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