Vfrdtyky

I have the issue that I cannot properly close mmap-s in Python after I created a pointer to them. My use case is that I open files (usually that is UIO-devices to work with hardware, but the issue also happens with normal files), memory-map them and then use them as buffers for ctypes-structures. Usually structs or arrays of data. A minimal example looks like this:

import ctypes as ct

import mmap

import os



fileno = os.open('/tmp/testfile', os.O_RDWR | os.O_SYNC)

map = mmap.mmap(fileno, 32768, flags=mmap.MAP_SHARED)

memory = (ct.c_uint32 * 8192).from_buffer(map)



# Use the memory object to do things here



del memory

map.close()

os.close(fileno)

Everything is fine at that point.

However, sometimes I need to call some C library functions that also need access to that memory so I have to pass in a pointer to them. I create that pointer using:

ptr = ct.cast(memory, ct.c_void_p)

All of that works very well except one thing. As soon as I create such a pointer, I can no longer close the memory map. Take this slightly extended example:

import ctypes as ct

import mmap

import os



fileno = os.open('/tmp/testfile', os.O_RDWR | os.O_SYNC)

map = mmap.mmap(fileno, 32768, flags=mmap.MAP_SHARED)

memory = (ct.c_uint32 * 8192).from_buffer(map)



# Use the memory object to do things here

ptr = ct.cast(memory, ct.c_void_p)

del ptr



del memory

map.close()

os.close(fileno)

Running this results in the following exception:

Traceback (most recent call last):

  File "pointer_test.py", line 14, in <module>

    map.close()

BufferError: cannot close exported pointers exist



Process finished with exit code 1

I've run some analysis (using gc.get_referrers) on what is referencing the map-instance and it turns out that there is a memoryview-instance still there. In the end, this traces back to the ctypes Array:

[<__main__.c_uint_Array_8192 object at 0x7f954bd1e0>,

 [{547965620704: <__main__.c_uint_Array_8192 object at 0x7f954bd1e0>,

   'ffffffff': <memory at 0x7f95621a08>},

  [<memory at 0x7f95621a08>,

   [<managedbuffer object at 0x7f95746d08>,

However, this does not really help me. I'd like to know how I can get rid of that pointer. I know that this is likely not fully safe since I could, of course, always have a copy of that pointer somewhere. But completely preventing closing the mmap after a pointer was created also does not seem like a good idea. Anyone knows how I can convince Python that I discarded all the pointers and it is now safe to close the mmap?