String Format

Format characters struct.unpack_from()

Format character	Description
d	signed char (8 bits)
i	unsigned char (8 bits)
h	signed short (16 bits)
H	unsigned short (16 bits)
I	signed int (32 bits)
L	unsigned int (32 bits)
q	signed long long (64 bits)
Q	unsigned long long (64 bits)
f	single precision (32-bit) floating-point number
d	double precision (64-bit) floating-point number
s	string of characters terminated by a null byte (ASCII)
p	pointer to a structure
b	singed byte (8 bits)
B	unsigned byte (8bits)
!	Big Endian
>	Little Endian

Alternative String Format Creation

For parsing packet data I used a simplified approach using a dictionary storing the most important Symbols.

string_format: Dict[int, str] = {
   1: '{}s',
   8: 'B',
   16: 'H',
   32: 'I',
   64: 'Q'
}

def bld_str_fmt(bit_list: List[int]) -> str:
   return '>' + ''.join([string_format[1].format(bits // 8) if bits not in string_format.keys() else string_format[bits] for bits in bit_list])

bld_str_fmt([8, 8, 16, 16, 16, 8, 8, 16, 32, 32])

The bld_str_fmt() function effectively clarifies the extraction of specific bit groups into contiguous segments using struct.unpack_from(). The build function consistently employs little-endian byte order, which is appropriate since all conversions are performed in this format. It identifies integers from the provided list and replaces them with corresponding values from the pre-defined dictionary. If the key is not found, the integer is divided by 8 (truncated integer division) and suffixed with “s”. For instance, while parsing an IPv6 address, which occupies 128 bits, dividing 128 by 8 yields 16, resulting in the format “16s”.