Re: [systemd-devel] sd-bus: get size of array container in D-Bus message
On Mi, 02.08.23 06:39, Stanislav Angelovič (stanislav.angelo...@protonmail.com) wrote: > Hi folks, > > I have a quick question: is there a way to get container size when > deserializing an array from a D-Bus message (be it an array of > trivial or non-trivial D-Bus types)? Say I enter a container with > sd_bus_message_enter_container, and then, before reading individual > elements, I'd like to get the number of elements in that array in > that message (so I can reserve storage on my side, etc.). The D-Bus marshalling format does not allow us to determine this efficiently: for arrays it only encodes the size in bytes in the serialization, not the number of entries. Given that array element sizes can have varying sizes (consider strings or variants) we thus cannot determine the array's number of elements efficiently ahead of time, except by iterating through the potentially huge array. You can implement a helper for that, but we don't provide an API for that, since it might make people write inefficient code, by assuming this was a fast call. Generally, consumers are better off using an exponential realloc() loop for growing their data structures as they parse messages. Yeah, this is not fun, but hey, this is C, so nothing is "fun". Lennart -- Lennart Poettering, Berlin
Re: [systemd-devel] sd-bus: get size of array container in D-Bus message
On Wed, 2 Aug 2023 at 20:00, Giacinto Cifelli wrote: > > On Wed, Aug 2, 2023 at 6:44 PM Stephen Hemminger > wrote: > > > > On Wed, 02 Aug 2023 06:39:47 + > > Stanislav Angelovič wrote: > > > > > Hi folks, > > > > > > I have a quick question: is there a way to get container size when > > > deserializing an array from a D-Bus message (be it an array of trivial or > > > non-trivial D-Bus types)? Say I enter a container with > > > sd_bus_message_enter_container, and then, before reading individual > > > elements, I'd like to get the number of elements in that array in that > > > message (so I can reserve storage on my side, etc.). > > > > > > If not, could such an API function be added? > > > > > > Thanks in advance. > > > > > > Stanislav. > > > > The dbus messsage just an encoding of a byte stream. There is nothing in > > the byte stream > > that indicates the length. > > yes, but the whole message is available locally, it could be parsed on > request, discarding the data, and keeping only the number of elements > in an array. For basic types there's https://www.freedesktop.org/software/systemd/man/sd_bus_message_read_array.html
Re: [systemd-devel] sd-bus: get size of array container in D-Bus message
On Wed, Aug 2, 2023 at 6:44 PM Stephen Hemminger wrote: > > On Wed, 02 Aug 2023 06:39:47 + > Stanislav Angelovič wrote: > > > Hi folks, > > > > I have a quick question: is there a way to get container size when > > deserializing an array from a D-Bus message (be it an array of trivial or > > non-trivial D-Bus types)? Say I enter a container with > > sd_bus_message_enter_container, and then, before reading individual > > elements, I'd like to get the number of elements in that array in that > > message (so I can reserve storage on my side, etc.). > > > > If not, could such an API function be added? > > > > Thanks in advance. > > > > Stanislav. > > The dbus messsage just an encoding of a byte stream. There is nothing in the > byte stream > that indicates the length. yes, but the whole message is available locally, it could be parsed on request, discarding the data, and keeping only the number of elements in an array.
Re: [systemd-devel] sd-bus: get size of array container in D-Bus message
On Wed, 02 Aug 2023 06:39:47 + Stanislav Angelovič wrote: > Hi folks, > > I have a quick question: is there a way to get container size when > deserializing an array from a D-Bus message (be it an array of trivial or > non-trivial D-Bus types)? Say I enter a container with > sd_bus_message_enter_container, and then, before reading individual > elements, I'd like to get the number of elements in that array in that > message (so I can reserve storage on my side, etc.). > > If not, could such an API function be added? > > Thanks in advance. > > Stanislav. The dbus messsage just an encoding of a byte stream. There is nothing in the byte stream that indicates the length.
[systemd-devel] sd-bus: get size of array container in D-Bus message
Hi folks, I have a quick question: is there a way to get container size when deserializing an array from a D-Bus message (be it an array of trivial or non-trivial D-Bus types)? Say I enter a container with sd_bus_message_enter_container, and then, before reading individual elements, I'd like to get the number of elements in that array in that message (so I can reserve storage on my side, etc.). If not, could such an API function be added? Thanks in advance. Stanislav.
