Python mmap ctypes - read only

Question

I think I have the opposite problem as described here. I have one process writing data to a log, and I want a second process to read it, but I don't want the 2nd process to be able to modify the contents. This is potentially a large file, and I need random access, so I'm using python's mmap module.

If I create the mmap as read/write (for the 2nd process), I have no problem creating ctypes object as a "view" of the mmap object using from_buffer. From a cursory look at the c-code, it looks like this is a cast, not a copy, which is what I want. However, this breaks if I make the mmap ACCESS_READ, throwing an exception that from_buffer requires write privileges.

I think I want to use ctypes from_address() method instead, which doesn't appear to need write access. I'm probably missing something simple, but I'm not sure how to get the address of the location within an mmap. I know I can use ACCESS_COPY (so write operations show up in memory, but aren't persisted to disk), but I'd rather keep things read only.

Any suggestions?

Answer 1

Ok, from looking at the mmap .c code, I don't believe it supports this use case. Also, I found that the performance pretty much sucks - for my use case. I'd be curious what kind performance others see, but I found that it took about 40 sec to walk through a binary file of 500 MB in Python. This is creating a mmap, then turning the location into a ctype object with from_buffer(), and using the ctypes object to decipher the size of the object so I could step to the next object. I tried doing the same thing directly in c++ from msvc. Obviously here I could cast directly into an object of the correct type, and it was fast - less than a second (this is with a core 2 quad and ssd).

I did find that I could get a pointer with the following

firstHeader = CEL_HEADER.from_buffer(map, 0) #CEL_HEADER is a ctypes Structure
pHeader = pointer(firstHeader)
#Now I can use pHeader[ind] to get a CEL_HEADER object 
#at an arbitrary point in the file

This doesn't get around the original problem - the mmap isn't read-only, since I still need to use from_buffer for the first call. In this config, it still took around 40 sec to process the whole file, so it looks like the conversion from a pointer into ctypes structs is killing the performance. That's just a guess, but I don't see a lot of value in tracking it down further.

I'm not sure my plan will help anyone else, but I'm going to try to create a c module specific to my needs based on the mmap code. I think I can use the fast c-code handling to index the binary file, then expose only small parts of the file at a time through calls into ctypes/python objects. Wish me luck.

Also, as a side note, Python 2.7.2 was released today (6/12/11), and one of the changes is an update to the mmap code so that you can use a python long to set the file offset. This lets you use mmap for files over 4GB on 32-bit systems. See Issue #4681 here

Answer 2

Ran into this same problem, we needed the from_buffer interface and wanted read only access. From the python docs https://docs.python.org/3/library/mmap.html "Assignment to an ACCESS_COPY memory map affects memory but does not update the underlying file." If it's acceptable for you to use an anonymous file backing you can use ACCESS_COPY

An example: open two cmd.exe or terminals and in one terminal:

mm_file_write = mmap.mmap(-1, 4096, access=mmap.ACCESS_WRITE, tagname="shmem")
mm_file_read = mmap.mmap(-1, 4096, access=mmap.ACCESS_COPY, tagname="shmem")

write = ctypes.c_int.from_buffer(mm_file_write)
read = ctypes.c_int.from_buffer(mm_file_read)
try:
    while True:
        value = int(input('enter an integer using mm_file_write: '))
        write.value = value
        print('updated value')
        value = int(input('enter an integer using mm_file_read: '))
        #read.value assignment doesnt update anonymous backed file
        read.value = value
        print('updated value')
except KeyboardInterrupt:
    print('got exit event')

In the other terminal do:

mm_file = mmap.mmap(-1, 4096, access=mmap.ACCESS_WRITE, tagname="shmem")
i = None
try:
    while True:
        new_i = struct.unpack('i', mm_file[:4])
        if i != new_i:
            print('i: {} => {}'.format(i, new_i))
            i = new_i
        time.sleep(0.1)
except KeyboardInterrupt:
    print('Stopped . . .')

And you will see that the second process does not receive updates when the first process writes using ACCESS_COPY