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| <a name="A-Sample-GDB-Session"></a> |
| <h2 class="chapter">1 A Sample <small>GDB</small> Session</h2> |
| |
| <p>You can use this manual at your leisure to read all about <small>GDB</small>. |
| However, a handful of commands are enough to get started using the |
| debugger. This chapter illustrates those commands. |
| </p> |
| |
| |
| <p>One of the preliminary versions of <small>GNU</small> <code>m4</code> (a generic macro |
| processor) exhibits the following bug: sometimes, when we change its |
| quote strings from the default, the commands used to capture one macro |
| definition within another stop working. In the following short <code>m4</code> |
| session, we define a macro <code>foo</code> which expands to <code>0000</code>; we |
| then use the <code>m4</code> built-in <code>defn</code> to define <code>bar</code> as the |
| same thing. However, when we change the open quote string to |
| <code><QUOTE></code> and the close quote string to <code><UNQUOTE></code>, the same |
| procedure fails to define a new synonym <code>baz</code>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">$ <b>cd gnu/m4</b> |
| $ <b>./m4</b> |
| <b>define(foo,0000)</b> |
| |
| <b>foo</b> |
| 0000 |
| <b>define(bar,defn(‘foo’))</b> |
| |
| <b>bar</b> |
| 0000 |
| <b>changequote(<QUOTE>,<UNQUOTE>)</b> |
| |
| <b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b> |
| <b>baz</b> |
| <b>Ctrl-d</b> |
| m4: End of input: 0: fatal error: EOF in string |
| </pre></div> |
| |
| <p>Let us use <small>GDB</small> to try to see what is going on. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">$ <b>gdb m4</b> |
| GDB is free software and you are welcome to distribute copies |
| of it under certain conditions; type "show copying" to see |
| the conditions. |
| There is absolutely no warranty for GDB; type "show warranty" |
| for details. |
| |
| GDB 7.10.1.20160210-cvs, Copyright 1999 Free Software Foundation, Inc... |
| (gdb) |
| </pre></div> |
| |
| <p><small>GDB</small> reads only enough symbol data to know where to find the |
| rest when needed; as a result, the first prompt comes up very quickly. |
| We now tell <small>GDB</small> to use a narrower display width than usual, so |
| that examples fit in this manual. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>set width 70</b> |
| </pre></div> |
| |
| <p>We need to see how the <code>m4</code> built-in <code>changequote</code> works. |
| Having looked at the source, we know the relevant subroutine is |
| <code>m4_changequote</code>, so we set a breakpoint there with the <small>GDB</small> |
| <code>break</code> command. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>break m4_changequote</b> |
| Breakpoint 1 at 0x62f4: file builtin.c, line 879. |
| </pre></div> |
| |
| <p>Using the <code>run</code> command, we start <code>m4</code> running under <small>GDB</small> |
| control; as long as control does not reach the <code>m4_changequote</code> |
| subroutine, the program runs as usual: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>run</b> |
| Starting program: /work/Editorial/gdb/gnu/m4/m4 |
| <b>define(foo,0000)</b> |
| |
| <b>foo</b> |
| 0000 |
| </pre></div> |
| |
| <p>To trigger the breakpoint, we call <code>changequote</code>. <small>GDB</small> |
| suspends execution of <code>m4</code>, displaying information about the |
| context where it stops. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"><b>changequote(<QUOTE>,<UNQUOTE>)</b> |
| |
| Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) |
| at builtin.c:879 |
| 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3)) |
| </pre></div> |
| |
| <p>Now we use the command <code>n</code> (<code>next</code>) to advance execution to |
| the next line of the current function. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>n</b> |
| 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ |
| : nil, |
| </pre></div> |
| |
| <p><code>set_quotes</code> looks like a promising subroutine. We can go into it |
| by using the command <code>s</code> (<code>step</code>) instead of <code>next</code>. |
| <code>step</code> goes to the next line to be executed in <em>any</em> |
| subroutine, so it steps into <code>set_quotes</code>. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>s</b> |
| set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") |
| at input.c:530 |
| 530 if (lquote != def_lquote) |
| </pre></div> |
| |
| <p>The display that shows the subroutine where <code>m4</code> is now |
| suspended (and its arguments) is called a stack frame display. It |
