How to use structure with dynamically changing size of data?

Question

Question for C only, C++ and vectors do not solve problem.

I have such structure:

typedef __packed struct Packet_s
{
  U8  head;
  U16 len;
  U32 id;
  U         


        

Answer 1

You can't have dynamic buffer in middle of a struct.

Another way to solve the problem is divide the struct to two pieces. For example (notice that data is here a flexible array member):

typedef __packed struct Packet_s
{
  U8  head;
  U16 len;
  U32 id;
  U8  data[];
} Packet_t, *Packet_p;

typedef __packed struct PacketEnd_s
{
  U8  end;
  U16 crc;
} PacketEnd_t, *PacketEnd_p;

Then use

Packet_t *pPacket = (Packet_t *)&Buffer;
PacketEnd_t *pPacketEnd = (PacketEnd_t *)( count pointer here by using pPacket->len );

Assuming that __packed allows to use unaligned access to members of __packed structs.