OpenCL HelloWorld OSX Linker Error - macos

Question

I tried to build the HelloWorld OpenCL example from the OpenCL Mac Programming Guide:
http://developer.apple.com/library/mac/documentation/Performance/Conceptual/OpenCL_MacProgGuide/OpenCL_MacProgGuide.pdf
through Xcode 4.6 on an iMac (Late 2012, OSX 10.8.3) and on my MacBook (Early 2008, OSX 10.7.5) and I'm getting what seems to be linking errors:
https://www.dropbox.com/s/iih87g0495qn4c6/Screen%20Shot%202013-06-04%20at%203.35.55%20PM.png (I tried copy-pasting but the formatting looks horrendous.)
I followed all the instructions in OpenCL Mac Programming Guide, dated 2012-07-23, but no cigar.
// mykernel.cl
kernel void square(global float* input, global float* output) {
size_t i = get_global_id(0);
output[i] = input[i] * input[i];
}
// main.c
// HelloWorld
#include <stdio.h>
#include <OpenCL/OpenCL.h>
#include "mykernel.cl.h"
static void print_device_info(cl_device_id device) {
char name[128];
char vendor[128];
clGetDeviceInfo(device, CL_DEVICE_NAME, 128, name, NULL);
clGetDeviceInfo(device, CL_DEVICE_VENDOR, 128, vendor, NULL);
fprintf(stdout, "%s : %s\n", vendor, name);
}
However, the older, longer version of the OpenCL HelloWorld from Apple, works correctly:
//
// File: hello.c
//
// Abstract: A simple "Hello World" compute example showing basic usage of OpenCL which
// calculates the mathematical square (X[i] = pow(X[i],2)) for a buffer of
// floating point values.
//
//
// Version: <1.0>
//
// Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc. ("Apple")
// in consideration of your agreement to the following terms, and your use,
// installation, modification or redistribution of this Apple software
// constitutes acceptance of these terms. If you do not agree with these
// terms, please do not use, install, modify or redistribute this Apple
// software.
//
// In consideration of your agreement to abide by the following terms, and
// subject to these terms, Apple grants you a personal, non - exclusive
// license, under Apple's copyrights in this original Apple software ( the
// "Apple Software" ), to use, reproduce, modify and redistribute the Apple
// Software, with or without modifications, in source and / or binary forms;
// provided that if you redistribute the Apple Software in its entirety and
// without modifications, you must retain this notice and the following text
// and disclaimers in all such redistributions of the Apple Software. Neither
// the name, trademarks, service marks or logos of Apple Inc. may be used to
// endorse or promote products derived from the Apple Software without specific
// prior written permission from Apple. Except as expressly stated in this
// notice, no other rights or licenses, express or implied, are granted by
// Apple herein, including but not limited to any patent rights that may be
// infringed by your derivative works or by other works in which the Apple
// Software may be incorporated.
//
// The Apple Software is provided by Apple on an "AS IS" basis. APPLE MAKES NO
// WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED
// WARRANTIES OF NON - INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A
// PARTICULAR PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION
// ALONE OR IN COMBINATION WITH YOUR PRODUCTS.
//
// IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR
// CONSEQUENTIAL DAMAGES ( INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION ) ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION
// AND / OR DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER
// UNDER THEORY OF CONTRACT, TORT ( INCLUDING NEGLIGENCE ), STRICT LIABILITY OR
// OTHERWISE, EVEN IF APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// Copyright ( C ) 2008 Apple Inc. All Rights Reserved.
//
////////////////////////////////////////////////////////////////////////////////
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <OpenCL/opencl.h>
////////////////////////////////////////////////////////////////////////////////
// Use a static data size for simplicity
//
#define DATA_SIZE (1024)
////////////////////////////////////////////////////////////////////////////////
// Simple compute kernel which computes the square of an input array
//
const char *KernelSource = "\n" \
"__kernel void square( \n" \
" __global float* input, \n" \
" __global float* output, \n" \
" const unsigned int count) \n" \
"{ \n" \
" int i = get_global_id(0); \n" \
" if(i < count) \n" \
" output[i] = input[i] * input[i]; \n" \
"} \n" \
"\n";
////////////////////////////////////////////////////////////////////////////////
int main(int argc, char** argv)
{
int err; // error code returned from api calls
float data[DATA_SIZE]; // original data set given to device
