An object that is accessed through a restrict-qualified pointer has a special association with that pointer. This association requires that all accesses to that object use, directly or indirectly, the value of that particular pointer. The intended use of the restrict qualifier is to promote optimization, and deleting all instances of the qualifier from a program does not change its meaning (that is, observable behavior). In the absence of this qualifier, other pointers can alias this object. Caching the value in an object designated through a restrict-qualified pointer is safe at the beginning of the block in which the pointer is declared, because no preexisting aliases may also be used to reference that object. The cached value must be restored to the object by the end of the block, where preexisting aliases again become available. New aliases may be formed within the block, but these must all depend on the value of the restrict-qualified pointer so that they can be identified and adjusted to refer to the cached value. For a restrict-qualified pointer at file scope, the block is the body of each function in the file [Walls 2006]. Developers should be aware that C++ does not support the restrict qualifier, but some C++ compiler vendors support an equivalent qualifier. Annex J of the
The C Standard [ISO/IEC 9899:2011] identifies the
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following undefined behavior
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:
A restrict-qualified pointer is assigned a value based on another restricted pointer whose associated block neither began execution before the block associated with this pointer, nor ended before the assignment (6.7.3.1).
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