How to add bytes to a capnp::word array?

[Zach La Celle]

Suppose I have some other process which reads a CapnProto message from the wire, and passes it to my code.  How can I:

- Put the standard C array full of data into an Array<::capnp::word> array?

- If that's not necessary, what's the best way to read this into a MessageReader (right now I'm using a FlatArrayMessageReader with a ::capnp::word array)

[Kenton Varda]

Hi Zach,

In order to convert a byte array to a word array without copying, you must first ensure that the bytes are aligned -- that is, they start on an 8-byte boundary (or 4-byte for 32-bit systems). If you try to use non-aligned data, you may see crashes or slowness (depending on CPU architecture).

Sometimes you can be sure that your data is aligned by virtue of the context -- for example, arrays allocated by malloc() are aligned. But if you don't know for sure, it's safest to make a copy. You can also check at runtime by converting the pointer into an integer and doing arithmetic.

*If* the bytes are aligned, then you can use reinterpret_cast to convert, like:

    kj::ArrayPtr<capnp::word> words(reinterpret_cast<const byte*>(bytes.begin()), bytes.size() / sizeof(capnp::word));

Alternatively, to make a copy, do:

    auto words = kj::heapArray<capnp::word>(bytes.size() / sizeof(capnp::word));

    memcpy(words.begin(), bytes.begin(), words.asBytes().size());

-Kenton

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[Zach La Celle]

Perfect, that's very helpful.  Thank you!