When developing graphics or compute pipelines with OpenCL, you might encounter a puzzling compilation error when initializing image descriptors: error: ‘cl_image_desc’ {aka ‘struct _cl_image_desc’} has no member named ‘mem_object’. This issue is particularly common when targeting OpenCL 1.2 or 2.0+ on Linux distributions, or when compiling against NVIDIA CUDA-bundled OpenCL headers.

Even though the official Khronos OpenCL specification documents cl_image_desc with an anonymous union containing both buffer and mem_object, actual header implementations often omit mem_object under standard compile flags. Here is why this happens and how to resolve it cleanly and portably.

Why Does This Compilation Error Happen?

In OpenCL 1.2, images backed by a buffer were introduced using the buffer field. In OpenCL 2.0, the specification expanded image creation from existing memory objects (such as another image or a buffer) and renamed the field conceptually to mem_object. To preserve backwards compatibility with OpenCL 1.2 code, the specification introduced an anonymous union:

union {
    cl_mem buffer;
    cl_mem mem_object;
};

However, C99 and earlier ISO C standards do not officially support anonymous unions or anonymous structs (they were introduced in C11). Because OpenCL headers are designed to compile with standard-compliant C89/C99 compilers without issuing warnings or errors, Khronos wrapped the anonymous union in conditional preprocessor guards:

#if defined(CL_VERSION_2_0) && __CL_HAS_ANON_UNION__
    __CL_ANON_UNION__ union {
#endif
      cl_mem                  buffer;
#if defined(CL_VERSION_2_0) && __CL_HAS_ANON_UNION__
      cl_mem                  mem_object;
    };
#endif

If your build environment targets OpenCL 1.2 by default (common in older headers or default defines), or if __CL_HAS_ANON_UNION__ fails compiler detection, the preprocessor strips out the union wrapper and the mem_object identifier entirely, leaving only cl_mem buffer;.

How to Fix and Write Portable Code

Depending on your project constraints and compatibility targets (such as NVIDIA CUDA toolkit headers), here are the best solutions.

Option 1: Use buffer Directly (Recommended for Maximum Portability)

If you want code that compiles without issue across any OpenCL header version (including legacy NVIDIA CUDA headers, Apple OpenCL, or older distro packages), simply assign your memory object to the buffer member:

cl_image_desc desc;
memset(&desc, 0, sizeof(desc));
desc.image_type = CL_MEM_OBJECT_IMAGE2D;
desc.image_width = 1920;
desc.image_height = 1080;
// Works everywhere: 'buffer' and 'mem_object' share the exact same offset
desc.buffer = my_backing_mem_object;

Why this is safe: Because buffer and mem_object occupy the exact same memory offset inside the struct, assigning to buffer is binary-identical to setting mem_object. It works whether the headers compile with the anonymous union or fall back to the single field.

Option 2: Target OpenCL 2.0+ Explicitly

If you prefer using mem_object for code clarity and your codebase targets modern OpenCL versions, ensure your compiler target definition is set to OpenCL 2.0 or higher before including the headers:

#define CL_TARGET_OPENCL_VERSION 200
#include <CL/cl.h>

Additionally, make sure your C compiler supports anonymous structs/unions. For GCC or Clang, ensure you are compiling with at least C11 (-std=c11) or using GNU extensions (-std=gnu99 or -std=gnu11).

Option 3: Write a Preprocessor Fallback

If you want your code to be semantically descriptive while maintaining 100% compatibility with older headers, you can implement a helper macro or conditional assignment:

#if defined(CL_VERSION_2_0) && (defined(__CL_HAS_ANON_UNION__) || defined(__CL_HAS_ANON_STRUCT__))
    desc.mem_object = my_backing_mem_object;
#else
    desc.buffer = my_backing_mem_object;
#endif

Summary

  • The error occurs because OpenCL headers guard anonymous unions to maintain backwards compatibility with strict C89/C99 compilers.
  • NVIDIA's OpenCL implementation often freezes headers at older target versions (such as OpenCL 1.2 or 3.0 with 1.2 conformance levels), causing mem_object to disappear.
  • The simplest and most robust fix is to populate the .buffer member. It points to the exact same offset in memory and avoids macro discrepancies entirely.