float results[DATA_SIZE]; // results returned from device
unsigned int correct; // number of correct results returned
size_t global; // global domain size for our calculation
size_t local; // local domain size for our calculation
cl_device_id device_id; // compute device id
cl_context context; // compute context
cl_command_queue commands; // compute command queue
cl_program program; // compute program
cl_kernel kernel; // compute kernel
cl_mem input; // device memory used for the input array
cl_mem output; // device memory used for the output array
// Fill our data set with random float values
//
int i = 0;
unsigned int count = DATA_SIZE;
for(i = 0; i < count; i++)
data[i] = rand() / (float)RAND_MAX;
// Connect to a compute device
//
int gpu = 1;
err = clGetDeviceIDs(NULL, gpu ? CL_DEVICE_TYPE_GPU : CL_DEVICE_TYPE_CPU, 1, &device_id, NULL);
if (err != CL_SUCCESS)
{
printf("Error: Failed to create a device group!\n");
return EXIT_FAILURE;
}
// Create a compute context
//
context = clCreateContext(0, 1, &device_id, NULL, NULL, &err);
if (!context)
{
printf("Error: Failed to create a compute context!\n");
return EXIT_FAILURE;
}
// Create a command commands
//
commands = clCreateCommandQueue(context, device_id, 0, &err);
if (!commands)
{
printf("Error: Failed to create a command commands!\n");
return EXIT_FAILURE;
}
// Create the compute program from the source buffer
//
program = clCreateProgramWithSource(context, 1, (const char **) & KernelSource, NULL, &err);
if (!program)
{
printf("Error: Failed to create compute program!\n");
return EXIT_FAILURE;
}
// Build the program executable
//
err = clBuildProgram(program, 0, NULL, NULL, NULL, NULL);
if (err != CL_SUCCESS)
{
size_t len;
char buffer[2048];
printf("Error: Failed to build program executable!\n");
clGetProgramBuildInfo(program, device_id, CL_PROGRAM_BUILD_LOG, sizeof(buffer), buffer, &len);
printf("%s\n", buffer);
exit(1);
}
// Create the compute kernel in the program we wish to run
//
kernel = clCreateKernel(program, "square", &err);
if (!kernel || err != CL_SUCCESS)
{
printf("Error: Failed to create compute kernel!\n");
exit(1);
}
// Create the input and output arrays in device memory for our calculation
//
input = clCreateBuffer(context, CL_MEM_READ_ONLY, sizeof(float) * count, NULL, NULL);
output = clCreateBuffer(context, CL_MEM_WRITE_ONLY, sizeof(float) * count, NULL, NULL);
if (!input || !output)
{
printf("Error: Failed to allocate device memory!\n");
exit(1);
}
// Write our data set into the input array in device memory
//
err = clEnqueueWriteBuffer(commands, input, CL_TRUE, 0, sizeof(float) * count, data, 0, NULL, NULL);
if (err != CL_SUCCESS)
{
printf("Error: Failed to write to source array!\n");
exit(1);
}
// Set the arguments to our compute kernel
//
err = 0;
err = clSetKernelArg(kernel, 0, sizeof(cl_mem), &input);
err |= clSetKernelArg(kernel, 1, sizeof(cl_mem), &output);
err |= clSetKernelArg(kernel, 2, sizeof(unsigned int), &count);
if (err != CL_SUCCESS)
{
printf("Error: Failed to set kernel arguments! %d\n", err);
exit(1);
}
// Get the maximum work group size for executing the kernel on the device
//
err = clGetKernelWorkGroupInfo(kernel, device_id, CL_KERNEL_WORK_GROUP_SIZE, sizeof(local), &local, NULL);
if (err != CL_SUCCESS)
{
printf("Error: Failed to retrieve kernel work group info! %d\n", err);
exit(1);
}
// Execute the kernel over the entire range of our 1d input data set
// using the maximum number of work group items for this device
//
global = count;
err = clEnqueueNDRangeKernel(commands, kernel, 1, NULL, &global, &local, 0, NULL, NULL);
if (err)
{
printf("Error: Failed to execute kernel!\n");
return EXIT_FAILURE;
}
// Wait for the command commands to get serviced before reading back results
//
clFinish(commands);
// Read back the results from the device to verify the output
//
err = clEnqueueReadBuffer( commands, output, CL_TRUE, 0, sizeof(float) * count, results, 0, NULL, NULL );
if (err != CL_SUCCESS)
{
printf("Error: Failed to read output array! %d\n", err);
exit(1);
}
// Validate our results
//
correct = 0;
for(i = 0; i < count; i++)
{
if(results[i] == data[i] * data[i])
correct++;
}
// Print a brief summary detailing the results
//
printf("Computed '%d/%d' correct values!\n", correct, count);
// Shutdown and cleanup
//
clReleaseMemObject(input);
clReleaseMemObject(output);
clReleaseProgram(program);
clReleaseKernel(kernel);
clReleaseCommandQueue(commands);
clReleaseContext(context);
return 0;
}

Answer 1

I have not used OpenCL, but the linker error is saying it can't find a main() function in your program. Is your first code listing the complete program? If so, you need to add a main() function to the main.c file that calls your print_device_info() function.