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<title>Debugging with GDB: Memory</title>
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<a name="Memory"></a>
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<div class="header">
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<p>
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Next: <a href="Auto-Display.html#Auto-Display" accesskey="n" rel="next">Auto Display</a>, Previous: <a href="Output-Formats.html#Output-Formats" accesskey="p" rel="prev">Output Formats</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</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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<hr>
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<a name="Examining-Memory"></a>
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<h3 class="section">10.6 Examining Memory</h3>
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<p>You can use the command <code>x</code> (for “examine”) to examine memory in
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any of several formats, independently of your program’s data types.
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</p>
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<a name="index-examining-memory"></a>
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<dl compact="compact">
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<dd><a name="index-x-_0028examine-memory_0029"></a>
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</dd>
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<dt><code>x/<var>nfu</var> <var>addr</var></code></dt>
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<dt><code>x <var>addr</var></code></dt>
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<dt><code>x</code></dt>
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<dd><p>Use the <code>x</code> command to examine memory.
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</p></dd>
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</dl>
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<p><var>n</var>, <var>f</var>, and <var>u</var> are all optional parameters that specify how
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much memory to display and how to format it; <var>addr</var> is an
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expression giving the address where you want to start displaying memory.
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If you use defaults for <var>nfu</var>, you need not type the slash ‘<samp>/</samp>’.
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Several commands set convenient defaults for <var>addr</var>.
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</p>
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<dl compact="compact">
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<dt><span class="roman"><var>n</var>, the repeat count</span></dt>
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<dd><p>The repeat count is a decimal integer; the default is 1. It specifies
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how much memory (counting by units <var>u</var>) to display. If a negative
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number is specified, memory is examined backward from <var>addr</var>.
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</p>
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</dd>
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<dt><span class="roman"><var>f</var>, the display format</span></dt>
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<dd><p>The display format is one of the formats used by <code>print</code>
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(‘<samp>x</samp>’, ‘<samp>d</samp>’, ‘<samp>u</samp>’, ‘<samp>o</samp>’, ‘<samp>t</samp>’, ‘<samp>a</samp>’, ‘<samp>c</samp>’,
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‘<samp>f</samp>’, ‘<samp>s</samp>’), and in addition ‘<samp>i</samp>’ (for machine instructions).
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The default is ‘<samp>x</samp>’ (hexadecimal) initially. The default changes
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each time you use either <code>x</code> or <code>print</code>.
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</p>
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</dd>
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<dt><span class="roman"><var>u</var>, the unit size</span></dt>
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<dd><p>The unit size is any of
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</p>
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<dl compact="compact">
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<dt><code>b</code></dt>
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<dd><p>Bytes.
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</p></dd>
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<dt><code>h</code></dt>
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<dd><p>Halfwords (two bytes).
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</p></dd>
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<dt><code>w</code></dt>
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<dd><p>Words (four bytes). This is the initial default.
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</p></dd>
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<dt><code>g</code></dt>
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<dd><p>Giant words (eight bytes).
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</p></dd>
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</dl>
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<p>Each time you specify a unit size with <code>x</code>, that size becomes the
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default unit the next time you use <code>x</code>. For the ‘<samp>i</samp>’ format,
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the unit size is ignored and is normally not written. For the ‘<samp>s</samp>’ format,
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the unit size defaults to ‘<samp>b</samp>’, unless it is explicitly given.
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Use <kbd>x /hs</kbd> to display 16-bit char strings and <kbd>x /ws</kbd> to display
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32-bit strings. The next use of <kbd>x /s</kbd> will again display 8-bit strings.
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Note that the results depend on the programming language of the
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current compilation unit. If the language is C, the ‘<samp>s</samp>’
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modifier will use the UTF-16 encoding while ‘<samp>w</samp>’ will use
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UTF-32. The encoding is set by the programming language and cannot
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be altered.
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</p>
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</dd>
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<dt><span class="roman"><var>addr</var>, starting display address</span></dt>
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<dd><p><var>addr</var> is the address where you want <small>GDB</small> to begin displaying
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memory. The expression need not have a pointer value (though it may);
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it is always interpreted as an integer address of a byte of memory.
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See <a href="Expressions.html#Expressions">Expressions</a>, for more information on expressions. The default for
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<var>addr</var> is usually just after the last address examined—but several
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other commands also set the default address: <code>info breakpoints</code> (to
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the address of the last breakpoint listed), <code>info line</code> (to the
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starting address of a line), and <code>print</code> (if you use it to display
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a value from memory).
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</p></dd>
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</dl>
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<p>For example, ‘<samp>x/3uh 0x54320</samp>’ is a request to display three halfwords
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(<code>h</code>) of memory, formatted as unsigned decimal integers (‘<samp>u</samp>’),
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starting at address <code>0x54320</code>. ‘<samp>x/4xw $sp</samp>’ prints the four
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words (‘<samp>w</samp>’) of memory above the stack pointer (here, ‘<samp>$sp</samp>’;
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see <a href="Registers.html#Registers">Registers</a>) in hexadecimal (‘<samp>x</samp>’).
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</p>
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<p>You can also specify a negative repeat count to examine memory backward
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from the given address. For example, ‘<samp>x/-3uh 0x54320</samp>’ prints three
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halfwords (<code>h</code>) at <code>0x54314</code>, <code>0x54328</code>, and <code>0x5431c</code>.
