Python equivalent of C struct (porting app form C to python)

Question

I'm porting a simple bluetooth app, which sends "magic" packet on L2Cap protocol to bluetooth device..

I have a problem with converting struct object in C to python equivalent..

In c:

/* command types */
#define CFGBT_TYPE_SETREQ           0x00
#define CFGBT_TYPE_SETRES           0x01
#define CFGBT_TYPE_GETREQ           0x02
#define CFGBT_TYPE_GETRES           0x03

/* varid types */
#define CFG_VARIDT_UINT16                   0x0000
#define CFG_VARIDT_UINT32                   0x1000
#define CFG_VARIDT_STRING16                 0x2000

typedef struct {
    uint8_t      type, status;
    uint16_t     varid;
    uint8_t      value[16];
} __attribute__((packed)) CFGBTFRAME;

static CFGBTFRAME c;

and then in app it's used like that:

        /* set up */
    c.type = CFGBT_TYPE_GETREQ;
    c.varid = strtol(argv[3], NULL, 0);
    c.status = 0;
    memset(c.value, 0, 16);
    /* send frame */
    write(s, &c, sizeof(c));

Can you point me out how to construct same packet/stuct-like structure using python?

I know I will probably need to use ctypes and create "empty" class, but how to get all this together?

do you just want to have a class which holds the same values as your struct, or do you want to have the same memory structure ? It might be helpful if you tell which goal you have. — rocksportrocker
– rocksportrocker, Commented Sep 19, 2011 at 8:34
We're not going to write your code for you. What, specifically, is your problem? You create a subclass of ctypes.Structure, you specify the fields, then you file_like_obj.write(yourobject) once you've populated the fields. — agf
– agf, Commented Sep 19, 2011 at 8:39
@agf I need to send identicaly same packet "data" as the C app, because receiving device only waits for that kind of packet.. if not there is no answer.. I'm, not waiting for code.. but seeing struct exaples converted to Python would be nice, on linkt where to find the answers.. — Michal
– Michal, Commented Sep 19, 2011 at 13:27

Michael F · Accepted Answer · 2011-09-19 09:35:20Z

3

You can go about it with the struct module to pack values into a byte string, for example:

>>> import struct
>>> type, status, varid, value = 1, 0, 16, b'Hello'
>>> buffer = struct.pack('>BBH16s', type, status, varid, value)
>>> buffer
b'\x01\x00\x00\x10Hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

Alternatively, you can use ctypes.Structure to define a class that will be representing your structure. It has the advantage of being easier to use with Python code, but you have to take into account alignment and padding issues and resolve them yourself (perhaps with struct).

answered Sep 19, 2011 at 9:35

Michael F

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Simon Bergot · Accepted Answer · 2011-09-19 09:20:39Z

0

If your goal is to group a set of key/value in an object, you may use dict or namedtuple.

a dict would be:

CFGBTFRAME = {
    'type' : myType,
    'status' : ...
}

access: CFGBTFRAME['type']

with namedtuple:

from collections import namedtuple
CFGBTFRAME = namedtuple('CFGBTFRAME', ['type', 'status', ...])
c = CFGBTFRAME()
c.type = myType

see http://docs.python.org/library/collections.html#collections.namedtuple for more information about namedtuple.

answered Sep 19, 2011 at 9:20

Simon Bergot

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1 Comment

Michal Over a year ago

my goal is not exactly to group data into key/value object.. I just need to send exactly the same packet data to receiving device as te C app does.... If not there will be no answer from it..

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