[New-bugs-announce] [issue21797] mmap read of single byte accesses more that just that byte

[Kevin Smith]

New submission from Kevin Smith:

I am using the mmap module on Linux with python 2.7.6 to access memory-mapped IO.  The device that I am accessing implements a hardware FIFO that holds 16-bit values and pops an entry when the MSB of the entry is read.  I am trying to use the mmap module to do an 8-bit read of the LSB then an 8-bit read of the MSB, which should pop the value.  However, I am finding that the LSB read is sometimes popping the value before I read the MSB.

I am mapping the memory like this:
        self.fd = os.open ("/dev/mem", os.O_RDWR | os.O_SYNC)
        self.mempage = mmap.mmap (self.fd, mmap_size, mmap.MAP_SHARED,
                mmap.PROT_READ | mmap.PROT_WRITE, offset = pc104_base)

Then trying to read a value like this:
        val = self.mempage[pos]

The LSB of the hardware device is a lower address than the MSB.  The read of the LSB seems to sometimes overlap into a read of the MSB, which causes the following read of the MSB to be incorrect, as the value has already been popped.  Sometimes the reads of the MSB are correct; I am not sure why this behavior is not consistent across all reads.

The reads for this device need to be 8-bit, as the bus the device is connected to only supports 8-bit reads.

I think that a single-byte access of the memory mapping should only access a single byte in the underlying memory region.

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components: Extension Modules
messages: 220877
nosy: FazJaxton
priority: normal
severity: normal
status: open
title: mmap read of single byte accesses more that just that byte
type: behavior
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21797>
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