Multiple Thread with Altera OpenCL

Altera SDK for OpenCL Programming Guide claims:

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The Altera Software Development Kit (SDK) for OpenCL (AOCL) host library is not thread-safe.

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I have a simple question.

In this context, what "not thread-safe" means strictly?

• Only main thread is allowed to call OpenCL API.

• Only one thread is allowed to call OpenCL API.

• Multiple thread can work only if there are appropriate exclusive lock not to call OpenCL API simultaneously.

Anybody knows ?

iitaku