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| <title>Debugging with GDB: Machine Code</title> |
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| <a name="Machine-Code"></a> |
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| Previous: <a href="Source-Path.html#Source-Path" accesskey="p" rel="prev">Source Path</a>, Up: <a href="Source.html#Source" accesskey="u" rel="up">Source</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> |
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| <a name="Source-and-Machine-Code"></a> |
| <h3 class="section">9.6 Source and Machine Code</h3> |
| <a name="index-source-line-and-its-code-address"></a> |
| |
| <p>You can use the command <code>info line</code> to map source lines to program |
| addresses (and vice versa), and the command <code>disassemble</code> to display |
| a range of addresses as machine instructions. You can use the command |
| <code>set disassemble-next-line</code> to set whether to disassemble next |
| source line when execution stops. When run under <small>GNU</small> Emacs |
| mode, the <code>info line</code> command causes the arrow to point to the |
| line specified. Also, <code>info line</code> prints addresses in symbolic form as |
| well as hex. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-info-line"></a> |
| </dd> |
| <dt><code>info line <var>linespec</var></code></dt> |
| <dd><p>Print the starting and ending addresses of the compiled code for |
| source line <var>linespec</var>. You can specify source lines in any of |
| the ways documented in <a href="Specify-Location.html#Specify-Location">Specify Location</a>. |
| </p></dd> |
| </dl> |
| |
| <p>For example, we can use <code>info line</code> to discover the location of |
| the object code for the first line of function |
| <code>m4_changequote</code>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) info line m4_changequote |
| Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350. |
| </pre></div> |
| |
| <a name="index-code-address-and-its-source-line"></a> |
| <p>We can also inquire (using <code>*<var>addr</var></code> as the form for |
| <var>linespec</var>) what source line covers a particular address: |
| </p><div class="smallexample"> |
| <pre class="smallexample">(gdb) info line *0x63ff |
| Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404. |
| </pre></div> |
| |
| <a name="index-_0024_005f-and-info-line"></a> |
| <a name="index-x-command_002c-default-address"></a> |
| <a name="index-x_0028examine_0029_002c-and-info-line"></a> |
| <p>After <code>info line</code>, the default address for the <code>x</code> command |
| is changed to the starting address of the line, so that ‘<samp>x/i</samp>’ is |
| sufficient to begin examining the machine code (see <a href="Memory.html#Memory">Examining Memory</a>). Also, this address is saved as the value of the |
| convenience variable <code>$_</code> (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience |
| Variables</a>). |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-disassemble"></a> |
| <a name="index-assembly-instructions"></a> |
| <a name="index-instructions_002c-assembly"></a> |
| <a name="index-machine-instructions"></a> |
| <a name="index-listing-machine-instructions"></a> |
| </dd> |
| <dt><code>disassemble</code></dt> |
| <dt><code>disassemble /m</code></dt> |
| <dt><code>disassemble /r</code></dt> |
| <dd><p>This specialized command dumps a range of memory as machine |
| instructions. It can also print mixed source+disassembly by specifying |
| the <code>/m</code> modifier and print the raw instructions in hex as well as |
| in symbolic form by specifying the <code>/r</code>. |
| The default memory range is the function surrounding the |
| program counter of the selected frame. A single argument to this |
| command is a program counter value; <small>GDB</small> dumps the function |
| surrounding this value. When two arguments are given, they should |
| be separated by a comma, possibly surrounded by whitespace. The |
| arguments specify a range of addresses to dump, in one of two forms: |
| </p> |
| <dl compact="compact"> |
| <dt><code><var>start</var>,<var>end</var></code></dt> |
| <dd><p>the addresses from <var>start</var> (inclusive) to <var>end</var> (exclusive) |
| </p></dd> |
| <dt><code><var>start</var>,+<var>length</var></code></dt> |
| <dd><p>the addresses from <var>start</var> (inclusive) to |
| <code><var>start</var>+<var>length</var></code> (exclusive). |
| </p></dd> |
| </dl> |
| |
| <p>When 2 arguments are specified, the name of the function is also |
| printed (since there could be several functions in the given range). |
| </p> |
| <p>The argument(s) can be any expression yielding a numeric value, such as |
| ‘<samp>0x32c4</samp>’, ‘<samp>&main+10</samp>’ or ‘<samp>$pc - 8</samp>’. |
| </p> |
| <p>If the range of memory being disassembled contains current program counter, |
| the instruction at that location is shown with a <code>=></code> marker. |
