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| 22 | <title>Invalid Dereference (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Invalid-Dereference"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Void-Pointers.html" accesskey="n" rel="next">Void Pointers</a>, Previous: <a href="Null-Pointers.html" accesskey="p" rel="prev">Null Pointers</a>, Up: <a href="Pointers.html" accesskey="u" rel="up">Pointers</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Dereferencing-Null-or-Invalid-Pointers"></span><h3 class="section">14.7 Dereferencing Null or Invalid Pointers</h3>
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| 64 |
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| 65 | <p>Trying to dereference a null pointer is an error. On most platforms,
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| 66 | it generally causes a signal, usually <code>SIGSEGV</code>
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| 67 | (see <a href="Signals.html">Signals</a>).
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| 68 | </p>
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| 69 | <div class="example">
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| 70 | <pre class="example">char *foo = NULL;
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| 71 | c = *foo; /* <span class="roman">This causes a signal and terminates.</span> */
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| 72 | </pre></div>
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| 73 |
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| 74 | <p>Likewise a pointer that has the wrong alignment for the target data type
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| 75 | (on most types of computer), or points to a part of memory that has
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| 76 | not been allocated in the process’s address space.
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| 77 | </p>
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| 78 | <p>The signal terminates the program, unless the program has arranged to
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| 79 | handle the signal (see <a href="https://www.gnu.org/software/libc/manual/html_node/Signal-Handling.html#Signal-Handling">The GNU C Library</a> in <cite>The GNU C Library Reference Manual</cite>).
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| 80 | </p>
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| 81 | <p>However, the signal might not happen if the dereference is optimized
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| 82 | away. In the example above, if you don’t subsequently use the value
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| 83 | of <code>c</code>, GCC might optimize away the code for <code>*foo</code>. You
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| 84 | can prevent such optimization using the <code>volatile</code> qualifier, as
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| 85 | shown here:
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">volatile char *p;
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| 89 | volatile char c;
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| 90 | c = *p;
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| 91 | </pre></div>
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| 92 |
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| 93 | <p>You can use this to test whether <code>p</code> points to unallocated
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| 94 | memory. Set up a signal handler first, so the signal won’t terminate
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| 95 | the program.
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| 96 | </p>
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| 97 |
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| 98 |
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| 99 |
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