CODEWITHSUNDEEP
c++cuda
John
John

Reputation: 1

CUDA - Using __constant__ variables and cudaMemcpyFromSymbol

I'm a bit new to CUDA so please forgive me if this is a dumb question. I've been reading/watching quite a few tutorials but they are all very confusing but I think I have the basic idea down. Anyway I'm trying to do the following: I want to initialize several constant variables on the device (hbar,kb,q,T,me). I then want to also have a variable muB that's a function of these other initialized constants. However, I believe to do that, I need to first copy the constant values to the host memory, compute muB and pass it back to the device memory. I attempt to do this in the main function. 

#include <stdio.h>
#include <stdlib.h>
#include <cmath>
#include <ctime>

#include <cuda.h>
#include <cuda_runtime.h>

using namespace std;

//Physical Constants
__constant__ float hbar = 1.054e-34;                   
__constant__ float kb = 1.38e-23;                       
__constant__ float q = 1.6e-19;                         
__constant__ float T = 300.0;                           
__constant__ float me = 9.1e-31;                        
__constant__ float muB = 0.0;                           

int main() {
    float tmp = 0.0f;

    float h_q, h_hbar, h_me;
    cudaMemcpyFromSymbol(&h_q, &q, sizeof(float));
    cudaMemcpyFromSymbol(&h_hbar, &hbar, sizeof(float));
    cudaMemcpyFromSymbol(&h_me, &me, sizeof(float));
    tmp = h_q*h_hbar / 2 / h_me;
    cudaMemcpyToSymbol(&muB, &tmp, sizeof(float));

    return 0;
}

The problem is, when I run this program, h_q, h_hbar, and h_me are all equal to -107374176 which is not what I initialized those constant variables to. tmp is also equal to -53687088.0. I know I must be doing something wrong but can't quite figure out what. Anyone have any insights or suggestions? Thanks

Upvotes: 0

Views: 1660

Answers (1)

Robert Crovella
Robert Crovella

Reputation: 151849

When using cudaMemcpyTo/FromSymbol, we use the symbol name directly, not with the ampersand. So instead of this:

cudaMemcpyFromSymbol(&h_q, &q, sizeof(float));
cudaMemcpyFromSymbol(&h_hbar, &hbar, sizeof(float));
cudaMemcpyFromSymbol(&h_me, &me, sizeof(float));
tmp = h_q*h_hbar / 2 / h_me;
cudaMemcpyToSymbol(&muB, &tmp, sizeof(float));

do this:

cudaMemcpyFromSymbol(&h_q, q, sizeof(float));
cudaMemcpyFromSymbol(&h_hbar, hbar, sizeof(float));
cudaMemcpyFromSymbol(&h_me, me, sizeof(float));
tmp = h_q*h_hbar / 2 / h_me;
cudaMemcpyToSymbol(muB, &tmp, sizeof(float));

Also, I'm pretty sure that if you did proper cuda error checking, each of those lines you wrote would throw an error.

Upvotes: 1

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