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</p>
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<p>Since the letters indicating unit sizes are all distinct from the
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letters specifying output formats, you do not have to remember whether
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unit size or format comes first; either order works. The output
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specifications ‘<samp>4xw</samp>’ and ‘<samp>4wx</samp>’ mean exactly the same thing.
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(However, the count <var>n</var> must come first; ‘<samp>wx4</samp>’ does not work.)
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</p>
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<p>Even though the unit size <var>u</var> is ignored for the formats ‘<samp>s</samp>’
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and ‘<samp>i</samp>’, you might still want to use a count <var>n</var>; for example,
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‘<samp>3i</samp>’ specifies that you want to see three machine instructions,
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including any operands. For convenience, especially when used with
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the <code>display</code> command, the ‘<samp>i</samp>’ format also prints branch delay
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slot instructions, if any, beyond the count specified, which immediately
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follow the last instruction that is within the count. The command
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<code>disassemble</code> gives an alternative way of inspecting machine
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instructions; see <a href="Machine-Code.html#Machine-Code">Source and Machine Code</a>.
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</p>
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<p>If a negative repeat count is specified for the formats ‘<samp>s</samp>’ or ‘<samp>i</samp>’,
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the command displays null-terminated strings or instructions before the given
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address as many as the absolute value of the given number. For the ‘<samp>i</samp>’
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format, we use line number information in the debug info to accurately locate
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instruction boundaries while disassembling backward. If line info is not
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available, the command stops examining memory with an error message.
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</p>
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<p>All the defaults for the arguments to <code>x</code> are designed to make it
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easy to continue scanning memory with minimal specifications each time
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you use <code>x</code>. For example, after you have inspected three machine
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instructions with ‘<samp>x/3i <var>addr</var></samp>’, you can inspect the next seven
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with just ‘<samp>x/7</samp>’. If you use <tt class="key">RET</tt> to repeat the <code>x</code> command,
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the repeat count <var>n</var> is used again; the other arguments default as
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for successive uses of <code>x</code>.
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</p>
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<p>When examining machine instructions, the instruction at current program
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counter is shown with a <code>=></code> marker. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) x/5i $pc-6
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0x804837f <main+11>: mov %esp,%ebp
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0x8048381 <main+13>: push %ecx
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0x8048382 <main+14>: sub $0x4,%esp
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=> 0x8048385 <main+17>: movl $0x8048460,(%esp)
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0x804838c <main+24>: call 0x80482d4 <puts@plt>
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</pre></div>
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<a name="index-_0024_005f_002c-_0024_005f_005f_002c-and-value-history"></a>
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<p>The addresses and contents printed by the <code>x</code> command are not saved
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in the value history because there is often too much of them and they
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would get in the way. Instead, <small>GDB</small> makes these values available for
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subsequent use in expressions as values of the convenience variables
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<code>$_</code> and <code>$__</code>. After an <code>x</code> command, the last address
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examined is available for use in expressions in the convenience variable
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<code>$_</code>. The contents of that address, as examined, are available in
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the convenience variable <code>$__</code>.
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</p>
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<p>If the <code>x</code> command has a repeat count, the address and contents saved
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are from the last memory unit printed; this is not the same as the last
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address printed if several units were printed on the last line of output.
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</p>
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<a name="addressable-memory-unit"></a><a name="index-addressable-memory-unit"></a>
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<p>Most targets have an addressable memory unit size of 8 bits. This means
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that to each memory address are associated 8 bits of data. Some
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targets, however, have other addressable memory unit sizes.
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Within <small>GDB</small> and this document, the term
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<em>addressable memory unit</em> (or <em>memory unit</em> for short) is used
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when explicitly referring to a chunk of data of that size. The word
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<em>byte</em> is used to refer to a chunk of data of 8 bits, regardless of
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the addressable memory unit size of the target. For most systems,
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addressable memory unit is a synonym of byte.
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</p>
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<a name="index-remote-memory-comparison"></a>
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<a name="index-target-memory-comparison"></a>
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<a name="index-verify-remote-memory-image"></a>
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<a name="index-verify-target-memory-image"></a>
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<p>When you are debugging a program running on a remote target machine
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(see <a href="Remote-Debugging.html#Remote-Debugging">Remote Debugging</a>), you may wish to verify the program’s image
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in the remote machine’s memory against the executable file you
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downloaded to the target. Or, on any target, you may want to check
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whether the program has corrupted its own read-only sections. The
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<code>compare-sections</code> command is provided for such situations.
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</p>
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<dl compact="compact">
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<dd><a name="index-compare_002dsections"></a>
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</dd>
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<dt><code>compare-sections <span class="roman">[</span><var>section-name</var><span class="roman">|</span><code>-r</code><span class="roman">]</span></code></dt>
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<dd><p>Compare the data of a loadable section <var>section-name</var> in the
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executable file of the program being debugged with the same section in
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the target machine’s memory, and report any mismatches. With no
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arguments, compares all loadable sections. With an argument of
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<code>-r</code>, compares all loadable read-only sections.
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</p>
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<p>Note: for remote targets, this command can be accelerated if the
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target supports computing the CRC checksum of a block of memory
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(see <a href="General-Query-Packets.html#qCRC-packet">qCRC packet</a>).
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</p></dd>
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</dl>
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