structures, pointers, and cudaMalloc/Memcpy

Accelerated Computing CUDA CUDA Programming and Performance
1

Hi, I have the following matrix struct:

typedef struct {

   int width;

   int height;

   int pitch;

   float *elements;

} Matrix;

I’ve created the host matrix, and I’m not sure how to copy it over into the device. Here is my code:

Matrix * d_alloc_matrix(Matrix const * A) {

cudaError_t err;

Matrix *Md;

//Get the size of struct

    int size_m = sizeof(*A);

err = cudaMalloc((void **) & Md, size_m);

//cudaMemcpy sets Md->elements to point to the same address as that of in Host(which is wrong)

    cudaMemcpy(Md, A, size_m, cudaMemcpyHostToDevice);

//need to cudaMalloc a new chunk of memory in the device

    int size_e = A->width * A->height * sizeof(float);

float *temp_addr;

cudaMalloc((void**) &temp_addr, size_e);

//Copy over the elements to our device storage

    err = cudaMemcpy(temp_addr, A->elements, size_e, cudaMemcpyHostToDevice);

//Set the Md->elements pointers to point to the same address as of temp_addr

    err = cudaMemcpy(&(Md->elements), &temp_addr , sizeof(float *), cudaMemcpyDeviceToDevice);

return Md;

}

I have another piece of code that transfers the matrix back to host:

void copy_matrix_DeviceToHost(Matrix *Md, Matrix *Mh) {

    int size = Mh->height * Mh->width * sizeof(float);

    cudaMemcpy(Mh->elements, Md->elements, size, cudaMemcpyDeviceToHost);

}

I’m segfaulting =( . Any tips?

2

You can’t access Md->elements because Md is on the device.
Malloc Md on the host and only Md->elements on the device. You can then pass *Md to kernels [post=‘1270181’]just like here[/post].