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202 lines
8.3 KiB
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202 lines
8.3 KiB
HTML
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<head>
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<title>Output Formats (Debugging with GDB)</title>
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Data.html#Data" rel="up" title="Data">
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<link href="Memory.html#Memory" rel="next" title="Memory">
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<link href="Arrays.html#Arrays" rel="prev" title="Arrays">
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</head>
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<body lang="en">
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<a name="Output-Formats"></a>
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<div class="header">
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<p>
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Next: <a href="Memory.html#Memory" accesskey="n" rel="next">Memory</a>, Previous: <a href="Arrays.html#Arrays" accesskey="p" rel="prev">Arrays</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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</div>
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<hr>
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<a name="Output-Formats-1"></a>
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<h3 class="section">10.5 Output Formats</h3>
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<a name="index-formatted-output"></a>
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<a name="index-output-formats"></a>
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<p>By default, <small>GDB</small> prints a value according to its data type. Sometimes
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this is not what you want. For example, you might want to print a number
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in hex, or a pointer in decimal. Or you might want to view data in memory
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at a certain address as a character string or as an instruction. To do
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these things, specify an <em>output format</em> when you print a value.
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</p>
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<p>The simplest use of output formats is to say how to print a value
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already computed. This is done by starting the arguments of the
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<code>print</code> command with a slash and a format letter. The format
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letters supported are:
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</p>
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<dl compact="compact">
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<dt><code>x</code></dt>
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<dd><p>Regard the bits of the value as an integer, and print the integer in
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hexadecimal.
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</p>
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</dd>
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<dt><code>d</code></dt>
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<dd><p>Print as integer in signed decimal.
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</p>
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</dd>
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<dt><code>u</code></dt>
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<dd><p>Print as integer in unsigned decimal.
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</p>
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</dd>
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<dt><code>o</code></dt>
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<dd><p>Print as integer in octal.
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</p>
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</dd>
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<dt><code>t</code></dt>
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<dd><p>Print as integer in binary. The letter ‘<samp>t</samp>’ stands for “two”.
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<a name="DOCF10" href="#FOOT10"><sup>10</sup></a>
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</p>
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</dd>
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<dt><code>a</code></dt>
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<dd><a name="index-unknown-address_002c-locating"></a>
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<a name="index-locate-address"></a>
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<p>Print as an address, both absolute in hexadecimal and as an offset from
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the nearest preceding symbol. You can use this format used to discover
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where (in what function) an unknown address is located:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) p/a 0x54320
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$3 = 0x54320 <_initialize_vx+396>
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</pre></div>
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<p>The command <code>info symbol 0x54320</code> yields similar results.
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See <a href="Symbols.html#Symbols">info symbol</a>.
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</p>
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</dd>
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<dt><code>c</code></dt>
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<dd><p>Regard as an integer and print it as a character constant. This
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prints both the numerical value and its character representation. The
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character representation is replaced with the octal escape ‘<samp>\nnn</samp>’
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for characters outside the 7-bit <small>ASCII</small> range.
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</p>
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<p>Without this format, <small>GDB</small> displays <code>char</code>,
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<code>unsigned char</code><!-- /@w -->, and <code>signed char</code><!-- /@w --> data as character
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constants. Single-byte members of vectors are displayed as integer
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data.
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</p>
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</dd>
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<dt><code>f</code></dt>
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<dd><p>Regard the bits of the value as a floating point number and print
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using typical floating point syntax.
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</p>
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</dd>
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<dt><code>s</code></dt>
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<dd><a name="index-printing-strings"></a>
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<a name="index-printing-byte-arrays"></a>
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<p>Regard as a string, if possible. With this format, pointers to single-byte
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data are displayed as null-terminated strings and arrays of single-byte data
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are displayed as fixed-length strings. Other values are displayed in their
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natural types.
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</p>
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<p>Without this format, <small>GDB</small> displays pointers to and arrays of
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<code>char</code>, <code>unsigned char</code><!-- /@w -->, and <code>signed char</code><!-- /@w --> as
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strings. Single-byte members of a vector are displayed as an integer
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array.
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</p>
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</dd>
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<dt><code>z</code></dt>
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<dd><p>Like ‘<samp>x</samp>’ formatting, the value is treated as an integer and
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printed as hexadecimal, but leading zeros are printed to pad the value
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to the size of the integer type.
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</p>
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</dd>
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<dt><code>r</code></dt>
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<dd><a name="index-raw-printing"></a>
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<p>Print using the ‘<samp>raw</samp>’ formatting. By default, <small>GDB</small> will
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use a Python-based pretty-printer, if one is available (see <a href="Pretty-Printing.html#Pretty-Printing">Pretty Printing</a>). This typically results in a higher-level display of the
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value’s contents. The ‘<samp>r</samp>’ format bypasses any Python
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pretty-printer which might exist.
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</p></dd>
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</dl>
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<p>For example, to print the program counter in hex (see <a href="Registers.html#Registers">Registers</a>), type
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</p>
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<div class="smallexample">
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<pre class="smallexample">p/x $pc
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</pre></div>
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<p>Note that no space is required before the slash; this is because command
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names in <small>GDB</small> cannot contain a slash.
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</p>
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<p>To reprint the last value in the value history with a different format,
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you can use the <code>print</code> command with just a format and no
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expression. For example, ‘<samp>p/x</samp>’ reprints the last value in hex.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT10" href="#DOCF10">(10)</a></h3>
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<p>‘<samp>b</samp>’ cannot be used because these format letters are also
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used with the <code>x</code> command, where ‘<samp>b</samp>’ stands for “byte”;
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see <a href="Memory.html#Memory">Examining Memory</a>.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Memory.html#Memory" accesskey="n" rel="next">Memory</a>, Previous: <a href="Arrays.html#Arrays" accesskey="p" rel="prev">Arrays</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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</div>
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