6

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?

4

2 回答 2

4
  1. 您Struct的格式与 C 结构不匹配。（最终 H应该是6H）
  2. struct.unpack( 6h, ..) 确实返回 6 个字段。（不是一个有 6 个元素的）

所以你的代码应该看起来像..

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:]
于 2013-08-06T14:16:20.680 回答
4

您可以尝试构建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)
于 2016-08-16T14:50:45.490 回答