Integer sizes on nano vs nano 33 iot

[MegaHolmes]

Having trouble passing structured data between devices over RFM69 radio. I've narrowed down the problem to the same data structure having different sizes on the 2 different devices. With the structure declaration below, DriveData is 5 bytes on a Nano BUT 6 bytes on the Nano 33 iot ! The size of a temp variable declared to be unit16_t on the Nano 33 iot is 2 bytes as expected, so I'm not sure where the extra byte comes from. If I declare BattV to be unit8_t, then DriveData is 4 bytes on both devices, as expected - and all is well. If instead I declare BattV as an int, then DriveData is 12 bytes on the Nano 33 iot, and 5 bytes on the Nano. I have come to understand that the sizes of the various dat types differs on unique hardware, but if int is 8 bytes on the iot, then why is DriveData 12 instead of 11 bytes? I have scaled my data to get by with a single byte in this case but this wont always be satisfactory. What is it that I don't know?

typedef struct {
  byte        DW;
  uint16_t    BattV;  
  byte       Hr;   
  byte       Min;  
} DWDataStructure;

DWDataStructure DriveData;

[guix]

Hello

Search about struct alignment, padding, packing...

[J-M-L]

if you can’t find it, look for GCC struct attributes like __attribute___ ((packed)) and align

[MegaHolmes]

Nice! Got the most out of this source:
https://developer.arm.com/documentation/100748/0616/Writing-Optimized-Code/Packing-data-structures
Arranging my members largest to smallest and using the packed attribute I get the same size structure using the unit16_t declaration for my integers without needing to scale down to a single byte. So everybody is talking and passing the correct data! I appreciate the help.

typedef struct __attribute__((packed)) {
  uint16_t    BattV;  
  uint16_t    PnlV;
  byte       Hr;   
  byte       Min;
  bool       DW;  
} DWDataStructure;

DWDataStructure DriveData;

[J-M-L]

Great

(Note that in C++ the typedef keyword is no longer needed)