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<title>Debugging with GDB: Machine Code</title>
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<a name="Machine-Code"></a>
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<div class="header">
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<p>
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Previous: <a href="Source-Path.html#Source-Path" accesskey="p" rel="previous">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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</div>
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<hr>
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<a name="Source-and-Machine-Code"></a>
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<h3 class="section">9.6 Source and Machine Code</h3>
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<a name="index-source-line-and-its-code-address"></a>
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<p>You can use the command <code>info line</code> to map source lines to program
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addresses (and vice versa), and the command <code>disassemble</code> to display
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a range of addresses as machine instructions. You can use the command
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<code>set disassemble-next-line</code> to set whether to disassemble next
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source line when execution stops. When run under <small>GNU</small> Emacs
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mode, the <code>info line</code> command causes the arrow to point to the
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line specified. Also, <code>info line</code> prints addresses in symbolic form as
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well as hex.
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</p>
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<dl compact="compact">
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<dd><a name="index-info-line"></a>
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</dd>
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<dt><code>info line</code></dt>
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<dt><code>info line <var>location</var></code></dt>
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<dd><p>Print the starting and ending addresses of the compiled code for
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source line <var>location</var>. You can specify source lines in any of
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the ways documented in <a href="Specify-Location.html#Specify-Location">Specify Location</a>. With no <var>location</var>
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information about the current source line is printed.
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</p></dd>
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</dl>
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<p>For example, we can use <code>info line</code> to discover the location of
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the object code for the first line of function
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<code>m4_changequote</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) info line m4_changequote
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Line 895 of "builtin.c" starts at pc 0x634c <m4_changequote> and \
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ends at 0x6350 <m4_changequote+4>.
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</pre></div>
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<a name="index-code-address-and-its-source-line"></a>
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<p>We can also inquire (using <code>*<var>addr</var></code> as the form for
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<var>location</var>) what source line covers a particular address:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) info line *0x63ff
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Line 926 of "builtin.c" starts at pc 0x63e4 <m4_changequote+152> and \
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ends at 0x6404 <m4_changequote+184>.
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</pre></div>
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<a name="index-_0024_005f-and-info-line"></a>
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<a name="index-x-command_002c-default-address"></a>
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<a name="index-x_0028examine_0029_002c-and-info-line"></a>
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<p>After <code>info line</code>, the default address for the <code>x</code> command
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is changed to the starting address of the line, so that ‘<samp>x/i</samp>’ is
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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
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convenience variable <code>$_</code> (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience
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Variables</a>).
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</p>
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<a name="index-info-line_002c-repeated-calls"></a>
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<p>After <code>info line</code>, using <code>info line</code> again without
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specifying a location will display information about the next source
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line.
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</p>
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<dl compact="compact">
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<dd><a name="index-disassemble"></a>
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<a name="index-assembly-instructions"></a>
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<a name="index-instructions_002c-assembly"></a>
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<a name="index-machine-instructions"></a>
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<a name="index-listing-machine-instructions"></a>
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</dd>
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<dt><code>disassemble</code></dt>
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<dt><code>disassemble /m</code></dt>
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<dt><code>disassemble /s</code></dt>
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<dt><code>disassemble /r</code></dt>
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<dd><p>This specialized command dumps a range of memory as machine
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instructions. It can also print mixed source+disassembly by specifying
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the <code>/m</code> or <code>/s</code> modifier and print the raw instructions in hex
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as well as in symbolic form by specifying the <code>/r</code> modifier.
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The default memory range is the function surrounding the
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program counter of the selected frame. A single argument to this
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command is a program counter value; <small>GDB</small> dumps the function
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surrounding this value. When two arguments are given, they should
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be separated by a comma, possibly surrounded by whitespace. The
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arguments specify a range of addresses to dump, in one of two forms:
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</p>
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<dl compact="compact">
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<dt><code><var>start</var>,<var>end</var></code></dt>
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<dd><p>the addresses from <var>start</var> (inclusive) to <var>end</var> (exclusive)
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</p></dd>
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<dt><code><var>start</var>,+<var>length</var></code></dt>
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<dd><p>the addresses from <var>start</var> (inclusive) to
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<code><var>start</var>+<var>length</var></code> (exclusive).
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</p></dd>
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</dl>
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<p>When 2 arguments are specified, the name of the function is also
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printed (since there could be several functions in the given range).
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</p>
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<p>The argument(s) can be any expression yielding a numeric value, such as
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‘<samp>0x32c4</samp>’, ‘<samp>&main+10</samp>’ or ‘<samp>$pc - 8</samp>’.
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</p>
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<p>If the range of memory being disassembled contains current program counter,
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the instruction at that location is shown with a <code>=></code> marker.
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</p></dd>
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</dl>
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<p>The following example shows the disassembly of a range of addresses of
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HP PA-RISC 2.0 code:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) disas 0x32c4, 0x32e4
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Dump of assembler code from 0x32c4 to 0x32e4:
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0x32c4 <main+204>: addil 0,dp
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0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26
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0x32cc <main+212>: ldil 0x3000,r31
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0x32d0 <main+216>: ble 0x3f8(sr4,r31)
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0x32d4 <main+220>: ldo 0(r31),rp
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0x32d8 <main+224>: addil -0x800,dp
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0x32dc <main+228>: ldo 0x588(r1),r26
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0x32e0 <main+232>: ldil 0x3000,r31
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End of assembler dump.
