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8.0 KiB
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166 lines
8.0 KiB
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<HTML><HEAD><TITLE>Built-In Commands - trace manual page</TITLE></HEAD><BODY>
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<DL>
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<DD><A HREF="trace.htm#M2" NAME="L1316">NAME</A>
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<DL><DD>trace - Monitor variable accesses</DL>
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<DD><A HREF="trace.htm#M3" NAME="L1317">SYNOPSIS</A>
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<DL>
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<DD><B>trace </B><I>option</I> ?<I>arg arg ...</I>?
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</DL>
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<DD><A HREF="trace.htm#M4" NAME="L1318">DESCRIPTION</A>
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<DL>
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<DD><A HREF="trace.htm#M5" NAME="L1319"><B>trace variable </B><I>name ops command</I></A>
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<DL>
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<DD><A HREF="trace.htm#M6" NAME="L1320"><B>r</B></A>
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<DD><A HREF="trace.htm#M7" NAME="L1321"><B>w</B></A>
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<DD><A HREF="trace.htm#M8" NAME="L1322"><B>u</B></A>
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</DL>
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<DD><A HREF="trace.htm#M9" NAME="L1323"><B>trace vdelete </B><I>name ops command</I></A>
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<DD><A HREF="trace.htm#M10" NAME="L1324"><B>trace vinfo </B><I>name</I></A>
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</DL>
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<DD><A HREF="trace.htm#M11" NAME="L1325">KEYWORDS</A>
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</DL><HR>
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<H3><A NAME="M2">NAME</A></H3>
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trace - Monitor variable accesses
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<H3><A NAME="M3">SYNOPSIS</A></H3>
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<B>trace </B><I>option</I> ?<I>arg arg ...</I>?<BR>
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<H3><A NAME="M4">DESCRIPTION</A></H3>
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This command causes Tcl commands to be executed whenever certain operations are
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invoked. At present, only variable tracing is implemented. The
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legal <I>option</I>'s (which may be abbreviated) are:
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<P>
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<DL>
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<P><DT><A NAME="M5"><B>trace variable </B><I>name ops command</I></A><DD>
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Arrange for <I>command</I> to be executed whenever variable <I>name</I>
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is accessed in one of the ways given by <I>ops</I>. <I>Name</I> may
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refer to a normal variable, an element of an array, or to an array
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as a whole (i.e. <I>name</I> may be just the name of an array, with no
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parenthesized index). If <I>name</I> refers to a whole array, then
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<I>command</I> is invoked whenever any element of the array is
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manipulated. If the variable does not exist, it will be created but
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will not be given a value, so it will be visible to <B>namespace which</B>
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queries, but not to <B><A HREF="../TkCmd/info.htm">info exists</A></B> queries.
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<P>
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<I>Ops</I> indicates which operations are of interest, and consists of
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one or more of the following letters:
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<P>
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<DL>
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<P><DT><A NAME="M6"><B>r</B></A><DD>
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Invoke <I>command</I> whenever the variable is read.
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<P><DT><A NAME="M7"><B>w</B></A><DD>
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Invoke <I>command</I> whenever the variable is written.
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<P><DT><A NAME="M8"><B>u</B></A><DD>
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Invoke <I>command</I> whenever the variable is unset. Variables
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can be unset explicitly with the <B><A HREF="../TkCmd/unset.htm">unset</A></B> command, or
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implicitly when procedures return (all of their local variables
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are unset). Variables are also unset when interpreters are
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deleted, but traces will not be invoked because there is no
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interpreter in which to execute them.
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<P></DL>
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<P>
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When the trace triggers, three arguments are appended to
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<I>command</I> so that the actual command is as follows:
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<PRE><I>command name1 name2 op</I></PRE>
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<I>Name1</I> and <I>name2</I> give the name(s) for the variable
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being accessed: if the variable is a scalar then <I>name1</I>
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gives the variable's name and <I>name2</I> is an empty string;
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if the variable is an array element then <I>name1</I> gives the
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name of the array and name2 gives the index into the array;
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if an entire array is being deleted and the trace was registered
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on the overall array, rather than a single element, then <I>name1</I>
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gives the array name and <I>name2</I> is an empty string.
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<I>Name1</I> and <I>name2</I> are not necessarily the same as the
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name used in the <B>trace variable</B> command: the <B><A HREF="../TkCmd/upvar.htm">upvar</A></B>
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command allows a procedure to reference a variable under a
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different name.
