CUDA: cudaMemcpy only works in emulation mode.

I am just starting to learn how to use CUDA. I am trying to run some simple example code:

float *ah, *bh, *ad, *bd;
ah = (float *)malloc(sizeof(float)*4);
bh = (float *)malloc(sizeof(float)*4);
cudaMalloc((void **) &ad, sizeof(float)*4);
cudaMalloc((void **) &bd, sizeof(float)*4);
... initialize ah ...

/* copy array on device */
cudaMemcpy(ad,ah,sizeof(float)*N,cudaMemcpyHostToDevice);
cudaMemcpy(bd,ad,sizeof(float)*N,cudaMemcpyDeviceToDevice);
cudaMemcpy(bh,bd,sizeof(float)*N,cudaMemcpyDeviceToHost);

When I run in emulation mode (nvcc -deviceemu) it runs fine (and actually copies the array). But when I run it in regular mode, it runs w/o error, but never copies the data. It's as if the cudaMemcpy lines are just ignored.

What am I doing wrong?

Thank you very much, Jason

From stackoverflow

• You should check for errors, ideally on each malloc and memcpy but just doing it once at the end will be sufficient (cudaGetErrorString(cudaGetLastError()).

  Just to check the obvious:

  • You do have a CUDA capable GPU, right? Run the deviceQuery SDK sample to check the device is working correctly and all the drivers are installed and working.
  • N (in the memcpy) is equal to 4 (in the malloc), right?

• i am having a similar problem. i have cuda driver 3.0 installed and cuda toolkit v3.0 installed and an nvidia g210m. when i run a very simple array reverse program normally (using nvcc file.cu) it runs but the output comes out wrong( no reversal). however the very same prog wen run under -deviceemu mode gives the right output. y is this so?

• See if you have a CUDA enabled device. Probably you can try running the code below and see what info you get:

  #include <cstdio>
  
  int main( void ) {
      cudaDeviceProp  prop;
  
      int count;
      cudaGetDeviceCount( &count );
      for (int i=0; i< count; i++) {
          cudaGetDeviceProperties( &prop, i );
          printf( "   --- General Information for device %d ---\n", i );
          printf( "Name:  %s\n", prop.name );
          printf( "Compute capability:  %d.%d\n", prop.major, prop.minor );
          printf( "Clock rate:  %d\n", prop.clockRate );
          printf( "Device copy overlap:  " );
          if (prop.deviceOverlap)
              printf( "Enabled\n" );
          else
              printf( "Disabled\n");
          printf( "Kernel execution timeout :  " );
          if (prop.kernelExecTimeoutEnabled)
              printf( "Enabled\n" );
          else
              printf( "Disabled\n" );
  
          printf( "   --- Memory Information for device %d ---\n", i );
          printf( "Total global mem:  %ld\n", prop.totalGlobalMem );
          printf( "Total constant Mem:  %ld\n", prop.totalConstMem );
          printf( "Max mem pitch:  %ld\n", prop.memPitch );
          printf( "Texture Alignment:  %ld\n", prop.textureAlignment );
  
          printf( "   --- MP Information for device %d ---\n", i );
          printf( "Multiprocessor count:  %d\n",
                      prop.multiProcessorCount );
          printf( "Shared mem per mp:  %ld\n", prop.sharedMemPerBlock );
          printf( "Registers per mp:  %d\n", prop.regsPerBlock );
          printf( "Threads in warp:  %d\n", prop.warpSize );
          printf( "Max threads per block:  %d\n",
                      prop.maxThreadsPerBlock );
          printf( "Max thread dimensions:  (%d, %d, %d)\n",
                      prop.maxThreadsDim[0], prop.maxThreadsDim[1],
                      prop.maxThreadsDim[2] );
          printf( "Max grid dimensions:  (%d, %d, %d)\n",
                      prop.maxGridSize[0], prop.maxGridSize[1],
                      prop.maxGridSize[2] );
          printf( "\n" );
      }
  }