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</pre></div>
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<p>Here is an example showing mixed source+assembly for Intel x86
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with <code>/m</code> or <code>/s</code>, when the program is stopped just after
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function prologue in a non-optimized function with no inline code.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) disas /m main
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Dump of assembler code for function main:
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5 {
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0x08048330 <+0>: push %ebp
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0x08048331 <+1>: mov %esp,%ebp
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0x08048333 <+3>: sub $0x8,%esp
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0x08048336 <+6>: and $0xfffffff0,%esp
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0x08048339 <+9>: sub $0x10,%esp
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6 printf ("Hello.\n");
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=> 0x0804833c <+12>: movl $0x8048440,(%esp)
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0x08048343 <+19>: call 0x8048284 <puts@plt>
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7 return 0;
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8 }
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0x08048348 <+24>: mov $0x0,%eax
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0x0804834d <+29>: leave
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0x0804834e <+30>: ret
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End of assembler dump.
|
</pre></div>
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<p>The <code>/m</code> option is deprecated as its output is not useful when
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there is either inlined code or re-ordered code.
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The <code>/s</code> option is the preferred choice.
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Here is an example for AMD x86-64 showing the difference between
|
<code>/m</code> output and <code>/s</code> output.
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This example has one inline function defined in a header file,
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and the code is compiled with ‘<samp>-O2</samp>’ optimization.
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Note how the <code>/m</code> output is missing the disassembly of
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several instructions that are present in the <code>/s</code> output.
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</p>
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<p><samp>foo.h</samp>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">int
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foo (int a)
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{
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if (a < 0)
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return a * 2;
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if (a == 0)
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return 1;
|
return a + 10;
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}
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</pre></div>
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<p><samp>foo.c</samp>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#include "foo.h"
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volatile int x, y;
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int
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main ()
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{
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x = foo (y);
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return 0;
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}
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</pre></div>
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<div class="smallexample">
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<pre class="smallexample">(gdb) disas /m main
|
Dump of assembler code for function main:
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5 {
|
|
6 x = foo (y);
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0x0000000000400400 <+0>: mov 0x200c2e(%rip),%eax # 0x601034 <y>
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0x0000000000400417 <+23>: mov %eax,0x200c13(%rip) # 0x601030 <x>
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7 return 0;
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8 }
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0x000000000040041d <+29>: xor %eax,%eax
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0x000000000040041f <+31>: retq
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0x0000000000400420 <+32>: add %eax,%eax
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0x0000000000400422 <+34>: jmp 0x400417 <main+23>
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End of assembler dump.
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(gdb) disas /s main
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Dump of assembler code for function main:
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foo.c:
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5 {
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6 x = foo (y);
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0x0000000000400400 <+0>: mov 0x200c2e(%rip),%eax # 0x601034 <y>
|
|
foo.h:
|
4 if (a < 0)
|
0x0000000000400406 <+6>: test %eax,%eax
|
0x0000000000400408 <+8>: js 0x400420 <main+32>
|
|
6 if (a == 0)
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7 return 1;
|
8 return a + 10;
|
0x000000000040040a <+10>: lea 0xa(%rax),%edx
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0x000000000040040d <+13>: test %eax,%eax
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0x000000000040040f <+15>: mov $0x1,%eax
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0x0000000000400414 <+20>: cmovne %edx,%eax
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|
foo.c:
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6 x = foo (y);
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0x0000000000400417 <+23>: mov %eax,0x200c13(%rip) # 0x601030 <x>
|
|
7 return 0;
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8 }
|
0x000000000040041d <+29>: xor %eax,%eax
|
0x000000000040041f <+31>: retq
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|
foo.h:
|
5 return a * 2;
|
0x0000000000400420 <+32>: add %eax,%eax
|
0x0000000000400422 <+34>: jmp 0x400417 <main+23>
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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
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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 location (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>’
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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>
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<dl compact="compact">
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<dd><a name="index-set-disassembler_002doptions"></a>
|
<a name="index-disassembler-options"></a>
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</dd>
|
<dt><code>set disassembler-options <var>option1</var>[,<var>option2</var>…]</code></dt>
|
<dd><p>This command controls the passing of target specific information to
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the disassembler. For a list of valid options, please refer to the
|
<code>-M</code>/<code>--disassembler-options</code> section of the ‘<samp>objdump</samp>’
|
manual and/or the output of <kbd>objdump --help</kbd>
|
(see <a href="http://sourceware.org/binutils/docs/binutils/objdump.html#objdump">objdump</a> in <cite>The GNU Binary Utilities</cite>).
|
The default value is the empty string.
|
</p>
|
<p>If it is necessary to specify more than one disassembler option, then
|
multiple options can be placed together into a comma separated list.
|
Currently this command is only supported on targets ARM, MIPS, PowerPC
|
and S/390.
|
</p>
|
<a name="index-show-disassembler_002doptions"></a>
|
</dd>
|
<dt><code>show disassembler-options</code></dt>
|
<dd><p>Show the current setting of the disassembler options.
|
</p></dd>
|
</dl>
|
|
<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>
|
|
|
<hr>
|
<div class="header">
|
<p>
|
Previous: <a href="Source-Path.html#Source-Path" accesskey="p" rel="previous">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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