OpenCL clGetPlatformIDs returning CL_PLATFORM_NOT_FOUND in multi-vendor environments

Direct Answer

The Khronos ICD loader treats each file in /etc/OpenCL/vendors as a pointer to a vendor library. It reads every .icd file, loads the referenced shared object, and merges the platforms that library exposes into a single list. clGetPlatformIDs returns CL_PLATFORM_NOT_FOUND only when that merged list is empty. With multiple valid profiles present, the behavior is deterministic: the loader enumerates the .icd files, loads each library independently, and concatenates the platforms they report. A conforming loader skips a library that fails to load and continues with the remaining entries rather than aborting the whole enumeration.

Confirmed Behavior vs. Likely Explanation

The confirmed, specification-aligned behavior is aggregation: the loader does not prioritize one vendor over another when multiple valid profiles exist. The likely explanation for your production failure is that the loader is finding zero valid entries, not that it is selecting the wrong vendor. Specific configurations that trigger CL_PLATFORM_NOT_FOUND despite installed drivers include:

• The vendors directory is missing or empty because the ICD aggregation layer was never installed, so vendor drivers exist on disk but are never discovered.
• Every .icd file points to a library path that no longer exists after a driver upgrade or GPU removal.
• The .icd files exist, but the referenced libraries have incompatible ABIs or missing dependencies, causing silent load failures.
• The environment variable OCL_ICD_VENDORS redirects the loader to a directory that contains no valid .icd files, overriding the system path entirely.
• Permission changes on /etc/OpenCL/vendors or the .icd files prevent the loader from reading them.

Verification Steps

1. List the vendors directory and inspect each .icd file to confirm it contains an absolute path to an existing, loadable library.
2. Check whether OCL_ICD_VENDORS is set in the production environment; if it is, inspect that directory instead of the system path.
3. Run the application with the loader's debug variable (commonly OCL_ICD_DEBUG=1 for the Khronos loader) to log which files and libraries are attempted.
4. Use ldd on the loader and each vendor library to detect missing dependencies or symbol mismatches.
5. Run clinfo or a minimal host program that calls clGetPlatformIDs to confirm the observed platform count matches the number of valid .icd entries.

One caveat: some vendor or distro-patched loaders abort on the first library load failure instead of skipping it. If a single broken .icd file suppresses all platforms, you are likely using a non-conforming loader, and the fix is to remove or repair the offending entry rather than rely on skip-and-continue behavior.

Missing Diagnostic

Is OCL_ICD_VENDORS set in your production environment? If it is, the recommendation changes from repairing the system vendors directory to auditing the override directory instead.