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| 22 | <title>Stack (GNU C Language Manual)</title>
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| 24 | <meta name="description" content="Stack (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="The-First-Example.html" rel="up" title="The First Example">
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| 33 | <link href="Iterative-Fibonacci.html" rel="next" title="Iterative Fibonacci">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Stack"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Iterative-Fibonacci.html" accesskey="n" rel="next">Iterative Fibonacci</a>, Previous: <a href="Recursive-Fibonacci.html" accesskey="p" rel="prev">Recursive Fibonacci</a>, Up: <a href="The-First-Example.html" accesskey="u" rel="up">The First Example</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="The-Stack_002c-And-Stack-Overflow"></span><h3 class="section">1.2 The Stack, And Stack Overflow</h3>
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| 64 | <span id="index-stack"></span>
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| 65 | <span id="index-stack-frame"></span>
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| 66 | <span id="index-stack-overflow"></span>
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| 67 | <span id="index-recursion_002c-drawbacks-of"></span>
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| 68 |
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| 69 | <span id="index-stack-frame-1"></span>
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| 70 | <p>Recursion has a drawback: there are limits to how many nested function
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| 71 | calls a program can make. In C, each function call allocates a block
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| 72 | of memory which it uses until the call returns. C allocates these
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| 73 | blocks consecutively within a large area of memory known as the
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| 74 | <em>stack</em>, so we refer to the blocks as <em>stack frames</em>.
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| 75 | </p>
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| 76 | <p>The size of the stack is limited; if the program tries to use too
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| 77 | much, that causes the program to fail because the stack is full. This
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| 78 | is called <em>stack overflow</em>.
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| 79 | </p>
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| 80 | <span id="index-crash"></span>
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| 81 | <span id="index-segmentation-fault"></span>
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| 82 | <p>Stack overflow on GNU/Linux typically manifests itself as the
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| 83 | <em>signal</em> named <code>SIGSEGV</code>, also known as a “segmentation
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| 84 | fault.” By default, this signal terminates the program immediately,
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| 85 | rather than letting the program try to recover, or reach an expected
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| 86 | ending point. (We commonly say in this case that the program
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| 87 | “crashes”). See <a href="Signals.html">Signals</a>.
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| 88 | </p>
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| 89 | <p>It is inconvenient to observe a crash by passing too large
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| 90 | an argument to recursive Fibonacci, because the program would run a
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| 91 | long time before it crashes. This algorithm is simple but
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| 92 | ridiculously slow: in calculating <code>fib (<var>n</var>)</code>, the number of
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| 93 | (recursive) calls <code>fib (1)</code> or <code>fib (2)</code> that it makes equals
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| 94 | the final result.
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| 95 | </p>
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| 96 | <p>However, you can observe stack overflow very quickly if you use
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| 97 | this function instead:
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| 98 | </p>
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| 99 | <div class="example">
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| 100 | <pre class="example">int
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| 101 | fill_stack (int n)
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| 102 | {
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| 103 | if (n <= 1) /* <span class="roman">This limits the depth of recursion.</span> */
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| 104 | return 1;
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| 105 | else
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| 106 | return fill_stack (n - 1);
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| 107 | }
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| 108 | </pre></div>
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| 109 |
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| 110 | <p>Under gNewSense GNU/Linux on the Lemote Yeeloong, without optimization
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| 111 | and using the default configuration, an experiment showed there is
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| 112 | enough stack space to do 261906 nested calls to that function. One
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| 113 | more, and the stack overflows and the program crashes. On another
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| 114 | platform, with a different configuration, or with a different
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| 115 | function, the limit might be bigger or smaller.
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| 116 | </p>
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| 117 | <hr>
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| 118 | <div class="header">
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| 119 | <p>
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| 120 | Next: <a href="Iterative-Fibonacci.html" accesskey="n" rel="next">Iterative Fibonacci</a>, Previous: <a href="Recursive-Fibonacci.html" accesskey="p" rel="prev">Recursive Fibonacci</a>, Up: <a href="The-First-Example.html" accesskey="u" rel="up">The First Example</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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| 121 | </div>
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| 122 |
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| 123 |
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| 124 |
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