In CUDA programming, you work in two distinct worlds: the host and the device. In general, device kernels cannot access host variables and host functions cannot access device variables, even though these variables are declared in the same file scope.
The CUDA runtime API can access both host and device variables, but it is up to you to provide the correct arguments to the correct functions so that they operate properly on the correct variables. Because the runtime API makes assumptions about the memory space of certain parameters, passing a host variable where it expects a device variable or vice versa will result in undefined behavior (likely crashing your application).
#include <cuda_runtime.h>
#include <stdio.h>
__device__ float devData;
int main(void) {
float value = 3.14f;
float *devPtr = NULL;
// Get the memory address of the global variable
cudaGetSymbolAddress((void **)&devPtr, devData)
// Now we can use typical cudaMemcpy
cudaMemcpy(dptr, &value, cudaMemcpyHostToDevice);
return 0;
}