| </p></dd> |
| </dl> |
| |
| <p>The following example shows the disassembly of a range of addresses of |
| HP PA-RISC 2.0 code: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) disas 0x32c4, 0x32e4 |
| Dump of assembler code from 0x32c4 to 0x32e4: |
| 0x32c4 <main+204>: addil 0,dp |
| 0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26 |
| 0x32cc <main+212>: ldil 0x3000,r31 |
| 0x32d0 <main+216>: ble 0x3f8(sr4,r31) |
| 0x32d4 <main+220>: ldo 0(r31),rp |
| 0x32d8 <main+224>: addil -0x800,dp |
| 0x32dc <main+228>: ldo 0x588(r1),r26 |
| 0x32e0 <main+232>: ldil 0x3000,r31 |
| End of assembler dump. |
| </pre></div> |
| |
| <p>Here is an example showing mixed source+assembly for Intel x86, when the |
| program is stopped just after function prologue: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) disas /m main |
| Dump of assembler code for function main: |
| 5 { |
| 0x08048330 <+0>: push %ebp |
| 0x08048331 <+1>: mov %esp,%ebp |
| 0x08048333 <+3>: sub $0x8,%esp |
| 0x08048336 <+6>: and $0xfffffff0,%esp |
| 0x08048339 <+9>: sub $0x10,%esp |
| |
| 6 printf ("Hello.\n"); |
| => 0x0804833c <+12>: movl $0x8048440,(%esp) |
| 0x08048343 <+19>: call 0x8048284 <puts@plt> |
| |
| 7 return 0; |
| 8 } |
| 0x08048348 <+24>: mov $0x0,%eax |
| 0x0804834d <+29>: leave |
| 0x0804834e <+30>: ret |
| |
| End of assembler dump. |
| </pre></div> |
| |
| <p>Here is another example showing raw instructions in hex for AMD x86-64, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) disas /r 0x400281,+10 |
| Dump of assembler code from 0x400281 to 0x40028b: |
| 0x0000000000400281: 38 36 cmp %dh,(%rsi) |
| 0x0000000000400283: 2d 36 34 2e 73 sub $0x732e3436,%eax |
| 0x0000000000400288: 6f outsl %ds:(%rsi),(%dx) |
| 0x0000000000400289: 2e 32 00 xor %cs:(%rax),%al |
| End of assembler dump. |
| </pre></div> |
| |
| <p>Addresses cannot be specified as a linespec (see <a href="Specify-Location.html#Specify-Location">Specify Location</a>). |
| So, for example, if you want to disassemble function <code>bar</code> |
| in file <samp>foo.c</samp>, you must type ‘<samp>disassemble 'foo.c'::bar</samp>’ |
| and not ‘<samp>disassemble foo.c:bar</samp>’. |
| </p> |
| <p>Some architectures have more than one commonly-used set of instruction |
| mnemonics or other syntax. |
| </p> |
| <p>For programs that were dynamically linked and use shared libraries, |
| instructions that call functions or branch to locations in the shared |
| libraries might show a seemingly bogus location—it’s actually a |
| location of the relocation table. On some architectures, <small>GDB</small> |
| might be able to resolve these to actual function names. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-set-disassembly_002dflavor"></a> |
| <a name="index-Intel-disassembly-flavor"></a> |
| <a name="index-AT_0026T-disassembly-flavor"></a> |
| </dd> |
| <dt><code>set disassembly-flavor <var>instruction-set</var></code></dt> |
| <dd><p>Select the instruction set to use when disassembling the |
| program via the <code>disassemble</code> or <code>x/i</code> commands. |
| </p> |
| <p>Currently this command is only defined for the Intel x86 family. You |
| can set <var>instruction-set</var> to either <code>intel</code> or <code>att</code>. |
| The default is <code>att</code>, the AT&T flavor used by default by Unix |
| assemblers for x86-based targets. |
| </p> |
| <a name="index-show-disassembly_002dflavor"></a> |
| </dd> |
| <dt><code>show disassembly-flavor</code></dt> |
| <dd><p>Show the current setting of the disassembly flavor. |
| </p></dd> |
| </dl> |
| |
| <dl compact="compact"> |
| <dd><a name="index-set-disassemble_002dnext_002dline"></a> |
| <a name="index-show-disassemble_002dnext_002dline"></a> |
| </dd> |
| <dt><code>set disassemble-next-line</code></dt> |
| <dt><code>show disassemble-next-line</code></dt> |
| <dd><p>Control whether or not <small>GDB</small> will disassemble the next source |
| line or instruction when execution stops. If ON, <small>GDB</small> will |
| display disassembly of the next source line when execution of the |
| program being debugged stops. This is <em>in addition</em> to |
| displaying the source line itself, which <small>GDB</small> always does if |
| possible. If the next source line cannot be displayed for some reason |
| (e.g., if <small>GDB</small> cannot find the source file, or there’s no line |
| info in the debug info), <small>GDB</small> will display disassembly of the |
| next <em>instruction</em> instead of showing the next source line. If |
| AUTO, <small>GDB</small> will display disassembly of next instruction only |
| if the source line cannot be displayed. This setting causes |
| <small>GDB</small> to display some feedback when you step through a function |
| with no line info or whose source file is unavailable. The default is |
| OFF, which means never display the disassembly of the next line or |
| instruction. |
| </p></dd> |
| </dl> |
| |
| |
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