IntelliSense: identifier "__syncthreads" is undefined


I’m trying to do a parallel reduction but cant get __syncthreads() to work.

I’m running:
Nsight 2.1
Visual Studio 2010

Here is the code:

#include “cuda_runtime.h”
#include “device_launch_parameters.h”
#include <stdio.h>
#include <device_functions.h>

global void

reduce0(int *g_idata, int *g_odata)
extern shared int sdata;

// load shared mem
unsigned int tid = threadIdx.x;
unsigned int i = blockIdx.x*blockDim.x + threadIdx.x;
sdata[tid] = g_idata[i];

// do reduction in shared mem
for(unsigned int s=1; s < blockDim.x; s *= 2) {
    // modulo arithmetic is slow!
    if ((tid % (2*s)) == 0) {
        sdata[tid] += sdata[tid + s];

// write result for this block to global mem
if (tid == 0) g_odata[blockIdx.x] = sdata[0];


int main()
//set parameters in host
const int threads =10;
const int blocks =1;
dim3 dimBlock(threads, 1, 1);
dim3 dimGrid(blocks, 1, 1);
int smemSize = threads * sizeof(int);
int in_list[threads];
int out_list[threads];
for(int i =0;i<threads; i++)
in_list[i] = i;

//set parameters i device
int *d_idata, *d_odata;

//run kernel
reduce0<<< dimGrid, dimBlock, smemSize >>>(d_idata, d_odata);

//copy back to host


return 0;


I get the following error when I compile the program, “IntelliSense: identifier “__syncthreads” is undefined”. Because I can’t sync my threads it constantly fails, due to race condition :( please help! I really need to get this to work.


Without arguing on the effectiveness of your reduction algorithm since I guess this is not the question, you have a bug here. Actually, you allow for your threads to overflow the shared memory area you defined.
By changing your
if ((tid % (2s)) == 0) {
sdata[tid] += sdata[tid + s];
if (((tid % (2
s)) == 0) && ((tid + s) < blockDim.x)) {
sdata[tid] += sdata[tid + s];
You avoid this pitfall.

Now, regarding your issue with __syncthreads(), honestly I don’t know. Are you sure you use the right compiler?