restrict.md

title	ms.custom	ms.date	ms.technology	ms.topic	f1_keywords	dev_langs	helpviewer_keywords	ms.assetid	author	ms.author	ms.workload
restrict | Microsoft Docs		02/09/2018	cpp-language	language-reference	restrict_cpp	C++	__declspec keyword [C++], restrict restrict __declspec keyword	f39cf632-68d8-4362-a497-2d4c15693689	mikeblome	mblome	cplusplus

restrict

Microsoft Specific

When applied to a function declaration or definition that returns a pointer type, restrict tells the compiler that the function returns an object that is not aliased, that is, referenced by any other pointers. This allows the compiler to perform additional optimizations.

Syntax

__declspec(restrict) pointer_return_type function();

Remarks

The compiler propagates __declspec(restrict). For example, the CRT malloc function has a __declspec(restrict) decoration, and therefore, the compiler assumes that pointers initialized to memory locations by malloc are also not aliased by previously existing pointers.

The compiler does not check that the returned pointer is not actually aliased. It is the developer's responsibility to ensure the program does not alias a pointer marked with the restrict __declspec modifier.

For similar semantics on variables, see __restrict.

For another annotation that applies to aliasing within a function, see __declspec(noalias).

For information about the restrict keyword that is part of C++ AMP, see restrict (C++ AMP).

Example

The following sample demonstrates the use of __declspec(restrict).

When __declspec(restrict) is applied to a function that returns a pointer, this tells the compiler that the memory pointed to by the return value is not aliased. In this example, the pointers mempool and memptr are global, so the compiler can't be sure that the memory they refer to is not aliased. However, they are used within ma and its caller init in a way that returns memory that isn't otherwise referenced by the program, so __decslpec(restrict) is used to help the optimizer. This is similar to how the CRT headers decorate allocation functions such as malloc by using __declspec(restrict) to indicate that they always return memory that cannot be aliased by existing pointers.

// declspec_restrict.c
// Compile with: cl /W4 declspec_restrict.c
#include <stdio.h>
#include <stdlib.h>

#define M 800
#define N 600
#define P 700

float * mempool, * memptr;

__declspec(restrict) float * ma(int size)
{
    float * retval;
    retval = memptr;
    memptr += size;
    return retval;
}

__declspec(restrict) float * init(int m, int n)
{
    float * a;
    int i, j;
    int k=1;

    a = ma(m * n);
    if (!a) exit(1);
    for (i=0; i<m; i++)
        for (j=0; j<n; j++)
            a[i*n+j] = 0.1f/k++;
    return a;
}

void multiply(float * a, float * b, float * c)
{
    int i, j, k;

    for (j=0; j<P; j++)
        for (i=0; i<M; i++)
            for (k=0; k<N; k++)
                c[i * P + j] =
                          a[i * N + k] *
                          b[k * P + j];
}

int main()
{
    float * a, * b, * c;

    mempool = (float *) malloc(sizeof(float) * (M*N + N*P + M*P));

    if (!mempool)
    {
        puts("ERROR: Malloc returned null");
        exit(1);
    }

    memptr = mempool;
    a = init(M, N);
    b = init(N, P);
    c = init(M, P);

    multiply(a, b, c);
}

END Microsoft Specific

See also

Keywords
__declspec
__declspec(noalias)