Unpack hex-encoded NSData

Question

on peripheral:didReceiveWriteRequest a CBATTRequest returns hex-encoded NSData via request.value.

This is what I have tried

// Define struct
typedef struct __attribute__((packed)) {
    UInt8  pktNo;
    UInt8  ctrlCmd;
    UInt8  txPowerRequest;
    UInt16 uuid;
    UInt16 userPayload; // how to store 15 octets?
} Packet;

   // Unpack 
   Packet *packet = (Packet *)request.value.bytes;
    if (packet) {
        UInt8  pktNo   = packet->pktNo;
        UInt8  cmd     = packet->ctrlCmd;
        UInt8  tx      = packet->txPowerRequest;
        UInt16 uuid    = packet->uuid;
        UInt16 payload = packet->userPayload;
        NSLog(@"pktNo: %hhu, cmd: %hhu, tx: %hhu, uuid: %hu, payload: %hu", pktNo, cmd, tx, uuid, payload);
    }

Console

pktNo: 121, cmd: 202, tx: 130, uuid: 48321, payload: 21421

First, these numbers look inaccurate, and I'm uncertain about what format this is even in, since the following analogous values I got from debugging tool don't seem to match.

Default: raw strings?

packet          Packet *    0x281af0cc0 0x0000000281af0cc0
pktNo           UInt8       'y'
ctrlCmd         UInt8       '\xca'
txPowerRequest  UInt8       '\x82'
uuid            UInt16      48321
userPayload     UInt16      21421

Answer 1

The hex string representation of your NSData is apparently:

<79ca82c1 bcad530e 016a1435 127993ee 01ef7579>

That translates to:

• The 0x79 is the PKT, 121 in decimal
• The 0xca is the CMD, 202 in decimal
• The 0x82 is the TXP, 130 in decimal
• The 0xbcc1 is the UUID, 48321 in decimal
• The PAYLOAD is ad 53 0e 01 6a 14 35 12 79 93 ee 01 ef 75 79

So, you can use:

typedef struct __attribute__((packed)) {
    UInt8  pktNo;
    UInt8  ctrlCmd;
    UInt8  txPowerRequest;
    UInt16 uuid;
    UInt8  userPayload[15];
} Packet;

Which you can populate with:

Packet packet = {};
assert(data.length == sizeof(Packet));
memcpy(&packet, data.bytes, sizeof(Packet));

---

Note, I didn’t just set the packet to a pointer to the bytes of the NSData, because if the NSData is deallocated, you don’t want to use a pointer to that deallocated memory. Instead, copy the bytes to your packet struct (perhaps checking to make sure the two match in size).

---

In the interest of full disclosure, the above makes a somewhat cavalier assumption that endianness of the payload matches that of device running your app. You theoretically might want to make the UUID a UInt8 uuid[2], instead, and then if you need the UUID value, recalculate it from those two octets.