Why Does cudaMemcpy Cause a Segmentation Fault When Assigning to a Class Member Pointer?

cudaMemcpy Segmentation Fault in Class Member Pointer Assignment

Problem

A cudaMemcpy operation attempting to assign a value to a class member pointer on the host caused a segmentation fault due to incorrect usage of device pointers.

cudaMemcpy(CurrentGrid->cdata[i], Grid_dev->cdata[i], size*sizeof(float), cudaMemcpyDeviceToHost);

Solution

The solution involves setting up the device pointer correctly for use on the host. Instead of directly copying the device pointer's value to the host, an intermediate pointer on the host is required.

float * A;
cudaMalloc((void**)&A, sizeof(float));
...
...
cudaMemcpy(&A, &(Grid_dev->cdata[i]), sizeof(float *), cudaMemcpyDeviceToHost);    
CurrentGrid->cdata[i] = new float[size];
cudaMemcpy(CurrentGrid->cdata[i], A, size*sizeof(float), cudaMemcpyDeviceToHost);            

Explanation

Device pointers cannot be dereferenced directly in cudaMemcpy calls on the host. Instead, the pointer value itself must be copied to an intermediate pointer on the host. This ensures that the host pointer points to a valid device memory location where the data can be accessed.

Prevention

To avoid memory leaks and to ensure proper usage of device pointers in cudaMemcpy operations, it is important to follow the correct procedure for setting up class members with pointers on the device. This includes using intermediate pointers on the host for copying pointer values, and using the host pointer for subsequent cudaMemcpy operations involving the device memory.

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