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    <p>Static variables (file scope and function static) are initialized to zero:</p> <pre><code>int x; // zero int y = 0; // also zero void foo() { static int x; // also zero } </code></pre> <p>Non-static variables (local variables) are <em>indeterminate</em>. Reading them prior to assigning a value results in undefined behavior.</p> <pre><code>void foo() { int x; printf("%d", x); // the compiler is free to crash here } </code></pre> <p>In practice, they tend to just have some nonsensical value in there initially - some compilers may even put in specific, fixed values to make it obvious when looking in a debugger - but strictly speaking, the compiler is free to do anything from crashing to summoning <a href="http://catb.org/jargon/html/N/nasal-demons.html" rel="noreferrer">demons through your nasal passages</a>.</p> <p>As for why it's undefined behavior instead of simply "undefined/arbitrary value", there are a number of CPU architectures that have additional flag bits in their representation for various types. A modern example would be <a href="http://blogs.msdn.com/oldnewthing/archive/2004/01/19/60162.aspx" rel="noreferrer">the Itanium, which has a "Not a Thing" bit in its registers</a>; of course, the C standard drafters were considering some older architectures.</p> <p>Attempting to work with a value with these flag bits set can result in a CPU exception in an operation that <em>really</em> shouldn't fail (eg, integer addition, or assigning to another variable). And if you go and leave a variable uninitialized, the compiler might pick up some random garbage with these flag bits set - meaning touching that uninitialized variable may be deadly.</p>
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    1. POWhat happens to a declared, uninitialized variable in C? Does it have a value?
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    1. POWhat happens to a declared, uninitialized variable in C? Does it have a value?
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    1. COoh no they aren't. They might be, in debug mode, when you aren't in front of a customer, on months with an R in, if you're lucky
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      1. USMartin Beckett
    2. COwhat aren't? the static initialization is required by the standard; see ISO/IEC 9899:1999 6.7.8 #10
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    3. COfirst example is fine as far as I can tell. I'm less as to why the compiler might crash in the second one though :)
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