How to use extern cuda device variables

Question

I need to write the code into several .cu files. But where should I define the device variables which are use for many .cu files.

An example

File common.h

__device__ int x;

File A.cu

__global__ void a() 

File B.cu

__global__ void b() 

a(),b() both use x. what should I do?

In C language, I should write something like extern device int x; Then I define device int x in another place. But in CUDA I can not do it. If I do, it tells me ‘..........’ previously declared here

Answer 1

EDIT : @talonmies was right (as usual). So I've deleted my comment about CUDA 4.1

Furthermore the compiling commands I gave were not quite right. So let me replace my answer with one that demonstrably works and has the proper instructions.

You need CUDA 5.0 and a compute capability 2.0 or greater device for this to work.

I'm sure there's probably a better way, but this seems to work for me:

com.h:

#ifndef DEVMAIN
extern __device__ int x;
#endif

a.cu:

#include "com.h"
__global__ void a(){

  x = -5;
}

b.cu:

#include <stdio.h>
#define DEVMAIN
#include "com.h"

extern __global__ void a();
__device__ int x;

__global__ void b(){

  x = 5;
}

int main() {
  int temp=7;
  cudaMemcpyToSymbol(x,&temp, sizeof(int));
  a<<<1,1>>>();
  cudaMemcpyFromSymbol(&temp,x,sizeof(int));
  printf("in host : %d\n",temp);
  b<<<1,1>>>();
  cudaMemcpyFromSymbol(&temp,x,sizeof(int));
  printf("in host2 : %d\n",temp);
  return 0;
}

compiling:

nvcc -arch=sm_20 -dc a.cu
nvcc -arch=sm_20 -dc b.cu
nvcc -arch=sm_20 -o ab a.o b.o

output:

$ ./ab
in host : -5
in host2 : 5
$

Sorry for my previous errors.