95 lines
4.9 KiB
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95 lines
4.9 KiB
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<title>Expressions - Debugging with GDB</title>
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<link rel="next" href="Ambiguous-Expressions.html#Ambiguous-Expressions" title="Ambiguous Expressions">
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<a name="Expressions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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</div>
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<h3 class="section">10.1 Expressions</h3>
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<p><a name="index-expressions-625"></a><code>print</code> and many other <span class="sc">gdb</span> commands accept an expression and
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compute its value. Any kind of constant, variable or operator defined
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by the programming language you are using is valid in an expression in
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<span class="sc">gdb</span>. This includes conditional expressions, function calls,
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casts, and string constants. It also includes preprocessor macros, if
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you compiled your program to include this information; see
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<a href="Compilation.html#Compilation">Compilation</a>.
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<p><a name="index-arrays-in-expressions-626"></a><span class="sc">gdb</span> supports array constants in expressions input by
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the user. The syntax is {<var>element</var>, <var>element</var><small class="dots">...</small>}. For example,
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you can use the command <code>print {1, 2, 3}</code> to create an array
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of three integers. If you pass an array to a function or assign it
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to a program variable, <span class="sc">gdb</span> copies the array to memory that
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is <code>malloc</code>ed in the target program.
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<p>Because C is so widespread, most of the expressions shown in examples in
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this manual are in C. See <a href="Languages.html#Languages">Using <span class="sc">gdb</span> with Different Languages</a>, for information on how to use expressions in other
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languages.
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<p>In this section, we discuss operators that you can use in <span class="sc">gdb</span>
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expressions regardless of your programming language.
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<p><a name="index-casts_002c-in-expressions-627"></a>Casts are supported in all languages, not just in C, because it is so
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useful to cast a number into a pointer in order to examine a structure
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at that address in memory.
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<!-- FIXME: casts supported-Mod2 true? -->
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<p><span class="sc">gdb</span> supports these operators, in addition to those common
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to programming languages:
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<dl>
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<dt><code>@</code><dd>‘<samp><span class="samp">@</span></samp>’ is a binary operator for treating parts of memory as arrays.
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See <a href="Arrays.html#Arrays">Artificial Arrays</a>, for more information.
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<br><dt><code>::</code><dd>‘<samp><span class="samp">::</span></samp>’ allows you to specify a variable in terms of the file or
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function where it is defined. See <a href="Variables.html#Variables">Program Variables</a>.
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<p><a name="index-g_t_0040_007b_0040var_007btype_007d_0040_007d-628"></a><a name="index-type-casting-memory-629"></a><a name="index-memory_002c-viewing-as-typed-object-630"></a><a name="index-casts_002c-to-view-memory-631"></a><br><dt><code>{</code><var>type</var><code>} </code><var>addr</var><dd>Refers to an object of type <var>type</var> stored at address <var>addr</var> in
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memory. The address <var>addr</var> may be any expression whose value is
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an integer or pointer (but parentheses are required around binary
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operators, just as in a cast). This construct is allowed regardless
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of what kind of data is normally supposed to reside at <var>addr</var>.
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</dl>
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</body></html>
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