jpfender jpfender - 3 months ago 19
Python Question

Unpacking nested C structs in Python

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?

Answer
  1. You Struct format does not match with C structure. (final H should be 6H)
  2. 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:]