I am in the process of changing over my C wrapper to work with Arrow 0.9.0. One change is that the plasma APIs now use arrow::Buffer instead of of raw pointers, in Create and Get.
In my C wrapper for Create, I need to get the mutable data pointer from the arrow::Buffer, after it is created. Here is my current implementation: https://github.com/plures/pxnd/blob/84e9403ef3811e0893e12076455d7cbc49c39358/libplasma/plasma.cc#L56-L75 I do something similar to the Python wrapper around plasma: https://github.com/apache/arrow/blob/f9f8320339692d4134d1ef42c32cb7c8d547593e/python/pyarrow/plasma.pyx#L300-L307 However, this fails to link, I think because the mutable_data method is defined in the class and so is inline. So it doesn't show up in the libarrow.dylib/libarrow.a archive. Let me know if I am missing something obvious, this is just my hypothesis for why it isn't working. Is there a way for a third party library to get the pointer to the data from a arrow::Buffer? On Wed, Mar 7, 2018 at 10:59 AM Wes McKinney <wesmck...@gmail.com> wrote: > Where are you going to make Plasma requests, in Python or in C? > > * If in Python, use the buffer protocol to access the memory in C > https://docs.python.org/3/c-api/buffer.html#bufferobjects > * If in C, then I would suggest adding extern "C" APIs directly in > libplasma.so so you can connect to the Plasma store and make requests > directly in C. > > I want to make 100% sure that you need to interact with the Plasma > client in C before I send you down that path. Here's an example of a C > API embedded in an otherwise C++ library > https://github.com/wesm/feather/blob/master/cpp/src/feather/feather-c.h. > > - Wes > > On Wed, Mar 7, 2018 at 10:53 AM, Saul Shanabrook <s.shanabr...@gmail.com> > wrote: > > Hey Wes, > > > > Since all our logic for writing and reading the data is written in C, we > > need to be get access to the memory in C as well. I think it makes sense > > then for me to work on creating a C++ wrapper around Plasma that exports > > what is necessary in C. Should that be in Arrow GLib? > > > > Best, > > Saul > > > > On Tue, Mar 6, 2018 at 11:49 PM Wes McKinney <wesmck...@gmail.com> > wrote: > > > >> hi Saul -- I think the easiest solution here is the buffer/memoryview > >> protocol. You won't have to touch the Cython or C++ API from pyarrow > >> if you do this. > >> > >> You can interact with a Buffer object like any other Python object > >> implementing the buffer protocol. See numpy.frombuffer as an example > >> of a function that interacts with such objects. I would suggest adding > >> a method to xnd method for this. > >> > >> If you need to interact with Plasma from C then things will be more > >> complicated -- Robert or Philipp should be able to advise in this > >> case. > >> > >> - Wes > >> > >> On Tue, Mar 6, 2018 at 4:55 PM, Saul Shanabrook <s.shanabr...@gmail.com > > > >> wrote: > >> > Hey Wes, > >> > > >> > I don't have much experience doing C + Python + Cython development, > so I > >> am > >> > probably missing something obvious, but reading the Cython docs, > >> > < > >> > https://cython.readthedocs.io/en/latest/src/userguide/external_C_code.html#public-declarations > >> > > >> > it > >> > seems like I can only access types marked as public from C code. When > I > >> > compile arrow locally, I do get some C++ headers for the plasma code, > >> but I > >> > don't think I can use them from C code either. > >> > > >> > Best, > >> > Saul > >> > > >> > > >> > > >> > On Tue, Mar 6, 2018 at 3:12 PM Wes McKinney <wesmck...@gmail.com> > wrote: > >> > > >> >> hi Saul, > >> >> > >> >> Are you able to use the buffer/memoryview protocol? Instances of > >> >> pyarrow.Buffer, like PlasmaBuffer, support this > >> >> > >> >> > >> > https://github.com/apache/arrow/blob/master/python/pyarrow/plasma.pyx#L182 > >> >> > >> >> - Wes > >> >> > >> >> On Tue, Mar 6, 2018 at 3:09 PM, Saul Shanabrook < > s.shanabr...@gmail.com > >> > > >> >> wrote: > >> >> > I am trying to use the Plasma store to back xnd objects. Xnd ( > >> >> > https://xnd.readthedocs.io/en/latest/xnd/index.html) is a > container > >> >> library > >> >> > in C that has Python bindings. I would like to get a pointer to the > >> >> > allocated memory after creating or get an object in Plasma. I see > that > >> >> this > >> >> > is supported in the C++ API ( > >> >> > > >> >> > >> > https://arrow.apache.org/docs/cpp/classplasma_1_1_plasma_client.html#ac18ab9cc792c620a97a3dcb165e0ecd7 > >> >> ) > >> >> > but not in the python API (as far as I can tell). Is it possible to > >> use > >> >> the > >> >> > C++ Plasma API from a C project? If not, would it make sense to > expose > >> >> > pointer access on the Python API using capsules > >> >> > https://docs.python.org/3.6/c-api/capsule.html > >> >> > <https://docs.python.org/3.6/c-api/capsule.html#capsules>? > >> >> > >> >
