Python3 Struct unpack format string

Question

I am using the python3 struct module to unpack byte data I extracted from a serial com. (With help) I've figured out how to unpack most the data into human readable form. I am having difficult with the format string on a group header struct group_hdr (please see attached screenshot document). I have a byte data (b). I know the character string for "word" is "H" but it's unclear to me from the document what phd_status is. It hasn't been defined anywhere else in the Data structure document. Any ideas?. Thank you in advance.

struct group_hdr
{
union phdb_status  status
word               label
}

subrecord = struct.unpack_from('<??H', b)

I don't think this is related to Python. If you can't find the definition of this union, then you won't be able to proceed until you do find it. Or read further and notice that the status field is 32 bit wide, so you can replace it with any 32-bit datatype for now. — ForceBru
– ForceBru, Commented Apr 22, 2020 at 8:12

Jongware · Accepted Answer · 2020-04-22 08:28:37Z

1

As is explained under Status, it is a simple bitfield with a width of 32 bits. The union is probably defined elsewhere in C (or a similar language) as

union phdb_status {
    unsigned int bit_0:1;
    unsigned int bit_1:1;
};

The following Python code will store your values:

status, label = struct.unpack_from('<IH', b)

and you can test the individual bits of status with status & 1 and status & 2.

answered Apr 22, 2020 at 8:28

Jongware

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