Warning: Implicit Backdoor
For those wondering how this works:<p>In C, like most languages, all binary operators on numbers – including arithmetic as well as comparisons – 'natively' require two operands of the same type.  When you pass operands of different types, the language usually converts one of the operands to the other's type.  (Sometimes it converts both operands to a different type instead.)<p>In this case, without the cast, the comparison will be performed by converting the `int` to `size_t`.  The dangerous case is when `num` is negative.  Since `size_t` is unsigned, the conversion will turn negative numbers into very high positive numbers, so `num > limit` will be true, and the function will safely reject the allocation.  On the other hand, with `num > (int)limit`, no conversion occurs; the comparison is performed between two signed integers, and `num > limit` will be false if `num` is negative.<p>It would be better to be more explicit.  Perhaps use two comparisons (the compiler will optimize them into one if possible):<p><pre><code>    if (num < 0 || num > limit)
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Or, better yet, make `allocatebufs` take `size_t` instead of `int` in the first place.  Taking `int` is risky even without the comparison bug: if someone calls the function with a `size_t` on a platform where `size_t` is larger than `int` (i.e. 64-bit platforms), the upper bits will be truncated.  For instance, if the passed-in value is 0x100000001, it will be truncated to 0x1, causing allocatebufs to return a much smaller buffer than expected.<p>If you're wondering about the exact rules for what types get converted to what, they're very complicated, unnecessarily so.  It mostly boils down to "larger type wins; in case of a tie, unsigned wins over signed".  But there are exceptions.  Here's the spec language:<p>- <a href="http://c0x.coding-guidelines.com/6.3.1.8.html" rel="nofollow">http://c0x.coding-guidelines.com/6.3.1.8.html</a><p>- <a href="http://c0x.coding-guidelines.com/6.3.1.1.html" rel="nofollow">http://c0x.coding-guidelines.com/6.3.1.1.html</a>