restrict

In the C programming language, restrict is a keyword that can be used in pointer declarations. By adding this type qualifier, a programmer hints to the compiler that for the lifetime of the pointer, only the pointer itself or a value directly derived from it (such as pointer + 1) will be used to access the object to which it points.
restrict limits the effects of pointer aliasing, aiding optimizations. If the declaration of intent is not followed and the object is accessed by an independent pointer, this will result in undefined behavior. The use of this type qualifier allows C code to achieve the same performance as the same program written in Fortran. It was introduced in the C99 standard.C++ does not have standard support for restrict, but many compilers have equivalents that usually work in both C++ and C, such as the GCC's and Clang's __restrict__, and Visual C++'s __declspec(restrict). In addition, __restrict is supported by those three compilers. The exact interpretation of these alternative keywords vary by the compiler:

In Unix-style compilers such as GCC and Clang, __restrict and __restrict__ means exactly the same as their C counterpart. Extensions include allowing it to be applied to reference types and this.
In Visual C++, multiple no-alias qualifiers are provided:
__declspec(restrict) applies to the function declaraction and hints that the returned pointer is not aliased.
__restrict is used in the same place as restrict, but the no-alias hint does not propagate as in restrict. It is also extended for union types.

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