Converting thrust::iterators to and from raw pointers - cuda

Question

I want to use Thrust library to calculate prefix sum of device array in CUDA.
My array is allocated with cudaMalloc(). My requirement is as follows:
main()
{
Launch kernel 1 on data allocated through cudaMalloc()
// This kernel will poplulate some data d.
Use thrust to calculate prefix sum of d.
Launch kernel 2 on prefix sum.
}
I want to use Thrust somewhere between my kernels so I need method to convert pointers to device iterators and back.What is wrong in following code?
int main()
{
int *a;
cudaMalloc((void**)&a,N*sizeof(int));
thrust::device_ptr<int> d=thrust::device_pointer_cast(a);
thrust::device_vector<int> v(N);
thrust::exclusive_scan(a,a+N,v);
return 0;
}

Answer 1

A complete working example from your latest edit would look like this:
#include <thrust/device_ptr.h>
#include <thrust/device_vector.h>
#include <thrust/scan.h>
#include <thrust/fill.h>
#include <thrust/copy.h>
#include <cstdio>
int main()
{
const int N = 16;
int * a;
cudaMalloc((void**)&a, N*sizeof(int));
thrust::device_ptr<int> d = thrust::device_pointer_cast(a);
thrust::fill(d, d+N, 2);
thrust::device_vector<int> v(N);
thrust::exclusive_scan(d, d+N, v.begin());
int v_[N];
thrust::copy(v.begin(), v.end(), v_);
for(int i=0; i<N; i++)
printf("%d %d\n", i, v_[i]);
return 0;
}
The things you got wrong:
N not defined anywhere
passing the raw device pointer a rather than the device_ptr d as the input iterator to exclusive_scan
passing the device_vector v to exclusive_scan rather than the appropriate iterator v.begin()
Attention to detail was all that is lacking to make this work. And work it does:
$ nvcc -arch=sm_12 -o thrust_kivekset thrust_kivekset.cu
$ ./thrust_kivekset
0 0
1 2
2 4
3 6
4 8
5 10
6 12
7 14
8 16
9 18
10 20
11 22
12 24
13 26
14 28
15 30
Edit:
thrust::device_vector.data() will return a thrust::device_ptr which points to the first element of the vector. thrust::device_ptr.get() will return a raw device pointer. Therefore
cudaMemcpy(v_, v.data().get(), N*sizeof(int), cudaMemcpyDeviceToHost);
and
thrust::copy(v, v+N, v_);
are functionally equivalent in this example.

Answer 2

Convert your raw pointer obtained from cudaMalloc() to a thrust::device_ptr using thrust::device_pointer_cast. Here's an example from the Thrust docs:
#include <thrust/device_ptr.h>
#include <thrust/fill.h>
#include <cuda.h>
int main(void)
{
size_t N = 10;
// obtain raw pointer to device memory
int * raw_ptr;
cudaMalloc((void **) &raw_ptr, N * sizeof(int));
// wrap raw pointer with a device_ptr
thrust::device_ptr<int> dev_ptr = thrust::device_pointer_cast(raw_ptr);
// use device_ptr in Thrust algorithms
thrust::fill(dev_ptr, dev_ptr + N, (int) 0);
// access device memory transparently through device_ptr
dev_ptr[0] = 1;
// free memory
cudaFree(raw_ptr);
return 0;
}
Use thrust::inclusive_scan or thrust::exclusive_scan to compute the prefix sum.
http://code.google.com/p/thrust/wiki/QuickStartGuide#Prefix-Sums