1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3<head> 4<title>The Tracing Facility</title> 5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 6<link href="theme/style.css" rel="stylesheet" type="text/css"> 7</head> 8 9<body> 10<table width="100%" border="0" cellspacing="2" background="theme/bkd2.gif"> 11 <tr> 12 <td width="21"> <h1></h1></td> 13 <td width="885"> <font face="Verdana, Arial, Helvetica, sans-serif"><b><font size="6">The 14 Tracing Facility</font></b></font></td> 15 <td width="96"><a href="http://www.boost.org"><img src="theme/wave.gif" width="93" height="68" align="right" border="0"></a></td> 16 </tr> 17</table> 18<br> 19<table border="0"> 20 <tr> 21 <td width="10"></td> 22 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 23 <td width="30"><a href="wave_driver.html"><img src="theme/l_arr.gif" border="0"></a></td> 24 <td width="30"><a href="acknowledgements.html"><img src="theme/r_arr.gif" border="0"></a></td> 25 </tr> 26</table> 27<p>If you ever had the need to debug a macro expansion you had to discover, that 28 your tools provide only little or no support for this task. For this reason 29 the <i>Wave</i> library has a tracing facility, which allows to get selectively 30 some information about the expansion of a certain macro or several macros. </p> 31<p>The tracing of macro expansions generates a possibly huge amount of information, 32 so it is recommended, that you explicitly enable/disable the tracing for the 33 macro in question only. This may be done with the help of a special, <tt>Wave</tt> 34 specific #pragma:</p> 35<pre><span class="preprocessor"> #pragma</span> wave trace(enable) <span class="comment">// enable the tracing</span> 36 <span class="comment">// the macro expansions here will be traced</span> 37 <span class="comment">// ...</span> 38<span class="preprocessor"> #pragma</span> wave trace(disable) <span class="comment">// disable the tracing</span></pre> 39<p>In C99 mode or when specifying the <tt>--variadics</tt> command line option 40 you may additionally use the <tt>operator _Pragma()</tt> variant to enable/disable 41 the tracing output:</p> 42<pre><span class="preprocessor"> #define</span> CONCAT(x, y) \ 43 <span class="preprocessor">_Pragma</span>(<span class="string">"wave trace(enable)"</span>) \ 44 x \ 45 <span class="preprocessor">_Pragma</span>(<span class="string">"wave trace(disable)"</span>) \ 46 <span class="keyword">##</span> y</pre> 47<p>This way you have the possibility to enable the tracing during the expansion 48 of a part of a macro only. In the sample shown there is traced the expansion 49 of the macro argument <tt>'x'</tt> only. Note, that the <tt>operator _Pragma()</tt> 50 directives expand to nothing inside the macro expansion result.</p> 51<p>To see, what the <tt>Wave</tt> driver generates while expanding a simple macro, 52 let's have a look at the tracing output for the following example:</p> 53<pre ><span class="comment"> // test.cpp</span> 54<span class="preprocessor"> #define</span> X(x) x<br><span class="preprocessor"> #define</span> Y() 2<br><span class="preprocessor"> #define</span> CONCAT_(x, y) x <span class="keyword">##</span> y 55<span class="preprocessor"> #define</span> CONCAT(x, y) CONCAT_(x, y) 56<span class="preprocessor"> #pragma</span> wave trace(enable) 57<span class="comment"> // this macro expansion is to be traced</span> 58 CONCAT(X(1), Y()) <span class="comment">// should expand to 12</span> 59<span class="preprocessor"> #pragma</span> wave trace(disable)</pre> 60<p>When preprocessed with <tt>'wave -t test.trace test.cpp'</tt> the <tt>Wave</tt> 61 driver generates a file <tt>test.trace</tt>, which contains (without the line 62 numbers in front of the lines):</p> 63<pre> 1: test.cpp:8:1: