR
Invoke \fIcommand\fR whenever the variable is accessed or modified via
the \fBarray\fR command, provided that \fIname\fR is not a scalar
variable at the time that the \fBarray\fR command is invoked.  If
\fIname\fR is a scalar variable, the access via the \fBarray\fR
command will not trigger the trace.
.TP
\fBread\fR
Invoke \fIcommand\fR whenever the variable is read.
.TP
\fBwrite\fR
Invoke \fIcommand\fR whenever the variable is written.
.TP
\fBunset\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 \fBread\fR, \fBwrite\fR, or \fBunset\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 remove variable\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
.RE
.TP
\fBtrace remove \fItype name opList command\fR
Where \fItype\fR is either \fBcommand\fR, \fBexecution\fR or \fBvariable\fR.
.RS
.TP
\fBtrace remove command\fI name opList command\fR
If there is a trace set on command \fIname\fR with the operations and
command given by \fIopList\fR and \fIcommand\fR, then the trace is
removed, so that \fIcommand\fR will never again be invoked.  Returns
an empty string.   If \fIname\fR doesn't exist, the command will throw
an error.
.TP
\fBtrace remove execution\fI name opList command\fR
If there is a trace set on command \fIname\fR with the operations and
command given by \fIopList\fR and \fIcommand\fR, then the trace is
removed, so that \fIcommand\fR will never again be invoked.  Returns
an empty string.   If \fIname\fR doesn't exist, the command will throw
an error.
.TP
\fBtrace remove variable\fI name opList command\fR
If there is a trace set on variable \fIname\fR with the operations and
command given by \fIopList\fR and \fIcommand\fR, then the trace is
removed, so that \fIcommand\fR will never again be invoked.  Returns
an empty string.
.RE
.TP
\fBtrace info \fItype name\fR
Where \fItype\fR is either \fBcommand\fR, \fBexecution\fR or \fBvariable\fR.
.RS
.TP
\fBtrace info command\fI name\fR
Returns a list containing one element for each trace currently set on
command \fIname\fR. Each element of the list is itself a list
containing two elements, which are the \fIopList\fR and \fIcommand\fR
associated with the trace.  If \fIname\fR doesn't have any traces set,
then the result of the command will be an empty string.  If \fIname\fR
doesn't exist, the command will throw an error.
.TP
\fBtrace info execution\fI name\fR
Returns a list containing one element for each trace currently set on
command \fIname\fR. Each element of the list is itself a list
containing two elements, which are the \fIopList\fR and \fIcommand\fR
associated with the trace.  If \fIname\fR doesn't have any traces set,
then the result of the command will be an empty string.  If \fIname\fR
doesn't exist, the command will throw an error.
.TP
\fBtrace info variable\fI name\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 \fIopList\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.
.RE
.PP
For backwards compatibility, three other subcommands are available:
.RS
.TP
\fBtrace variable \fIname ops command\fR
This is equivalent to \fBtrace add variable \fIname ops command\fR.
.TP
\fBtrace vdelete \fIname ops command\fR
This is equivalent to \fBtrace remove variable \fIname ops command\fR
.TP 
\fBtrace vinfo \fIname\fR 
This is equivalent to \fBtrace info variable \fIname\fR
.RE
.PP
These subcommands are deprecated and will likely be removed in a
future version of Tcl.  They use an older syntax in which \fBarray\fR,
\fBread\fR, \fBwrite\fR, \fBunset\fR are replaced by \fBa\fR, \fBr\fR,
\fBw\fR and \fBu\fR respectively, and the \fIops\fR argument is not a
list, but simply a string concatenation of the operations, such as
\fBrwua\fR.

.SH "SEE ALSO"
set(n), unset(n)

.SH KEYWORDS
read, command, rename, variable, write, trace, unset