reading 24 bit binary data in python not working

Question

I have single value(val=2) written as 24 bit data using matlab as:

fid = fopen('.\t1.bin'), 'wb');
fwrite(fid, val, 'bit24', 0);

In a bin viewer, I can see that data (value 2) is stored as 02 00 00. I need to read value as single integer in python. my code below does not work:

    struct_fmt = '=xxx'       
    struct_unpack = struct.Struct(struct_fmt).unpack_from
    with open('.\\t1.bin', mode='rb') as file:
        fileContent = file.read()                
        res = struct_unpack(fileContent)

I also tried

val = struct.unpack('>I',fileContent)

but it give error:

unpack requires a string argument of length 4

What am i doing wrong?
Thanks
sedy

@BenjaminHodgson Well, there is no error but "res" reads 0, while it should be 2 — user915783
– user915783, Commented Oct 14, 2015 at 18:57
You can always read byte by byte and "assemble" the result as a 32 bit value. — heltonbiker
– heltonbiker, Commented Oct 14, 2015 at 19:21

bgporter · Accepted Answer · 2015-10-14 19:22:41Z

6

In the Python struct module format characters, x is defined as a pad byte. The format string that you have there says to read in 3 bytes and then discard them.

There's not already a format specifier to handle 24-bit data, so build one yourself:

>>> def unpack_24bit(bytes):
...    return bytes[0] | (bytes[1] << 8) | (bytes[2] << 16)
...
>>> bytes
'\x02\x00\x00'
>>> unpack_24bit(struct.unpack('BBB', bytes))
2

edited Oct 14, 2015 at 19:22

answered Oct 14, 2015 at 19:13

bgporter

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Comments

heltonbiker · Accepted Answer · 2015-10-14 19:34:21Z

You can always convert integers to bytes and vice-versa, by accessing individual bytes. Just take care of endianness.

The code below uses a random integer to convert to 24 bit binary and back.

import struct, random

# some number in the range of [0, UInt24.MaxValue]
originalvalue = int (random.random() * 255 ** 3)   

# take each one of its 3 component bytes with bitwise operations
a = (originalvalue & 0xff0000) >> 16
b = (originalvalue & 0x00ff00) >> 8
c = (originalvalue & 0x0000ff)

# byte array to be passed to "struct.pack"
originalbytes = (a,b,c)
print originalbytes

# convert to binary string
binary = struct.pack('3B', *originalbytes)

# convert back from binary string to byte array
rebornbytes = struct.unpack('3B', binary)   ## this is what you want to do!
print rebornbytes

# regenerate the integer
rebornvalue = a << 16 | b << 8 | c
print rebornvalue

westman379 · Accepted Answer · 2024-05-31 18:22:18Z

0

You can use rawutil library to get uint24 value. Here is my function for that:

import rawutil

def get_uint24(input_bytes: bytes, endianess: str) -> int:
  result = rawutil.unpack(endianess + "U", input_bytes)[0]
  return result

answered May 31, 2024 at 18:22

westman379

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