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139 lines
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<title>Output Formats - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Data.html#Data" title="Data">
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<link rel="prev" href="Arrays.html#Arrays" title="Arrays">
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<link rel="next" href="Memory.html#Memory" title="Memory">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<div class="node">
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<a name="Output-Formats"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Memory.html#Memory">Memory</a>,
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Previous: <a rel="previous" accesskey="p" href="Arrays.html#Arrays">Arrays</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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<hr>
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</div>
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<h3 class="section">10.5 Output Formats</h3>
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<p><a name="index-formatted-output-649"></a><a name="index-output-formats-650"></a>By default, <span class="sc">gdb</span> 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 <dfn>output format</dfn> when you print a value.
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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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<dl>
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<dt><code>x</code><dd>Regard the bits of the value as an integer, and print the integer in
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hexadecimal.
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<br><dt><code>d</code><dd>Print as integer in signed decimal.
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<br><dt><code>u</code><dd>Print as integer in unsigned decimal.
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<br><dt><code>o</code><dd>Print as integer in octal.
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<br><dt><code>t</code><dd>Print as integer in binary. The letter ‘<samp><span class="samp">t</span></samp>’ stands for “two”.
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<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>
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<br><dt><code>a</code><dd><a name="index-unknown-address_002c-locating-651"></a><a name="index-locate-address-652"></a>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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<pre class="smallexample"> (gdb) p/a 0x54320
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$3 = 0x54320 <_initialize_vx+396>
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</pre>
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<p class="noindent">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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<br><dt><code>c</code><dd>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><span class="samp">\nnn</span></samp>’
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for characters outside the 7-bit <span class="sc">ascii</span> range.
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<p>Without this format, <span class="sc">gdb</span> 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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<br><dt><code>f</code><dd>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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<br><dt><code>s</code><dd><a name="index-printing-strings-653"></a><a name="index-printing-byte-arrays-654"></a>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>Without this format, <span class="sc">gdb</span> 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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<br><dt><code>z</code><dd>Like ‘<samp><span class="samp">x</span></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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<br><dt><code>r</code><dd><a name="index-raw-printing-655"></a>Print using the ‘<samp><span class="samp">raw</span></samp>’ formatting. By default, <span class="sc">gdb</span> 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><span class="samp">r</span></samp>’ format bypasses any Python
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pretty-printer which might exist.
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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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<pre class="smallexample"> p/x $pc
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</pre>
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<p class="noindent">Note that no space is required before the slash; this is because command
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names in <span class="sc">gdb</span> cannot contain a slash.
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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><span class="samp">p/x</span></samp>’ reprints the last value in hex.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> ‘<samp><span class="samp">b</span></samp>’ cannot be used because these format letters are also
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used with the <code>x</code> command, where ‘<samp><span class="samp">b</span></samp>’ stands for “byte”;
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see <a href="Memory.html#Memory">Examining Memory</a>.</p>
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<hr></div>
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</body></html>
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