On Mon, Nov 28, 2016 at 6:45 PM, Lennart Poettering <lenn...@poettering.net> wrote:
> On Sat, 26.11.16 16:01, Stanislav Angelovič (angelovi...@gmail.com) wrote: > > > Hi, > > > > A quick question: Is it possible to append contents (the serialized data) > > of an sd-bus message to another sd-bus message? Conversely, is it > possible > > to extract a part of sd-bus message contents into a separate > > message? > > There's sd_bus_message_copy() which can copy the contents (or a part > of it) of a message into another message. > > There's currently no API that exposes the raw binary bits, and they > aren't particularly useful anyway on dbus1 as they are not safe > regarding alignment and you cannot copy them byte-by-byte to arbitrary > memory locations. > > Details: > > > > In the process of creating a method call message, I would like to do > this: > > 1. Create a temporary, empty message A. > > 2. Serialize data into the message A. > > 3. Create a method call message B. > > 4. Serialize some user data into the message B, plus copy the data of the > > message A into the message B. And send the message B. > > sd_bus_message_copy() should work fine for that. > > Analogously, when deserializing data from the message B upon its > reception > > on the server side, I would like to do this: > > 1. Extract user data from the message B, plus copy selected data into a > > separate, empty message A. > > 2. Destroy the message B. > > 3. In some other context, deserialize real data from the message A. > > > > Why am I asking is because we like sd-bus and are building a C++ binding > on > > top of it. We stumbled upon a problem when modelling Variant. The typical > > flow in such a case is that upon e.g. deserialization, first the Variant > > instances are created from the message contents, and then at some later > > phase, users want to extract real data from the Variant. In this phase, > our > > C++ binding would perform the deserialization from that temporry message > > (message A) that the Variant would hold. > > > > For example, if the method call message carries an array of Variants, we > > first deserialize the message into std::vector<Variant>, where each > Variant > > instance keeps the temporary sd-bus message with the variant data, and > when > > the user asks for a concrete data from that Variant, the deserialization > > from that temporary message takes place. > > > > This is the way it's done in e.g. dbus-c++, the C++ binding for libdbus. > > libdbus API allows creating "empty" messages, and allows copying the data > > among messages with the possibility of reading from them at the same > time. > > Is it possible to create an empty message using sd-bus? Regarding > copying, > > we know there's the sd_bus_message_copy function, but once we create the > > copy message, we cannot read from it later because of the assertion that > > the message is sealed, which in reality is not. > > Hmm, so internally there's bus_message_seal() which I figure we could > make publically available, so that you can create a new msg, copy the > data to it, seal it, and read it back. > Yes, exactly, I need to create a message copy, but also be able to read from it afterwards. If invoking bus_message_seal() is sufficient to make the message readable after creating it, it would be so welcome to have it in the public API. Additionally, we'd need a simple factory function for a message that won't have a context of a method call, a signal or an error message. Just plain message that will be used for storing some data and later, via sd_bus_message_copy(), copying the data into a 'real' message. There is message_new function in sd-bus, but it's internal. Would it be possible to make it public, just like lidbbus provides dbus_message_new()? > > > Is there a way to solve this in sd-bus? Perhaps another way than I > > described? Many thanks in advance for your response! > > There's no nicer way right now. But I think we could add a > sd_bus_message_dup() or so that duplicates a message, but does so > efficiently, i.e. shares the memory for the actual payload. This way > you could duplicate a message to simply get a new read ptr on the > same, immutable message. I think that would suit your usecase best? > sd_bus_message_dup() as you describe it could perfectly be an option, but -- similarly to sd_bus_message_copy() -- it would have to allow not only duplication of the entire message, but also a part of it. This could perhaps be handled -- consistently to sd_bus_message_copy() -- with an additional 'complete' boolean parameter. The behavior would be the same to sd_bus_message_copy, but the result would not be a real copy, but just a view/reference to (if complete==false, then a part of) the original message. So in my use case example if the message has signature "av", where the size of the array is 3, then I would like to create 3 messages out of it, each pointing to the corresponding variant data in the original message. And -- importantly -- be able to read from those 3 messages afterwards (the solution to that could be public sd_bus_message_seal(), as you proposed). The first phase of reading from the original message would be done by the C++ binding (creation of 3 messages), the second phase by the user of the binding (taking data of a concrete type from these messages). To sum up, we'd need just two simple things: - be able to create a plain message, a-ka sd_bus_message *sd_bus_message_new(sd_bus *bus, uint8_t type); - be able to read back from a message after we've created it (regardless of whether created by copying from another message, or anew) and appended some data into it, which now fails on the "sealed==true" expectation. Support for efficient, shallow copy (sd_bus_message_dup()) would also be very nice to have. I like this idea. Thanks a lot for your support! > > Lennart > > -- > Lennart Poettering, Red Hat >
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