How to get the serialized length of a message?

[vlo...@gmail.com]

Is there a convenient way to peek at the size a message will take up on the wire? My thinking is to create a custom output stream that I `capnp::writeMessage` to that just sums everything but I feel like this could be done more efficiently within capn'n'proto, just couldn't find anything like that at first glance. Is there an obvious API missing? Would there be any interest in me putting up a diff with such an API?

[Kenton Varda]

I think this is what you want:

https://github.com/capnproto/capnproto/blob/3f0fee61c65475c8debfdf8c01f96c2f7e7eeb14/c++/src/capnp/serialize.h#L102

:)

-Kenton

On Wed, May 6, 2020 at 2:54 PM <vlo...@gmail.com> wrote:

Is there a convenient way to peek at the size a message will take up on the wire? My thinking is to create a custom output stream that I `capnp::writeMessage` to that just sums everything but I feel like this could be done more efficiently within capn'n'proto, just couldn't find anything like that at first glance. Is there an obvious API missing? Would there be any interest in me putting up a diff with such an API?

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[Vitali Lovich]

The optometrist tells me I have 20/20 vision but I think it must clearly be a lie at this point :). Any objection to a contribution to return the size in bytes?

[Kenton Varda]

Eh, I like having the APIs be in words in hopes that it reminds people to think about alignment. When allocating space for a capnp message, you should always allocate an array of words, not an array of bytes.

Seems not too hard for the caller to multiply by sizeof(capnp::word) if needed?

-Kenton

[Vitali Lovich]

That’s fair. I’m having some corruption issues so I’m trying to layer a framing message around cap’n’proto that has the length of the payload (+ checksums) which is where the length in bytes comes in useful.