| shows a summary of the stack. We can use the <code>backtrace</code> |
| command (which can also be spelled <code>bt</code>), to see where we are |
| in the stack as a whole: the <code>backtrace</code> command displays a |
| stack frame for each active subroutine. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>bt</b> |
| #0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>") |
| at input.c:530 |
| #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) |
| at builtin.c:882 |
| #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 |
| #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) |
| at macro.c:71 |
| #4 0x79dc in expand_input () at macro.c:40 |
| #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195 |
| </pre></div> |
| |
| <p>We step through a few more lines to see what happens. The first two |
| times, we can use ‘<samp>s</samp>’; the next two times we use <code>n</code> to avoid |
| falling into the <code>xstrdup</code> subroutine. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>s</b> |
| 0x3b5c 532 if (rquote != def_rquote) |
| (gdb) <b>s</b> |
| 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ |
| def_lquote : xstrdup(lq); |
| (gdb) <b>n</b> |
| 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ |
| : xstrdup(rq); |
| (gdb) <b>n</b> |
| 538 len_lquote = strlen(rquote); |
| </pre></div> |
| |
| <p>The last line displayed looks a little odd; we can examine the variables |
| <code>lquote</code> and <code>rquote</code> to see if they are in fact the new left |
| and right quotes we specified. We use the command <code>p</code> |
| (<code>print</code>) to see their values. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>p lquote</b> |
| $1 = 0x35d40 "<QUOTE>" |
| (gdb) <b>p rquote</b> |
| $2 = 0x35d50 "<UNQUOTE>" |
| </pre></div> |
| |
| <p><code>lquote</code> and <code>rquote</code> are indeed the new left and right quotes. |
| To look at some context, we can display ten lines of source |
| surrounding the current line with the <code>l</code> (<code>list</code>) command. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>l</b> |
| 533 xfree(rquote); |
| 534 |
| 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ |
| : xstrdup (lq); |
| 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ |
| : xstrdup (rq); |
| 537 |
| 538 len_lquote = strlen(rquote); |
| 539 len_rquote = strlen(lquote); |
| 540 } |
| 541 |
| 542 void |
| </pre></div> |
| |
| <p>Let us step past the two lines that set <code>len_lquote</code> and |
| <code>len_rquote</code>, and then examine the values of those variables. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>n</b> |
| 539 len_rquote = strlen(lquote); |
| (gdb) <b>n</b> |
| 540 } |
| (gdb) <b>p len_lquote</b> |
| $3 = 9 |
| (gdb) <b>p len_rquote</b> |
| $4 = 7 |
| </pre></div> |
| |
| <p>That certainly looks wrong, assuming <code>len_lquote</code> and |
| <code>len_rquote</code> are meant to be the lengths of <code>lquote</code> and |
| <code>rquote</code> respectively. We can set them to better values using |
| the <code>p</code> command, since it can print the value of |
| any expression—and that expression can include subroutine calls and |
| assignments. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>p len_lquote=strlen(lquote)</b> |
| $5 = 7 |
| (gdb) <b>p len_rquote=strlen(rquote)</b> |
| $6 = 9 |
| </pre></div> |
| |
| <p>Is that enough to fix the problem of using the new quotes with the |
| <code>m4</code> built-in <code>defn</code>? We can allow <code>m4</code> to continue |
| executing with the <code>c</code> (<code>continue</code>) command, and then try the |
| example that caused trouble initially: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>c</b> |
| Continuing. |
| |
| <b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b> |
| |
| baz |
| 0000 |
| </pre></div> |
| |
| <p>Success! The new quotes now work just as well as the default ones. The |
| problem seems to have been just the two typos defining the wrong |
| lengths. We allow <code>m4</code> exit by giving it an EOF as input: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"><b>Ctrl-d</b> |
| Program exited normally. |
| </pre></div> |
| |
| <p>The message ‘<samp>Program exited normally.</samp>’ is from <small>GDB</small>; it |
| indicates <code>m4</code> has finished executing. We can end our <small>GDB</small> |
| session with the <small>GDB</small> <code>quit</code> command. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) <b>quit</b> |
| </pre></div> |
| |
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