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This code resembles the test for wraparound from the sprint() function as implemented for the Plan 9 operating system. If buf + len < buf evaluates to true, len is assigned the remaining space minus one byte. However, because the expression buf + len < buf constitutes undefined behavior, compilers can assume this condition will never occur and optimize away the entire conditional statement.
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If adding or subtracting an integer to a pointer results in a reference to an element outside the array or one past the last element of the array object, the behavior is undefined but frequently leads to a buffer overflow or buffer underrun, which can often be exploited to run arbitrary code. Iterators and standard template library containers exhibit the same behavior and caveats as pointers and arrays.
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