'\" '\" 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. '\" '\" RCS: @(#) $Id: trace.n,v 1.2 2001/09/14 01:42:29 zlaski 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