1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- Copyright (C) 1988-2021 Free Software Foundation, Inc. 4 5Permission is granted to copy, distribute and/or modify this document 6under the terms of the GNU Free Documentation License, Version 1.3 or 7any later version published by the Free Software Foundation; with the 8Invariant Sections being "Free Software" and "Free Software Needs 9Free Documentation", with the Front-Cover Texts being "A GNU Manual," 10and with the Back-Cover Texts as in (a) below. 11 12(a) The FSF's Back-Cover Text is: "You are free to copy and modify 13this GNU Manual. 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This chapter illustrates those commands. 76</p> 77 78 79<p>One of the preliminary versions of <small>GNU</small> <code>m4</code> (a generic macro 80processor) exhibits the following bug: sometimes, when we change its 81quote strings from the default, the commands used to capture one macro 82definition within another stop working. In the following short <code>m4</code> 83session, we define a macro <code>foo</code> which expands to <code>0000</code>; we 84then use the <code>m4</code> built-in <code>defn</code> to define <code>bar</code> as the 85same thing. However, when we change the open quote string to 86<code><QUOTE></code> and the close quote string to <code><UNQUOTE></code>, the same 87procedure fails to define a new synonym <code>baz</code>: 88</p> 89<div class="smallexample"> 90<pre class="smallexample">$ <b>cd gnu/m4</b> 91$ <b>./m4</b> 92<b>define(foo,0000)</b> 93 94<b>foo</b> 950000 96<b>define(bar,defn(‘foo’))</b> 97 98<b>bar</b> 990000 100<b>changequote(<QUOTE>,<UNQUOTE>)</b> 101 102<b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b> 103<b>baz</b> 104<b>Ctrl-d</b> 105m4: End of input: 0: fatal error: EOF in string 106</pre></div> 107 108<p>Let us use <small>GDB</small> to try to see what is going on. 109</p> 110<div class="smallexample"> 111<pre class="smallexample">$ <b>gdb m4</b> 112GDB is free software and you are welcome to distribute copies 113 of it under certain conditions; type "show copying" to see 114 the conditions. 115There is absolutely no warranty for GDB; type "show warranty" 116 for details. 117 118GDB 10.2.90.20210621-git, Copyright 1999 Free Software Foundation, Inc... 119(gdb) 120</pre></div> 121 122<p><small>GDB</small> reads only enough symbol data to know where to find the 123rest when needed; as a result, the first prompt comes up very quickly. 124We now tell <small>GDB</small> to use a narrower display width than usual, so 125that examples fit in this manual. 126</p> 127<div class="smallexample"> 128<pre class="smallexample">(gdb) <b>set width 70</b> 129</pre></div> 130 131<p>We need to see how the <code>m4</code> built-in <code>changequote</code> works. 132Having looked at the source, we know the relevant subroutine is 133<code>m4_changequote</code>, so we set a breakpoint there with the <small>GDB</small> 134<code>break</code> command. 135</p> 136<div class="smallexample"> 137<pre class="smallexample">(gdb) <b>break m4_changequote</b> 138Breakpoint 1 at 0x62f4: file builtin.c, line 879. 139</pre></div> 140 141<p>Using the <code>run</code> command, we start <code>m4</code> running under <small>GDB</small> 142control; as long as control does not reach the <code>m4_changequote</code> 143subroutine, the program runs as usual: 144</p> 145<div class="smallexample"> 146<pre class="smallexample">(gdb) <b>run</b> 147Starting program: /work/Editorial/gdb/gnu/m4/m4 148<b>define(foo,0000)</b> 149 150<b>foo</b> 1510000 152</pre></div> 153 154<p>To trigger the breakpoint, we call <code>changequote</code>. <small>GDB</small> 155suspends execution of <code>m4</code>, displaying information about the 156context where it stops. 157</p> 158<div class="smallexample"> 159<pre class="smallexample"><b>changequote(<QUOTE>,<UNQUOTE>)</b> 160 161Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) 162 at builtin.c:879 163879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3)) 164</pre></div> 165 166<p>Now we use the command <code>n</code> (<code>next</code>) to advance execution to 167the next line of the current function. 168</p> 169<div class="smallexample"> 170<pre class="smallexample">(gdb) <b>n</b> 171882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ 172 : nil, 173</pre></div> 174 175<p><code>set_quotes</code> looks like a promising subroutine. We can go into it 176by using the command <code>s</code> (<code>step</code>) instead of <code>next</code>. 177<code>step</code> goes to the next line to be executed in <em>any</em> 178subroutine, so it steps into <code>set_quotes</code>. 179</p> 180<div class="smallexample"> 181<pre class="smallexample">(gdb) <b>s</b> 182set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") 183 at input.c:530 184530 if (lquote != def_lquote) 185</pre></div> 186 187<p>The display that shows the subroutine where <code>m4</code> is now 188suspended (and its arguments) is called a stack frame display. It 189shows a summary of the stack. We can use the <code>backtrace</code> 190command (which can also be spelled <code>bt</code>), to see where we are 191in the stack as a whole: the <code>backtrace</code> command displays a 192stack frame for each active subroutine. 