# kernel template

**URL:** <https://forums.developer.nvidia.com/t/kernel-template/29362>\
**Category:** Teaching & Curriculum Support\
**Created:** [May 9, 2013, 1:30am UTC](https://forums.developer.nvidia.com/t/kernel-template/29362 "2013-05-09T01:30:01Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Essviv](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@Essviv](https://forums.developer.nvidia.com/u/Essviv)\
**Post date:** [May 9, 2013, 1:30am UTC](https://forums.developer.nvidia.com/t/kernel-template/29362/1 "2013-05-09T01:30:01Z")

</div>

I’m new to cuda programming,^-^. i wrote a template kernel function and when i called it in my main.cpp, i encountered a undefined error. My code is quite simple and as follows:

////////////////////////////////////////////////////////////////////////  
[//kernel.cu](https://kernel.cu)  
template  
**global** void addKernel(const T_a,const T_b,int n,T\* result)  
{  
int nIndexX=threadIdx.x+blockIdx.x_blockDim.x;  
int nIndexY=threadIdx.y+blockIdx.y_blockDim.y;  
int nIndex=nIndexY_blockDim.x_gridDim.x+nIndexX;  
/_int nIndex=threadIdx.x;_/  
if(nIndex\<n)  
result[nIndex]=a[nIndex]+b[nIndex];  
}

template  
void addByGPU(const T_a,const T_b,int n,T\* result)  
{  
if(a==NULL || b==NULL || result==NULL)  
return;

```
cout<<"GPU execution starts."<<endl;

int nSize=sizeof(T)*n;
cudaError_t status;

T* nA_d,*nB_d,*nC_d;
status=cudaMalloc(&nA_d,nSize);
status=cudaMalloc(&nB_d,nSize);
status=cudaMalloc(&nC_d,nSize);

status=cudaMemcpy(nA_d,a,nSize,cudaMemcpyHostToDevice);
status=cudaMemcpy(nB_d,b,nSize,cudaMemcpyHostToDevice);

dim3 blocksPerGrid(1024/16,1024/16,1);
dim3 threadsPerBlock(16,16,1);
addKernel<T><<<blocksPerGrid,threadsPerBlock>>>(nA_d,nB_d,n,nC_d);
status=cudaMemcpy(result,nC_d,nSize,cudaMemcpyDeviceToHost);
status=cudaDeviceSynchronize();

status=cudaFree(nA_d);
status=cudaFree(nB_d);
status=cudaFree(nC_d);

status = cudaDeviceReset();
return;

```

}

[//main.cpp](https://main.cpp)  
#include \<stdlib.h\>  
#include \<time.h\>  
#include   
using namespace std;

//template extern void addByGPU(const T_a,const T_b,int n,T\* result);  
//extern void addByGPU(const int_a,const int_b,int n,int\* result);

int main(void)  
{  
// MathOp mathOp;

```
int nCount=4*4*256*256;
int* a=new int[nCount];
int* b=new int[nCount];

srand(time(0));
for(int i=0;i<nCount;i++)
{
	a[i]=rand();
	b[i]=rand();
}

int*c=new int[nCount];
addByGPU<int>(a,b,nCount,c);
//int* c=mathOp.add(a,b,nCount);
for(int i=0;i<nCount;i++)
{
	if(a[i]+b[i]!=c[i])
	{
		cout<<"Error!"<<endl;
		break;
	}
}

cout<<"Completed..."<<endl;

delete [] a;
delete [] b;
delete [] c;
return 0;

```

}  
////////////////////////////////////////////////////////////////

i tried to add the line “extern \*\*\*\*” at the beginning of the main.cpp file, but it still doesn’t work.  
it turns out that the declaration and the definition of the template function in c++ must in the same file so the compiler can determine the exact type while compiling. i figure it out that if i move the main function into the cu file, then everything will be fine. but i wanna know if any alternative exists avoiding doing that. it makes the code structure so wired. Any suggestion would be appreciated.

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<div class="post-metadata">

**Author:** ![Tiomat](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@Tiomat](https://forums.developer.nvidia.com/u/Tiomat)\
**Post date:** [May 9, 2013, 3:06pm UTC](https://forums.developer.nvidia.com/t/kernel-template/29362/2 "2013-05-09T15:06:52Z")

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It is a general c++ issue that templates need to be completely visible to the compiler when it hits any call off to a template function. The way that I get around it is I put each of my templated kernels in a .cuh file, and simply #include that file at the top of each cpp file that requires them (or .cu files too). This way keeps kernels out of ‘normal’ c++ code as well as allowing each individual file to stay relatively small.

So in your example you would move your kernel into a seperate file called perhaps “AddKernel.cuh” (fully include guarded of course), and then at the top of your cpp file add #include “AddKernel.cuh”.

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<div class="post-metadata">

**Author:** ![Mati86](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@Mati86](https://forums.developer.nvidia.com/u/Mati86)\
**Post date:** [July 6, 2013, 3:52pm UTC](https://forums.developer.nvidia.com/t/kernel-template/29362/3 "2013-07-06T15:52:13Z")

</div>

In Device Query of Ge Force GT 640 …

Maximum sizes of each dimension of a grid: 2147483647 x 65535 x 65535

does it mean i can launch a grid with size 2147483647 x 65535 x 65535 …

can anyone answer