Data unchanged when passing data to Python in multiprocessing shared memory

[Dennis Lee Bieber]

On Wed, 2 Feb 2022 19:16:19 +0100 (CET), Jen Kris <jenkris at tutanota.com>
declaimed the following:

>It's not clear to me from the struct module whether it can actually auto-detect endianness.  I think it must be specified, just as I had to do with int.from_bytes().  In my case endianness was dictated by how the four bytes were populated, starting with the zero bytes on the left. 

	Which is why I also suggested maybe looking at the various network/host
translation calls. They are in the socket module of Python, and should also
be available in most C standard libraries...

https://docs.python.org/3/library/socket.html#other-functions

"""

socket.ntohl(x)

    Convert 32-bit positive integers from network to host byte order. On
machines where the host byte order is the same as network byte order, this
is a no-op; otherwise, it performs a 4-byte swap operation.

socket.ntohs(x)

    Convert 16-bit positive integers from network to host byte order. On
machines where the host byte order is the same as network byte order, this
is a no-op; otherwise, it performs a 2-byte swap operation.

    Changed in version 3.10: Raises OverflowError if x does not fit in a
16-bit unsigned integer.

socket.htonl(x)

    Convert 32-bit positive integers from host to network byte order. On
machines where the host byte order is the same as network byte order, this
is a no-op; otherwise, it performs a 4-byte swap operation.

socket.htons(x)

    Convert 16-bit positive integers from host to network byte order. On
machines where the host byte order is the same as network byte order, this
is a no-op; otherwise, it performs a 2-byte swap operation.

    Changed in version 3.10: Raises OverflowError if x does not fit in a
16-bit unsigned integer.
"""

https://docs.python.org/3/library/struct.html
"""
Byte Order, Size, and Alignment

By default, C types are represented in the machine’s native format and byte
order, and properly aligned by skipping pad bytes if necessary (according
to the rules used by the C compiler).

Alternatively, the first character of the format string can be used to
indicate the byte order, size and alignment of the packed data, according
to the following table:

Character	Byte order	Size	Alignment
	@		native		native	native
	=		native		standard	none
	<		little-endian	standard	none
	>		big-endian	standard	none
	!		network (= big-endian)			<<<<<<
						standard	none	<<<<<<

If the first character is not one of these, '@' is assumed.
"""

	Since all the programs in your situation are running on the same
machine, it would appear that at least one of them is NOT formatting
integers in native host mode -- and I don't think it is Python.


https://www.tutorialspoint.com/unix_sockets/network_byte_orders.htm
"""
These functions are macros and result in the insertion of conversion source
code into the calling program. On little-endian machines, the code will
change the values around to network byte order. On big-endian machines, no
code is inserted since none is needed; the functions are defined as null.
"""



-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
	wlfraed at ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/