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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: trace.n,v 1.1.1.1 2002-01-16 10:25:25 markom Exp $

'\"

.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