Interpretations

Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-2001 IEEE Standard Standard for Information Technology -- Portable Operating System Interface (POSIX®)

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Interpretation Request #74
Topic: makecontext() uc_link Relevant Sections: XSH makecontext() Page: 742 Line: 24473

The makecontext() description provides the only real detail about the uc_link element of ucontext_t. In the getcontext() man page it is only said that if the pointer is NULL this is the main thread etc.

Now, the complete text of the uc_link description is this: The uc_link member is used to determine the context that shall be resumed when the context being modified by makecontext( ) returns. The application shall ensure that the uc_link member is initialized prior to the call to makecontext( ).

There is no word on how the ucontext_t structure pointed to by uc_link has to be initialized. Some people try to interpret this as if it allows any kind of initialization. From the text in getcontext() it is clear, though, that this is not intended: The ucp argument should be created either by a prior call to getcontext() or makecontext(), or by being passed as an argument to a signal handler

In the case of uc_link the signal handler part goes away but it is possible to use swapcontext(). One problem is that the ucontext_t structure only needs to be fully initialized at the time the link is used. I suggest adding this requirement to the uc_link description.

Add at the end of line 24475: If uc_link is not null the structure pointed to by it at the time the pointer is used must be initialized by a prior call to getcontext(), makecontext(), or swapcontext().

Interpretation Response #74
The standard does not speak to this issue, and as such no conformance distinction can be made between alternative implementations based on this. This is being referred to the sponsor.

Rationale for Interpretation
None.