Rules for Extrinsic Procedures

Extrinsic procedures have a number of constraints placed on them so they behave in the same way as `regular' HPF procedures. An EXTRINSIC must:

• fully terminate before control returns,
• not permanently change the mapping of an object,
• obey any INTENT,
• ensure that non-local replicated variables have a consistent value,
• not permanently modify the number of available processors.

All these rules aim to make sure that the invocation of an EXTRINSIC is seamless. Guards are set against the extrinsic spawning processes which remain active after control has returned to HPF; if this was not enforced these processes could interact with execution in a destructive way.

The extrinsic should also leave the global state of the program in the same way as it found it. In other words, if any remapping takes place within the extrinsic, then it must be undone before control returns to HPF. The extrinsic must also ensure that scalar variables, which HPF goes to great lengths to ensure are equal across every processor, do not become corrupted.

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