cudaMemcpyToSymbol not doing anything

Accelerated Computing CUDA CUDA Programming and Performance
1

Hi, I’m trying to copy a struct that looks something like this:
struct.cuh

struct MyStruct{
    float value;
};

into a GPU variable:
variable.cuh

#include "struct.cuh"

__constant__ MyStruct myStruct;

So far I’ve tried this:
setter.cu

#include "struct.cuh"
#include "variable.cuh"

void SetStruct(MyStruct* data){
    printf("value [CPU] %d\n", data->value);
    CUDA_SAFE_CALL(cudaMemcpyToSymbol(myStruct, data, sizeof(MyStruct)));
    printf("value [GPU] %d\n", myStruct.value);
}

but this prints the following:

value [CPU]: 100
value [GPU]: 0

the expected result is this:

value [CPU]: 100
value [GPU]: 100

I seriously do not know what is going wrong. Any help would be greatly appreciated.
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2

You can’t print device memory from host code.

If you want to print it out, you could launch a kernel and printf from there.

3

Yes I’ve tried doing that, but the result is the same - “value [GPU]: 0”

4 
#include <stdio.h>
#include <stdlib.h>

struct MyStruct{
    float value;
};

__constant__ MyStruct myStruct;

__global__ void printKernel (void)
{
    printf ("GPU: myStruct.value = %15.8e\n", myStruct.value);
}

int main (void)
{
    for (float data = 1.0f; data < 5.0f; data += 1.0f) {
        printf ("CPU: Setting myStruct.value to %15.8e\n", data);
        cudaMemcpyToSymbol(myStruct, &data, sizeof myStruct);
        printKernel<<<1,1>>>();
        cudaDeviceSynchronize(); // flush device-side print buffer
    }
    return EXIT_SUCCESS;
}

prints something like

CPU: Setting myStruct.value to 1.00000000e+000
GPU: myStruct.value =  1.00000000e+00
CPU: Setting myStruct.value to 2.00000000e+000
GPU: myStruct.value =  2.00000000e+00
CPU: Setting myStruct.value to 3.00000000e+000
GPU: myStruct.value =  3.00000000e+00
CPU: Setting myStruct.value to 4.00000000e+000
GPU: myStruct.value =  4.00000000e+00
5

I think I know what the problem was: I was passing a pointer to the SetStruct function. After changing it to pass the struct as a reference I get the expected result.