CONCAT(X(1), Y()) 64 2: test.cpp:5:9: see macro definition: CONCAT(x, y) 65 3: invoked with 66 4: [ 67 5: x = X(1) 68 6: y = Y() 69 7: ] 70 8: [ 71 9: test.cpp:2:9: see macro definition: X(x) 72 10: invoked with 73 11: [ 74 12: x = 1 75 13: ] 76 14: [ 77 15: 1 78 16: rescanning 79 17: [ 80 18: 1 81 19: ] 82 20: ] 83 21: test.cpp:3:9: see macro definition: Y() 84 22: [ 85 23: 2 86 24: rescanning 87 25: [ 88 26: 2 89 27: ] 90 28: ] 91 29: CONCAT_(1, 2) 92 30: rescanning 93 31: [ 94 32: test.cpp:4:9: see macro definition: CONCAT_(x, y) 95 33: invoked with 96 34: [ 97 35: x = 1 98 36: y = 2 99 37: ] 100 38: [ 101 39: 12 102 40: rescanning 103 41: [ 104 42: 12 105 43: ] 106 44: ] 107 45: 12 108 46: ] 109 47: ] 110</pre> 111<p>The generated trace output is very verbose, but allows to follow every step 112 of the actual macro expansion process. The first line in this tracing example 113 contains the reference to the position, from where the macro expansion was initiated. 114 Additionally the following information is contained for every single macro expansion:</p> 115<ul> 116 <li>The reference to the position (line and column numbers), where the macro to expand was defined first 117 (see lines 2, 9, 21 and 32).</li> 118 <li>The real parameters supplied for this macro expansion (see lines 3, 10 and 119 33), this information is traced inside the <tt>invoked with</tt> block, where 120 the corresponding formal and actual parameters are listed.</li> 121 <li>The expansion of the given arguments (if any and if these are defined as 122 macros). This repeats the full tracing information for the argument macro 123 expansion, only indented by one level. Note though, that the macro expansion 124 of the actual arguments is traced, regardless of the fact, whether this argument 125 is really to be inserted into the replacement list after its expansion 126 or as it was initially supplied (see C++ Standard [16.3.1.1]: "A parameter 127 in the replacement list, unless preceded by a <tt>#</tt> or <tt>##</tt> preprocessing 128 token or followed by a <tt>##</tt> preprocessing token, is replaced by the 129 corresponding argument after all macros contained therein have been expanded" 130 <a href="references.html#iso_cpp">[1]</a>). </li> 131 <li>The result of the argument substitution (see lines 15, 23, 29 and 39), i.e. 132 the substituted replacement list.</li> 133 <li>The rescanning process, which again includes the full subsequent macro expansion 134 process of all found macros (see C++ Standard [16.3.4.1]: "After all 135 parameters in the replacement list have been substituted, the resulting preprocessing 136 token sequence is rescanned with all subsequent preprocessing tokens of the 137 source file for more macro names to replace." <a href="references.html#iso_cpp">[1]</a>).</li> 138 <li>The result of the actual macro expansion (this is the last line inside the 139 corresponding rescanning block - see lines 18, 26, 42 and 45).</li> 140</ul> 141<p>Every found macro to expand will add an additional indentation level inside 142 the trace output.</p> 143<table border="0"> 144 <tr> 145 <td width="10"></td> 146 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 147 <td width="30"><a href="wave_driver.html"><img src="theme/l_arr.gif" border="0"></a></td> 148 <td width="30"><a href="acknowledgements.html"><img src="theme/r_arr.gif" border="0"></a></td> 149 </tr> 150</table> 151<hr size="1"> 152<p class="copyright">Copyright © 2003-2011 Hartmut Kaiser<br> 153 <br> 154<font size="2">Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p> 155<p class="copyright"><span class="updated">Last updated: 156 <!-- #BeginDate format:fcAm1m -->Tuesday, March 21, 2006 9:25<!-- #EndDate --> 157 </span> 158</p> 159</body> 160</html> 161 162
