Unpacking nested C structs in Python
nested, python, struct, unpack
Solution
- 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:]
Problem
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?