NAME

       Unified Addressing -

   Functions
       cudaError_t cudaPointerGetAttributes (struct cudaPointerAttributes *attributes, const void *ptr)
           Returns attributes about a specified pointer.

Detailed Description

       \brief unified addressing functions of the CUDA runtime API (cuda_runtime_api.h)

       This section describes the unified addressing functions of the CUDA runtime application programming
       interface.

Overview

       CUDA devices can share a unified address space with the host. For these devices there is no distinction
       between a device pointer and a host pointer -- the same pointer value may be used to access memory from
       the host program and from a kernel running on the device (with exceptions enumerated below).

Supported Platforms

       Whether or not a device supports unified addressing may be queried by calling cudaGetDeviceProperties()
       with the device property cudaDeviceProp::unifiedAddressing.

       Unified addressing is automatically enabled in 64-bit processes .

       Unified addressing is not yet supported on Windows Vista or Windows 7 for devices that do not use the TCC
       driver model.

Looking Up Information from Pointer Values

       It is possible to look up information about the memory which backs a pointer value. For instance, one may
       want to know if a pointer points to host or device memory. As another example, in the case of device
       memory, one may want to know on which CUDA device the memory resides. These properties may be queried
       using the function cudaPointerGetAttributes()

       Since pointers are unique, it is not necessary to specify information about the pointers specified to
       cudaMemcpy() and other copy functions. The copy direction cudaMemcpyDefault may be used to specify that
       the CUDA runtime should infer the location of the pointer from its value.

Automatic Mapping of Host Allocated Host Memory

       All host memory allocated through all devices using cudaMallocHost() and cudaHostAlloc() is always
       directly accessible from all devices that support unified addressing. This is the case regardless of
       whether or not the flags cudaHostAllocPortable and cudaHostAllocMapped are specified.

       The pointer value through which allocated host memory may be accessed in kernels on all devices that
       support unified addressing is the same as the pointer value through which that memory is accessed on the
       host. It is not necessary to call cudaHostGetDevicePointer() to get the device pointer for these
       allocations.

       Note that this is not the case for memory allocated using the flag cudaHostAllocWriteCombined, as
       discussed below.

Direct Access of Peer Memory

       Upon enabling direct access from a device that supports unified addressing to another peer device that
       supports unified addressing using cudaDeviceEnablePeerAccess() all memory allocated in the peer device
       using cudaMalloc() and cudaMallocPitch() will immediately be accessible by the current device. The device
       pointer value through which any peer's memory may be accessed in the current device is the same pointer
       value through which that memory may be accessed from the peer device.

Exceptions, Disjoint Addressing

       Not all memory may be accessed on devices through the same pointer value through which they are accessed
       on the host. These exceptions are host memory registered using cudaHostRegister() and host memory
       allocated using the flag cudaHostAllocWriteCombined. For these exceptions, there exists a distinct host
       and device address for the memory. The device address is guaranteed to not overlap any valid host pointer
       range and is guaranteed to have the same value across all devices that support unified addressing.

       This device address may be queried using cudaHostGetDevicePointer() when a device using unified
       addressing is current. Either the host or the unified device pointer value may be used to refer to this
       memory in cudaMemcpy() and similar functions using the cudaMemcpyDefault memory direction.

Function Documentation

   cudaError_t cudaPointerGetAttributes (struct cudaPointerAttributes * attributes, const void * ptr)
       Returns in *attributes the attributes of the pointer ptr. If pointer was not allocated in, mapped by or
       registered with context supporting unified addressing cudaErrorInvalidValue is returned.

       The cudaPointerAttributes structure is defined as:

           struct cudaPointerAttributes {
               enum cudaMemoryType memoryType;
               int device;
               void *devicePointer;
               void *hostPointer;
               int isManaged;
           }

        In this structure, the individual fields mean

       • memoryType identifies the physical location of the memory associated with pointer ptr. It can be
         cudaMemoryTypeHost for host memory or cudaMemoryTypeDevice for device memory.

       • device is the device against which ptr was allocated. If ptr has memory type cudaMemoryTypeDevice then
         this identifies the device on which the memory referred to by ptr physically resides. If ptr has memory
         type cudaMemoryTypeHost then this identifies the device which was current when the allocation was made
         (and if that device is deinitialized then this allocation will vanish with that device's state).

       • devicePointer is the device pointer alias through which the memory referred to by ptr may be accessed
         on the current device. If the memory referred to by ptr cannot be accessed directly by the current
         device then this is NULL.

       • hostPointer is the host pointer alias through which the memory referred to by ptr may be accessed on
         the host. If the memory referred to by ptr cannot be accessed directly by the host then this is NULL.

       • isManaged indicates if the pointer ptr points to managed memory or not.

       Parameters:
           attributes - Attributes for the specified pointer
           ptr - Pointer to get attributes for

       Returns:
           cudaSuccess, cudaErrorInvalidDevice, cudaErrorInvalidValue

       See also:
           cudaGetDeviceCount, cudaGetDevice, cudaSetDevice, cudaChooseDevice, cuPointerGetAttributes

Author

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