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POInitialise a variable to its own undefined value
 

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  1. POInitialise a variable to its own undefined value
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    <p>In C, does initialising a variable to it's own value make sense? If yes, what for?</p> <p>Allow me to elaborate. In Git sources there are some examples of initialising a variable to it's own undefined value, as seen in <a href="https://github.com/git/git/blob/f84667def209e4a84e37e8488a08e9eca3f208c1/transport.c#L109" rel="nofollow">transport.c</a> or <a href="https://github.com/git/git/blob/f84667def209e4a84e37e8488a08e9eca3f208c1/wt-status.c#L267" rel="nofollow">wt-status.c</a>. I removed assignments from those declarations and run tests. Seeing no regressions, I thought that those assignments were redundant.</p> <p>On the other hand, I did some simple tests with GCC 4.6 and Clang 2.9.</p> <pre><code>#include &lt;stdio.h&gt; int main() { printf("print to increase probability of registers being non-zero\n"); int status = status; return printf("%i\n", status); } </code></pre> <p>Compiling with <code>-Wall -std=c99</code> and various <code>-O</code> levels prints no warnings and shows that <code>status == 0</code>. Clang with a non-zero optimisation level prints some garbage values though. It makes me infer that results of such expressions are undefined.</p> <p>I can imagine that such assignment can suppress an <em>uninitialised variable</em> warning, but it's not the case for the examples taken from Git. Removing assignments doesn't introduce any warnings.</p> <p>Are such assignments an undefined behaviour? If not, what do you use them for?</p> <hr> <p>I've suggested the change on the Git mailing list. <a href="http://article.gmane.org/gmane.comp.version-control.git/185969" rel="nofollow">Here's what I've learned</a>.</p>
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    1. COMaybe the examples in git don't suppress an uninitialized variable warning for *your* system, but on systems that other developers regularly use. With GCC, uninitialized variable warnings are affected by optimization flags, and Clang produces a fairly different set of warnings even though it is a "drop-in replacement" for GCC.
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      1. USDietrich Epp
    2. COI would initialize such variables to 0 (or -1) rather than drop the initialization altogether. Someone added the code for a purpose; I can respect the purpose even though I find the mechanism used odd.
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      1. USJonathan Leffler
    3. CO@JonathanLeffler, not only is it odd, on platforms with trap representations this yields undefined behavior.
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      1. USJens Gustedt
 

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