207 lines
13 KiB
HTML
207 lines
13 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Memory - Debugging with GDB</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="Debugging with GDB">
|
|
<meta name="generator" content="makeinfo 4.13">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="up" href="Data.html#Data" title="Data">
|
|
<link rel="prev" href="Output-Formats.html#Output-Formats" title="Output Formats">
|
|
<link rel="next" href="Auto-Display.html#Auto-Display" title="Auto Display">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being ``Free Software'' and ``Free Software Needs
|
|
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
|
|
and with the Back-Cover Texts as in (a) below.
|
|
|
|
(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
|
developing GNU and promoting software freedom.''
|
|
-->
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<style type="text/css"><!--
|
|
pre.display { font-family:inherit }
|
|
pre.format { font-family:inherit }
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
|
pre.smallexample { font-size:smaller }
|
|
pre.smalllisp { font-size:smaller }
|
|
span.sc { font-variant:small-caps }
|
|
span.roman { font-family:serif; font-weight:normal; }
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<a name="Memory"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Auto-Display.html#Auto-Display">Auto Display</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Output-Formats.html#Output-Formats">Output Formats</a>,
|
|
Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<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-656"></a>
|
|
|
|
<a name="index-x-_0040r_007b_0028examine-memory_0029_007d-657"></a>
|
|
<dl><dt><code>x/</code><var>nfu</var> <var>addr</var><dt><code>x </code><var>addr</var><dt><code>x</code><dd>Use the <code>x</code> command to examine memory.
|
|
</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><span class="samp">/</span></samp>’.
|
|
Several commands set convenient defaults for <var>addr</var>.
|
|
|
|
<dl>
|
|
<dt><var>n</var><span class="roman">, the repeat count</span><dd>The repeat count is a decimal integer; the default is 1. It specifies
|
|
how much memory (counting by units <var>u</var>) to display. If a negative
|
|
number is specified, memory is examined backward from <var>addr</var>.
|
|
<!-- This really is **decimal**; unaffected by 'set radix' as of GDB -->
|
|
<!-- 4.1.2. -->
|
|
|
|
<br><dt><var>f</var><span class="roman">, the display format</span><dd>The display format is one of the formats used by <code>print</code>
|
|
(‘<samp><span class="samp">x</span></samp>’, ‘<samp><span class="samp">d</span></samp>’, ‘<samp><span class="samp">u</span></samp>’, ‘<samp><span class="samp">o</span></samp>’, ‘<samp><span class="samp">t</span></samp>’, ‘<samp><span class="samp">a</span></samp>’, ‘<samp><span class="samp">c</span></samp>’,
|
|
‘<samp><span class="samp">f</span></samp>’, ‘<samp><span class="samp">s</span></samp>’), and in addition ‘<samp><span class="samp">i</span></samp>’ (for machine instructions).
|
|
The default is ‘<samp><span class="samp">x</span></samp>’ (hexadecimal) initially. The default changes
|
|
each time you use either <code>x</code> or <code>print</code>.
|
|
|
|
<br><dt><var>u</var><span class="roman">, the unit size</span><dd>The unit size is any of
|
|
|
|
<dl>
|
|
<dt><code>b</code><dd>Bytes.
|
|
<br><dt><code>h</code><dd>Halfwords (two bytes).
|
|
<br><dt><code>w</code><dd>Words (four bytes). This is the initial default.
|
|
<br><dt><code>g</code><dd>Giant words (eight bytes).
|
|
</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><span class="samp">i</span></samp>’ format,
|
|
the unit size is ignored and is normally not written. For the ‘<samp><span class="samp">s</span></samp>’ format,
|
|
the unit size defaults to ‘<samp><span class="samp">b</span></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><span class="samp">s</span></samp>’
|
|
modifier will use the UTF-16 encoding while ‘<samp><span class="samp">w</span></samp>’ will use
|
|
UTF-32. The encoding is set by the programming language and cannot
|
|
be altered.
|
|
|
|
<br><dt><var>addr</var><span class="roman">, starting display address</span><dd><var>addr</var> is the address where you want <span class="sc">gdb</span> 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).
