Unpacking nested C structs in Python

Question

I am trying to unpack a C struct that is handed to my Python program in binary form and includes another nested struct. The relevant part of the C header looks like this:

typedef struct {
    uint8_t seq;
    uint8_t type;
    uint16_t flags;
    uint16_t upTimestamp;
}__attribute__ ((packed)) mps_packet_header;

typedef struct {
    mps_packet_header header;
    int16_t x[6];
    int16_t y[6];
    int16_t z[6];
    uint16_t lowTimestamp[6];
}__attribute__((packed)) mps_acc_packet_t;
typedef mps_acc_packet_t accpacket_t;

Now, in my Python program, I want to use struct.unpack to unpack an accpacket. However, I don't know what the format string for the unpack should be as the accpacket contains a nested mps_packet_header. I have tried just inserting the format string for the mps_packet_header at the beginning and then continuing with the rest of the accpacket:

s = struct.Struct('= B B H H 6h 6h 6h H')
seq, _type, flags, upTimestamp, x, y, z, lowTimestamp = s.unpack(packet_data)

However, this is apparently not correct; the format string has a calcsize of 44, while the struct itself has a size of 54.

How do I formulate a correct format string for this struct?

falsetru · Accepted Answer · 2013-08-06 14:16:20Z

4

You Struct format does not match with C structure. (final H should be 6H)
struct.unpack(6h, ..) does return 6 fields. (not one with 6 elements)

So your code should looks like ..

s = struct.Struct('= B B H H 6h 6h 6h 6H')
fields = s.unpack(packet_data)
seq, _type, flags, upTimestamp = fields[:4]
x = fields[4:10]
y = fields[10:16]
z = fields[16:22]
lowTimestamp = fields[22:]

answered Aug 6, 2013 at 14:16

falsetru

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1 Comment

Martijn Pieters Over a year ago

Good catch; I missed the final quantifier. With that in place the .size gives 54.

igiroux · Accepted Answer · 2016-08-16 14:50:45Z

You can try construct http://construct.readthedocs.io/en/latest/

import construct as cstruct
def mps_packet_header(name):
    return cstruct.Struct(
        name,
        cstruct.UNInt8('seq'),
        cstruct.UNInt8('type'),
        cstruct.UNInt16('flags'),
        cstruct.UNInt16('upTimestamp'),
    )

mps_acc_packet_t = cstruct.Struct(
    'mps_acc_packet_t',
    mps_packet_header('header')
    cstruct.Array(6, cstruct.NInt16('x')),
    cstruct.Array(6, cstruct.NInt16('y')),
    cstruct.Array(6, cstruct.NInt16('z')),
    cstruct.Array(6, cstruct.UNInt16('lowTimestamp')),
)

accpacket_t = mps_acc_packet_t
...
...
packet_data = ....
packet = accpacket_t.parse(packet_data)
print(packet)
print(packet.header)
print(packet.x, packet.y, packet.z)

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