Hi Sachin, MallocMessageBuilder is really intended to allocate space with malloc. If you are using mmap, then you are not allocating using malloc, and MallocMessageBuilder probably isn't the right way to go.
Instead, you should implement a custom subclass of MessageBuilder. Then, when more space is needed, Cap'n Proto will call your custom allocateSegment() implementation which can mmap more space (or throw an exception, etc.). Note also that by implementing a custom MessageBuilder subclass, you can access the constructors of MessageBuilder that allow you to initialize it with an existing message to be modified in-place. MallocMessageBuilder only works for creating new messages from scratch. (However, note that there are security concerns -- if you don't trust the message content, you should always start a new message and copy the old message into it. See the comments in the header file for more info.) -Kenton On Wed, Jan 31, 2018 at 7:04 AM, Sachin Kumar <sac...@gmail.com> wrote: > Some updates from my own reading of other message on this group: > > I figured out how to construct a kj::ArrayPtr<word> from a raw char* > pointer claimed from shared memory. Moreover, the raw buffer I get from my > mmap'ed file is 64-bit aligned. So this means there should be no issues > backing a MallocMessageBuilder with this memory, correct? > > What still eludes me if this is an intended use case for MallocMessageBuilder, > particularly since I need to ensure that the MallocMessageBuilder does not > write more than the fixed size buffer I claimed, and that the > MallocMessageBuilder does not take ownership of the backed memory -- i.e. > does not try to deallocate it. > > Any suggestions on this use case? > > > On Wednesday, January 31, 2018 at 12:44:28 AM UTC-5, Sachin Kumar wrote: >> >> Hi, >> >> I'm trying to use MallocMessageBuilder that's backed by a fixed size >> chunk of mmap'ed memory. So, in pseudo-code: >> >> char* buf = claim_mmap_memory(1024); >> ::capnp::MallocMessageBuilder message(buf); >> ... build message >> >> The questions I have: >> >> 1) Am I correct in trying to do this with MallocMessageBuilder given that >> the memory claimed is fixed size (1024 bytes in the above example). I can >> generally assume that the message should *not* exceed 1024 -- perhaps I can >> programmatically check the size used as the message is built and throw an >> exception if it exceeds 1024? >> >> 2) Is it efficient to instance a malloc message builder like this in a >> tight loop repeatedly to send many messages? >> >> 3) How do I convert the raw char* buffer into a kj::ArrayPtr<word> so >> that I can pass into the constructor of MallocMessageBuilder? >> >> >> Thanks, >> >> Sachin >> >> -- > You received this message because you are subscribed to the Google Groups > "Cap'n Proto" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to capnproto+unsubscr...@googlegroups.com. > Visit this group at https://groups.google.com/group/capnproto. > -- You received this message because you are subscribed to the Google Groups "Cap'n Proto" group. To unsubscribe from this group and stop receiving emails from it, send an email to capnproto+unsubscr...@googlegroups.com. Visit this group at https://groups.google.com/group/capnproto.
