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I should know that, C edition

2016-03-26 , , , Comment

I had drinks with friends a couple of weeks ago and I mentioned a Hacker News discussion that included a link to a thread involving Rajiv Kurian in a comment on undefined behavior1 in C (the thread isn’t itself all that interesting but Krister Walfridsson blog post on compiler optimization is). They’re more experienced and/or better with C than I am and we sipped our beers and laughed at my and at our mutual expense for not knowing some of these. I don’t write much C for work in the past several years but I do make a point to re-educate myself.

Today I was reading Bruce Dawson’s blog and one of the bug reports he filed for Chromium against the Microsoft VC++ compiler included an array initializer construct I hadn’t seen (to some relief that I’m not a complete twit, he hadn’t either). I can’t be the only one who learned C before C99 (or before C90…) and didn’t know this syntax exists, from the GCC manual:

6.27 Designated Initializers

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well. This extension is not implemented in GNU C++.

To specify an array index, write ‘[index] =’ before the element value. For example,

    int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

    int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

and note that:

If the value in it has side-effects, the side-effects happen only once, not for each initialized field by the range initializer.

Have a sip and laugh, but I know it now.

1 re-read What Every C Programmer Should Know About Undefined Behavior #1/3

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