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mapped? 

        # IO::Buffer.mapped?

(from ruby core)
### Implementation from Buffer
---
    mapped? -> true or false

---

If the buffer is *mapped*, meaning it references memory mapped by the
buffer.

Mapped buffers are either anonymous, if created by ::new with the
IO::Buffer::MAPPED flag or if the size was at least
IO::Buffer::PAGE_SIZE, or backed by a file if created with ::map.

Mapped buffers can usually be resized, and such an operation will
typically invalidate all slices, but not always.

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