Increment and access global counter CUDA

Question

I need to make my kernel communicate with the host. I tried to use a global counter (better ways are well accepted), but the following code prints always 0. What am I doing wrong? (I tried both commented and uncommented ways).

#include <stdio.h>
#include <cuda_runtime.h>

//__device__ int count[1] = {0};
__device__ int count = 0;

__global__ void inc() {
    //count[0]++;
    atomicAdd(&count, 1);
}

int main(void) {
    inc<<<1,10>>>();
    cudaDeviceSynchronize();
    //int *c;
    int c;
    cudaMemcpyFromSymbol(&c, count, sizeof(int), cudaMemcpyDeviceToHost);
    printf("%d\n", c);
    return 0;
}

score 4 · Accepted Answer · answered Oct 22 '18 at 22:50

Anytime you are having trouble with a CUDA code, I strongly encourage you to use proper CUDA error checking and run your code with cuda-memcheck, before asking others for help. Even if you don't understand the error output, providing it in your question will be useful for those trying to help you.

If you had done so, you would have received a report that cudaMemcpyFromSymbol is throwing an invalid argument error.

If you study the documentation for that function call, you will see that the 4th parameter is not the direction parameter, but is the offset parameter. So providing cudaMemcpyDeviceToHost is incorrect for the offset parameter. Since cudaMemcpyFromSymbol is always a device->host transfer, providing the direction argument is redundant, and since it is provided a default, is unnecessary. Your code works correctly for me simply by eliminating that:

$ cat t1414.cu
#include <stdio.h>
#include <cuda_runtime.h>

//__device__ int count[1] = {0};
__device__ int count = 0;

__global__ void inc() {
    //count[0]++;
    atomicAdd(&count, 1);
}

int main(void) {
    inc<<<1,10>>>();
    cudaDeviceSynchronize();
    //int *c;
    int c;
    cudaMemcpyFromSymbol(&c, count, sizeof(int));
    printf("%d\n", c);
    return 0;
}
$ nvcc -o t1414 t1414.cu
$ cuda-memcheck ./t1414
========= CUDA-MEMCHECK
10
========= ERROR SUMMARY: 0 errors
$

Increment and access global counter CUDA

1 Answers1