|
|
</dl>
|
|
|
|
<p>For example, ‘<samp><span class="samp">x/3uh 0x54320</span></samp>’ is a request to display three halfwords
|
|
(<code>h</code>) of memory, formatted as unsigned decimal integers (‘<samp><span class="samp">u</span></samp>’),
|
|
starting at address <code>0x54320</code>. ‘<samp><span class="samp">x/4xw $sp</span></samp>’ prints the four
|
|
words (‘<samp><span class="samp">w</span></samp>’) of memory above the stack pointer (here, ‘<samp><span class="samp">$sp</span></samp>’;
|
|
see <a href="Registers.html#Registers">Registers</a>) in hexadecimal (‘<samp><span class="samp">x</span></samp>’).
|
|
|
|
<p>You can also specify a negative repeat count to examine memory backward
|
|
from the given address. For example, ‘<samp><span class="samp">x/-3uh 0x54320</span></samp>’ prints three
|
|
halfwords (<code>h</code>) at <code>0x54314</code>, <code>0x54328</code>, and <code>0x5431c</code>.
|
|
|
|
<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><span class="samp">4xw</span></samp>’ and ‘<samp><span class="samp">4wx</span></samp>’ mean exactly the same thing.
|
|
(However, the count <var>n</var> must come first; ‘<samp><span class="samp">wx4</span></samp>’ does not work.)
|
|
|
|
<p>Even though the unit size <var>u</var> is ignored for the formats ‘<samp><span class="samp">s</span></samp>’
|
|
and ‘<samp><span class="samp">i</span></samp>’, you might still want to use a count <var>n</var>; for example,
|
|
‘<samp><span class="samp">3i</span></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><span class="samp">i</span></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>If a negative repeat count is specified for the formats ‘<samp><span class="samp">s</span></samp>’ or ‘<samp><span class="samp">i</span></samp>’,
|
|
the command displays null-terminated strings or instructions before the given
|
|
address as many as the absolute value of the given number. For the ‘<samp><span class="samp">i</span></samp>’
|
|
format, we use line number information in the debug info to accurately locate
|
|
instruction boundaries while disassembling backward. If line info is not
|
|
available, the command stops examining memory with an error message.
|
|
|
|
<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><span class="samp">x/3i </span><var>addr</var></samp>’, you can inspect the next seven
|
|
with just ‘<samp><span class="samp">x/7</span></samp>’. If you use <RET> 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>When examining machine instructions, the instruction at current program
|
|
counter is shown with a <code>=></code> marker. For example:
|
|
|
|
<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 $0x8048460,(%esp)
|
|
0x804838c <main+24>: call 0x80482d4 <puts@plt>
|
|
</pre>
|
|
<p><a name="index-g_t_0040code_007b_0024_005f_007d_002c-_0040code_007b_0024_005f_005f_007d_002c-and-value-history-658"></a>The addresses and contents printed by the <code>x</code> command are not saved
|
|
in the value history because there is often too much of them and they
|
|
would get in the way. Instead, <span class="sc">gdb</span> makes these values available for
|
|
subsequent use in expressions as values of the convenience variables
|
|
<code>$_</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>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-659"></a>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 <span class="sc">gdb</span> and this document, the term
|
|
<dfn>addressable memory unit</dfn> (or <dfn>memory unit</dfn> for short) is used
|
|
when explicitly referring to a chunk of data of that size. The word
|
|
<dfn>byte</dfn> 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-660"></a><a name="index-target-memory-comparison-661"></a><a name="index-verify-remote-memory-image-662"></a><a name="index-verify-target-memory-image-663"></a>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.
|
|
|
|
|
|
<a name="index-compare_002dsections-664"></a>
|
|
<dl><dt><code>compare-sections </code><span class="roman">[</span><var>section-name</var><span class="roman">|</span><code>-r</code><span class="roman">]</span><dd>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>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="qCRC-packet.html#qCRC-packet">qCRC packet</a>).
|
|
</dl>
|
|
|
|
</body></html>
|
|
|