CODEWITHSUNDEEP
c++ubuntulinkeropencl
user615457
user615457

Reputation: 837

library is linked but reference is undefined

I'm trying to compile an openCL program on Ubuntu with an NVIDIA card that worked once before,

#include <CL/cl.h>
#include <iostream>
#include <vector>

using namespace std;

int main() {
  cl_platform_id platform;
  cl_device_id device;
  cl_context context;
  cl_command_queue command_queue;
  cl_int error;

  if(clGetPlatformIDs(1, &platform, NULL) != CL_SUCCESS) {
    cout << "platform error" << endl;
  }

  if(clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, &device, NULL) != CL_SUCCESS) {
    cout << "device error" << endl;
  }

  context = clCreateContext(NULL, 1, &device, NULL, NULL, &error);
  if(error != CL_SUCCESS) {
    cout << "context error" << endl;
  }

  command_queue = clCreateCommandQueue(context, device, 0, &error);
  if(error != CL_SUCCESS) {
    cout << "command queue error" << endl;
  }

  return 0;
}

I compile it like so,

g++ -I/usr/local/cuda/include -L/usr/lib/nvidia-current -lOpenCL opencl.cpp

and I get this result

/tmp/ccAdS9ig.o: In function `main':
opencl.cpp:(.text+0x1a): undefined reference to `clGetPlatformIDs'
opencl.cpp:(.text+0x3d): undefined reference to `clGetDeviceIDs'
opencl.cpp:(.text+0x65): undefined reference to `clCreateContext'
opencl.cpp:(.text+0x85): undefined reference to `clCreateCommandQueue'
collect2: ld returned 1 exit status

but nm -D /usr/lib/nvidia-current/libOpenCL.so tells me that libOpenCL.so at least contains clGetPlatformIDs

0000000000002400 T clGetKernelWorkGroupInfo
0000000000002140 T clGetMemObjectInfo
0000000000002e80 T clGetPlatformIDs
0000000000002de0 T clGetPlatformInfo
0000000000002310 T clGetProgramBuildInfo
00000000000022f0 T clGetProgramInfo
00000000000021f0 T clGetSamplerInfo

Am I missing something.

Upvotes: 40

Views: 63006

Answers (4)

Denis Reichelt
Denis Reichelt

Reputation: 145

In my case I compiled a c++ application but linked a C library. The included header file for that lib did not specify the function prototypes as extern "C" so the linker was searching for a decoarated function name instead of the plain C name. Specifying extern "C" around the C header includes solved this problem for me.

extern "C" {
#include "cheader.h"
}

Upvotes: 6

Jesse
Jesse

Reputation: 370

Alternatively you can add the header and library path to your global variables.

export CPLUS_INCLUDE_PATH=$CPLUS_INCLUDE_PATH:/usr/local/cuda/include
export LIBRARY_PATH=$LIBRARY_PATH:/usr/lib/nvidia-current

You can also try to set

export PATH=$PATH:/usr/local/cuda/bin

It should be possible to run now

g++ opencl.cpp

Upvotes: 0

user995502
user995502

Reputation:

when you are linking, the order of your libraries and source files makes a difference. for example for your case,

g++ -I/usr/local/cuda/include -L/usr/lib/nvidia-current -lOpenCL opencl.cpp

functions defined in the OpenCL library might not be loaded, since there nothing before them asking for a look-up. however if you use,

g++ opencl.cpp -I/usr/local/cuda/include -L/usr/lib/nvidia-current -lOpenCL

then any requests for functions will be found in the OpenCL library and they will be loaded.

Upvotes: 47

Jaffa
Jaffa

Reputation: 12700

From the gcc man page:

   -llibrary
   -l library
       Search the library named library when linking.  (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

       It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified.  Thus, foo.o
       -lz bar.o searches library z after file foo.o but before bar.o.  If bar.o refers to functions in z, those functions may not be loaded.

       The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a.  The linker then uses this file as if it had been specified
       precisely by name.

So try to specify the -lOpenCL after the file argument in your compile command.

You also search for symbols in libOpenCL.so, which is a shared library file. With your command you link your program agains a static library, in the format libOpenCL.a.

Upvotes: 31

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