Issue with clEnqueueReadBuffer Function in OpenCL - in an Array Sum Sample

Question

everyone, I'm a beginner on OpenCL and I wrote some simple code in C which sums two arrays. Here is part of the code:

// Create Kernel.

cl_kernel kernelSum = clCreateKernel( myProgram, "sum", &error );


// Set Input Array.

size_t arraySize = 1000;

char* a = ( char* ) malloc( sizeof( char ) * arraySize );
char* b = ( char* ) malloc( sizeof( char ) * arraySize );
char* c = ( char* ) malloc( sizeof( char ) * arraySize );

for (int i = 0; i < arraySize; i += 1)
{
    a[ i ] = 1;
    b[ i ] = 2;
    c[ i ] = -1;
}


// Set Buffers.

cl_mem a_buffer = clCreateBuffer(
    myContext,
    CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR,
    arraySize * sizeof( char ), a,
    &error );

cl_mem b_buffer = clCreateBuffer(
    myContext,
    CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR,
    arraySize * sizeof( char ), b,
    &error );

cl_mem c_buffer = clCreateBuffer(
    myContext,
    CL_MEM_WRITE_ONLY,
    arraySize * sizeof( char ), NULL,
    &error );

printf( "Buffers created.\n" );


// Setting Kernel Arguments.

error = clSetKernelArg( kernelSum, 0, sizeof( cl_mem ), &a_buffer );

error |= clSetKernelArg( kernelSum, 1, sizeof( cl_mem ), &b_buffer );

error |= clSetKernelArg( kernelSum, 2, sizeof( cl_mem ), &c_buffer );

printf( "Arguments Set.\n" );


// Enqueue kernels to execute.

cl_event event;

size_t globalWorkOffset = 0;

size_t globalWorkSize[ 1 ] = { arraySize };

size_t localWorkSize[ 1 ] = { 1 };

clEnqueueNDRangeKernel(
    myCommandQueue,
    kernelSum,
    1,                  // work_dim
    0,              // global work offset
    globalWorkSize,
    localWorkSize,              // local work offset
    0, NULL,
    &event
    );

printf( "Kernel Enqueued.\n" );

error = clEnqueueReadBuffer(
    myCommandQueue,
    c_buffer,
    CL_TRUE,                        // blocking option
    ( size_t ) 0, arraySize * sizeof( char ),   // offset, data_size
    c,                              // host_ptr
    0, NULL,
    &event );

if ( error != CL_SUCCESS )
{
    printf( "Buffer Reading Back Failed.\n" );
    exit( 1 );
}

However, I got incorrect result : all the numbers in "c" array are zeros. I thought it has something to do with clEnqueueReadBuffer, or perhaps not. Any ideas about this issue? Expecting your suggestions! :-)

Answer 1

Your call to clEnqueueReadBuffer will not wait for the kernel to finish. It will most likely execute simultaneously with the kernel. Change the call to:

error = clEnqueueReadBuffer(
    myCommandQueue,
    c_buffer,
    CL_TRUE,                        // blocking option
    ( size_t ) 0, arraySize * sizeof( char ),   // offset, data_size
    c,                              // host_ptr
    1, &event,
    NULL );

This will cause clEnqueueReadBuffer to wait for the kernel event to finish before starting to read the buffer.

Answer 2

Everyone. All your suggestions are really helpful. It's with your help I finally found what I've done wrong : clBuildProgram( myProgram, 1, &device, NULL, NULL, &error ); This is my original code. The official definition of this function is : cl_int clBuildProgram(cl_program program, cl_uint num_devices, const cl_device_id *device_list, const char *options, void (CL_CALLBACK *pfn_notify) (cl_program program, void *user_data), void *user_data) I found I set the last of the argument list as "&error"; I got this function mixed with some others whose argument list usually ends with an error number. Here, I should have call it like this: clBuildProgram( myProgram, 1, &device, NULL, NULL, NULL ); The last one set to NULL. Then the program run correctly.

Thank you all very much! :-)