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