C Interview Questions and Answers


What was noalias and what ever happened to it?

noalias was another type qualifier, in the same syntactic class as const and volatile,
which was intended to assert that an object was not pointed to (``aliased'')
by other pointers. The primary application, which is an important one, would have
been for the formal parameters of functions designed to perform computations on
large arrays. A compiler cannot usually take advantage of vectorization or other
parallelization hardware (on supercomputers which have it) unless it can ensure
that the source and destination arrays do not overlap.

The noalias keyword was not backed up by any ``prior art,'' and it was
introduced late in the review and approval process. It was surprisingly difficult
to define precisely and explain coherently, and sparked widespread, acrimonious
debate, including a scathing pan by Dennis Ritchie. It had far-ranging implications,
particularly for several standard library interfaces, for which easy fixes were
not readily apparent.

Because of the criticism and the difficulty of defining noalias well, the Committee
declined to adopt it, in spite of its superficial attractions. (When writing a standard,
features cannot be introduced halfway; their full integration, and all implications,
must be understood.) The need for an explicit mechanism to support parallel implementation
of non-overlapping operations remains unfilled (although some work is being done
on the problem)

Posted by:Richards