193</p> 194<div class="smallexample"> 195<pre class="smallexample">(gdb) <b>bt</b> 196#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") 197 at input.c:530 198#1 0x6344 in m4_changequote (argc=3, argv=0x33c70) 199 at builtin.c:882 200#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 201#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) 202 at macro.c:71 203#4 0x79dc in expand_input () at macro.c:40 204#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195 205</pre></div> 206 207<p>We step through a few more lines to see what happens. The first two 208times, we can use ‘<samp>s</samp>’; the next two times we use <code>n</code> to avoid 209falling into the <code>xstrdup</code> subroutine. 210</p> 211<div class="smallexample"> 212<pre class="smallexample">(gdb) <b>s</b> 2130x3b5c 532 if (rquote != def_rquote) 214(gdb) <b>s</b> 2150x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ 216def_lquote : xstrdup(lq); 217(gdb) <b>n</b> 218536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ 219 : xstrdup(rq); 220(gdb) <b>n</b> 221538 len_lquote = strlen(rquote); 222</pre></div> 223 224<p>The last line displayed looks a little odd; we can examine the variables 225<code>lquote</code> and <code>rquote</code> to see if they are in fact the new left 226and right quotes we specified. We use the command <code>p</code> 227(<code>print</code>) to see their values. 228</p> 229<div class="smallexample"> 230<pre class="smallexample">(gdb) <b>p lquote</b> 231$1 = 0x35d40 "<QUOTE>" 232(gdb) <b>p rquote</b> 233$2 = 0x35d50 "<UNQUOTE>" 234</pre></div> 235 236<p><code>lquote</code> and <code>rquote</code> are indeed the new left and right quotes. 237To look at some context, we can display ten lines of source 238surrounding the current line with the <code>l</code> (<code>list</code>) command. 239</p> 240<div class="smallexample"> 241<pre class="smallexample">(gdb) <b>l</b> 242533 xfree(rquote); 243534 244535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ 245 : xstrdup (lq); 246536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ 247 : xstrdup (rq); 248537 249538 len_lquote = strlen(rquote); 250539 len_rquote = strlen(lquote); 251540 } 252541 253542 void 254</pre></div> 255 256<p>Let us step past the two lines that set <code>len_lquote</code> and 257<code>len_rquote</code>, and then examine the values of those variables. 258</p> 259<div class="smallexample"> 260<pre class="smallexample">(gdb) <b>n</b> 261539 len_rquote = strlen(lquote); 262(gdb) <b>n</b> 263540 } 264(gdb) <b>p len_lquote</b> 265$3 = 9 266(gdb) <b>p len_rquote</b> 267$4 = 7 268</pre></div> 269 270<p>That certainly looks wrong, assuming <code>len_lquote</code> and 271<code>len_rquote</code> are meant to be the lengths of <code>lquote</code> and 272<code>rquote</code> respectively. We can set them to better values using 273the <code>p</code> command, since it can print the value of 274any expression—and that expression can include subroutine calls and 275assignments. 276</p> 277<div class="smallexample"> 278<pre class="smallexample">(gdb) <b>p len_lquote=strlen(lquote)</b> 279$5 = 7 280(gdb) <b>p len_rquote=strlen(rquote)</b> 281$6 = 9 282</pre></div> 283 284<p>Is that enough to fix the problem of using the new quotes with the 285<code>m4</code> built-in <code>defn</code>? We can allow <code>m4</code> to continue 286executing with the <code>c</code> (<code>continue</code>) command, and then try the 287example that caused trouble initially: 288</p> 289<div class="smallexample"> 290<pre class="smallexample">(gdb) <b>c</b> 291Continuing. 292 293<b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b> 294 295baz 2960000 297</pre></div> 298 299<p>Success! The new quotes now work just as well as the default ones. The 300problem seems to have been just the two typos defining the wrong 301lengths. We allow <code>m4</code> exit by giving it an EOF as input: 302</p> 303<div class="smallexample"> 304<pre class="smallexample"><b>Ctrl-d</b> 305Program exited normally. 306</pre></div> 307 308<p>The message ‘<samp>Program exited normally.</samp>’ is from <small>GDB</small>; it 309indicates <code>m4</code> has finished executing. We can end our <small>GDB</small> 310session with the <small>GDB</small> <code>quit</code> command. 311</p> 312<div class="smallexample"> 313<pre class="smallexample">(gdb) <b>quit</b> 314</pre></div> 315 316<hr> 317<div class="header"> 318<p> 319Next: <a href="Invocation.html#Invocation" accesskey="n" rel="next">Invocation</a>, Previous: <a href="Summary.html#Summary" accesskey="p" rel="previous">Summary</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> 320</div> 321 322 323 324</body> 325</html> 326