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<TITLE>Magic-7.3 Command Reference</TITLE>
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<H1> <IMG SRC=graphics/magic_title2.gif ALT="Magic VLSI Layout Tool Version 7.3">
<IMG SRC=graphics/magic_OGL_sm.gif ALIGN="top" ALT="*"> </H1>
<H2>tag</H2>
<HR>
Register a tag callback command.
<HR>
<H3>Usage:</H3>
<BLOCKQUOTE>
<B>tag</B> <I>command_name</I> [<I>procedure</I>] <BR><BR>
<BLOCKQUOTE>
where <I>command_name</I> is the name of any <B>magic</B>
command, and <I>procedure</I> is any valid Tcl procedure.
</BLOCKQUOTE>
</BLOCKQUOTE>
<H3>Summary:</H3>
<BLOCKQUOTE>
The <B>tag</B> command is registers callback procedures to be
executed after the indicated <B>magic</B> command has been
executed. The primary use of the <B>tag</B> command is to
register procedures that update the GUI window in response
to commands that are typed on the command line or generated
from macro calls. <P>
In keeping with standard methods for Tcl callback functions,
certain "escape sequences" beginning with the percent ("<B>%</B>")
character are allowed to be embedded in the callback function
<I>procedure</I>, and are substituted prior to execution, with
the substitutions defined as follows:
<BLOCKQUOTE>
<DL>
<DT> <B>%W</B>
<DD> Substitute the Tk path of the layout window from which
or in reference to which <I>command_name</I> was invoked.
<DT> <B>%r</B>
<DD> Substitute the previous Tcl result string, but do not reset
the Tcl result of the execution of <I>procedure</I>.
<DT> <B>%R</B>
<DD> Substitute the previous Tcl result string and reset the Tcl
result from the execution of <I>procedure</I> such that the
the result of <I>command_name</I> becomes the result of
<I>procedure</I>.
<DT> <B>%</B>[<B>0</B>-<B>5</B>]
<DD> Substitute the zeroth to fifth argument to the original command.
<DT> <B>%%</B>
<DD> Substitute a single percent character.
<DT> <B>%</B><I>char</I>
<DD> where <I>char</I> is any character not defined above: No action,
print exactly as written.
</DL>
</BLOCKQUOTE>
When a tag callback is used, the return value seen by the interpreter
is the return value of the function <I>procedure</I>, not the return
value of the tagged command <I>command_name</I>. The escape sequence
<B>%R</B> can be used to force the result of <I>command_name</I> to
become the result of <I>procedure</I> (unless <I>procedure</I>
produces an error condition, in which case the error is returned).
The escape sequence <I>%r</I> passes the result of <I>command_name</I>
as an argument to the procedure, which may choose to return it as
a result, or not. <P>
If no <I>procedure</I> is present, then the <B>tag</B> command
returns whatever procedure string is attached to the indicated
<I>command_name</I>, if any. This can be used as a way to prevent
infinite recursion inside a tag callback; for example,
<BLOCKQUOTE>
<B>set savetag [tag callback </B><I>command</I><B>]</B> <BR>
<I>(procedure calls, which may include a call to </I>command<I>)</I> <BR>
<B>tag</B> <I>command</I> <B>$savetag</B>
</BLOCKQUOTE>
Another way to avoid infinite recursion is to check the procedure
depth from within the tag callback procedure using the Tcl command
"<B>info level</B>", to avoid executing the callback procedure if
the level is not zero. <P>
Only one tag callback is allowed per command name. However, only
one is needed as that procedure may call as many other procedures
as it wants to.
</BLOCKQUOTE>
<H3>Implementation Notes:</H3>
<BLOCKQUOTE>
<B>tag</B> is implemented as a built-in command in <B>magic</B>,
but only in the Tcl version.
</BLOCKQUOTE>
<P><IMG SRC=graphics/line1.gif><P>
<TABLE BORDER=0>
<TR>
<TD> <A HREF=commands.html>Return to command index</A>
</TR>
</TABLE>
<P><I>Last updated:</I> October 8, 2004 at 10:15pm <P>
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