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<I>Op</I> indicates what operation is being performed on the
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variable, and is one of <B>r</B>, <B>w</B>, or <B>u</B> as
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defined above.
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<P>
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<I>Command</I> executes in the same context as the code that invoked
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the traced operation: if the variable was accessed as part of a
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Tcl procedure, then <I>command</I> will have access to the same
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local variables as code in the procedure. This context may be
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different than the context in which the trace was created.
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If <I>command</I> invokes a procedure (which it normally does) then
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the procedure will have to use <B><A HREF="../TkCmd/upvar.htm">upvar</A></B> or <B><A HREF="../TkCmd/uplevel.htm">uplevel</A></B> if it
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wishes to access the traced variable.
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Note also that <I>name1</I> may not necessarily be the same as the name
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used to set the trace on the variable; differences can occur if
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the access is made through a variable defined with the <B><A HREF="../TkCmd/upvar.htm">upvar</A></B>
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command.
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<P>
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For read and write traces, <I>command</I> can modify
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the variable to affect the result of the traced operation.
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If <I>command</I> modifies the value of a variable during a
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read or write trace, then the new value will be returned as the
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result of the traced operation.
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The return value from <I>command</I> is ignored except that
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if it returns an error of any sort then the traced operation
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also returns an error with
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the same error message returned by the trace command
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(this mechanism can be used to implement read-only variables, for
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example).
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For write traces, <I>command</I> is invoked after the variable's
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value has been changed; it can write a new value into the variable
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to override the original value specified in the write operation.
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To implement read-only variables, <I>command</I> will have to restore
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the old value of the variable.
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<P>
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While <I>command</I> is executing during a read or write trace, traces
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on the variable are temporarily disabled.
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This means that reads and writes invoked by
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<I>command</I> will occur directly, without invoking <I>command</I>
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(or any other traces) again.
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However, if <I>command</I> unsets the variable then unset traces
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will be invoked.
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<P>
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When an unset trace is invoked, the variable has already been
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deleted: it will appear to be undefined with no traces.
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If an unset occurs because of a procedure return, then the
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trace will be invoked in the variable context of the procedure
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being returned to: the stack frame of the returning procedure
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will no longer exist.
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Traces are not disabled during unset traces, so if an unset trace
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command creates a new trace and accesses the variable, the
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trace will be invoked.
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Any errors in unset traces are ignored.
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<P>
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If there are multiple traces on a variable they are invoked
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in order of creation, most-recent first.
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If one trace returns an error, then no further traces are
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invoked for the variable.
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If an array element has a trace set, and there is also a trace
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set on the array as a whole, the trace on the overall array
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is invoked before the one on the element.
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<P>
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Once created, the trace remains in effect either until the
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trace is removed with the <B>trace vdelete</B> command described
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below, until the variable is unset, or until the interpreter
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is deleted.
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Unsetting an element of array will remove any traces on that
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element, but will not remove traces on the overall array.
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<P>This command returns an empty string.
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<P><DT><A NAME="M9"><B>trace vdelete </B><I>name ops command</I></A><DD>
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If there is a trace set on variable <I>name</I> with the
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operations and command given by <I>ops</I> and <I>command</I>,
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then the trace is removed, so that <I>command</I> will never
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again be invoked.
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Returns an empty string.
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<P><DT><A NAME="M10"><B>trace vinfo </B><I>name</I></A><DD>
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Returns a list containing one element for each trace
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currently set on variable <I>name</I>.
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Each element of the list is itself a list containing two
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elements, which are the <I>ops</I> and <I>command</I> associated
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with the trace.
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If <I>name</I> doesn't exist or doesn't have any traces set, then
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the result of the command will be an empty string.
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<P></DL>
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<H3><A NAME="M11">KEYWORDS</A></H3>
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<A href="../Keywords/R.htm#read">read</A>, <A href="../Keywords/V.htm#variable">variable</A>, <A href="../Keywords/W.htm#write">write</A>, <A href="../Keywords/T.htm#trace">trace</A>, <A href="../Keywords/U.htm#unset">unset</A>
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<HR><PRE>
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<A HREF="../copyright.htm">Copyright</A> © 1993 The Regents of the University of California.
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<A HREF="../copyright.htm">Copyright</A> © 1994-1996 Sun Microsystems, Inc.
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<A HREF="../copyright.htm">Copyright</A> © 1995-1997 Roger E. Critchlow Jr.</PRE>
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</BODY></HTML>
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