trace.n

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'\"
'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\" Copyright (c) 2000 Ajuba Solutions.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" RCS: @(#) $Id: trace.n,v 1.13 2003/02/04 00:56:20 hobbs Exp $
'\" 
.so man.macros
.TH trace n "8.4" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
trace \- Monitor variable accesses, command usages and command executions
.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.  The legal \fIoption\fR's (which may be abbreviated) are:
.TP
\fBtrace add \fItype name ops ?args?\fR
Where \fItype\fR is \fBcommand\fR, \fBexecution\fR, or \fBvariable\fR.
.RS
.TP
\fBtrace add command\fR \fIname ops command\fR
Arrange for \fIcommand\fR to be executed whenever command \fIname\fR
is modified in one of the ways given by the list \fIops\fR.  \fIName\fR will be
resolved using the usual namespace resolution rules used by
procedures.  If the command does not exist, an error will be thrown.
.RS
.PP
\fIOps\fR indicates which operations are of interest, and is a list of
one or more of the following items:
.TP
\fBrename\fR
Invoke \fIcommand\fR whenever the command is renamed.  Note that
renaming to the empty string is considered deletion, and will not
be traced with '\fBrename\fR'.
.TP
\fBdelete\fR
Invoke \fIcommand\fR when the command is deleted.  Commands can be
deleted explicitly by using the \fBrename\fR command to rename the
command to an empty string.  Commands are also deleted when the
interpreter is deleted, but traces will not be invoked because there is no
interpreter in which to execute them.
.PP
When the trace triggers, depending on the operations being traced, a 
number of arguments are appended to \fIcommand\fR so that the actual 
command is as follows:
.CS
\fIcommand oldName newName op\fR
.CE
\fIOldName\fR and \fInewName\fR give the traced command's current
(old) name, and the name to which it is being renamed (the empty
string if this is a 'delete' operation).
\fIOp\fR indicates what operation is being performed on the
command, and is one of \fBrename\fR or \fBdelete\fR as
defined above.  The trace operation cannot be used to stop a command
from being deleted.  Tcl will always remove the command once the trace
is complete.  Recursive renaming or deleting will not cause further traces 
of the same type to be evaluated, so a delete trace which itself
deletes the command, or a rename trace which itself renames the
command will not cause further trace evaluations to occur.
Both \fIoldName\fR and \fInewName\fR are fully qualified with any namespace(s)
in which they appear.
.RE
.TP
\fBtrace add execution\fR \fIname ops command\fR
Arrange for \fIcommand\fR to be executed whenever command \fIname\fR
is modified in one of the ways given by the list \fIops\fR.  \fIName\fR will be
resolved using the usual namespace resolution rules used by
procedures.  If the command does not exist, an error will be thrown.
.RS
.PP
\fIOps\fR indicates which operations are of interest, and is a list of
one or more of the following items:
.TP
\fBenter\fR
Invoke \fIcommand\fR whenever the command \fIname\fR is executed,
just before the actual execution takes place.
.TP
\fBleave\fR
Invoke \fIcommand\fR whenever the command \fIname\fR is executed,
just after the actual execution takes place.
.TP
\fBenterstep\fR
Invoke \fIcommand\fR for every tcl command which is executed 
inside the procedure \fIname\fR, just before the actual execution
takes place.  For example if we have 'proc foo {} { puts "hello" }',
then a \fIenterstep\fR trace would be 
invoked just before \fIputs "hello"\fR is executed.
Setting a \fIenterstep\fR trace on a \fIcommand\fR
will not result in an error and is simply ignored.
.TP
\fBleavestep\fR
Invoke \fIcommand\fR for every tcl command which is executed 
inside the procedure \fIname\fR, just after the actual execution
takes place.
Setting a \fIleavestep\fR trace on a \fIcommand\fR
will not result in an error and is simply ignored.
.PP
When the trace triggers, depending on the operations being traced, a 
number of arguments are appended to \fIcommand\fR so that the actual 
command is as follows:
.PP
For \fBenter\fR and \fBenterstep\fR operations:
.CS
\fIcommand command-string op\fR
.CE
\fICommand-string\fR gives the complete current command being 
executed (the traced command for a \fBenter\fR operation, an 
arbitrary command for a \fBenterstep\fR operation), including
all arguments in their fully expanded form.
\fIOp\fR indicates what operation is being performed on the
command execution, and is one of \fBenter\fR or \fBenterstep\fR as
defined above.  The trace operation can be used to stop the
command from executing, by deleting the command in question.  Of
course when the command is subsequently executed, an 'invalid command'
error will occur.
.PP
For \fBleave\fR and \fBleavestep\fR operations:
.CS
\fIcommand command-string code result op\fR
.CE
\fICommand-string\fR gives the complete current command being 
executed (the traced command for a \fBenter\fR operation, an 
arbitrary command for a \fBenterstep\fR operation), including
all arguments in their fully expanded form.
\fICode\fR gives the result code of that execution, and \fIresult\fR
the result string.
\fIOp\fR indicates what operation is being performed on the
command execution, and is one of \fBleave\fR or \fBleavestep\fR as
defined above.  
Note that the creation of many \fBenterstep\fR or
\fBleavestep\fR traces can lead to unintuitive results, since the
invoked commands from one trace can themselves lead to further
command invocations for other traces.

\fICommand\fR executes in the same context as the code that invoked
the traced operation: thus the \fIcommand\fR, if invoked from a procedure,
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 upvar or uplevel commands if it wishes
to access the local variables of the code which invoked the trace operation.

While \fIcommand\fR is executing during an execution trace, traces
on \fIname\fR are temporarily disabled. This allows the \fIcommand\fR
to execute \fIname\fR in its body without invoking any other traces again.
If an error occurs while executing the \fIcommand\fR body, then the
\fIcommand\fR name as a whole will return that same error.

When multiple traces are set on \fIname\fR, then for \fIenter\fR
and \fIenterstep\fR operations, the traced commands are invoked
in the reverse order of how the traces were originally created;
and for \fIleave\fR and \fIleavestep\fR operations, the traced
commands are invoked in the original order of creation.

The behavior of execution traces is currently undefined for a command 
\fIname\fR imported into another namespace.
.RE
.TP
\fBtrace add variable\fI name ops command\fR
Arrange for \fIcommand\fR to be executed whenever variable \fIname\fR
is accessed in one of the ways given by the list \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.  If the variable does not exist, it will be created but
will not be given a value, so it will be visible to \fBnamespace which\fR
queries, but not to \fBinfo exists\fR queries.
.RS
.PP
\fIOps\fR indicates which operations are of interest, and is a list of
one or more of the following items: