Keil structure padding

[tmloos]

Hello,

I am using the nRF52840 DK with Keil to develop.  I have a structure defined as follows:

typedef struct
{
    uint16 packetType;
    uint32 packetLength;
    uint32 uuid[2];
    uint8  part;
    uint8  totalParts;
    //data here
    //...
    //uint16 crc
} multipart_header;

I have verified that uint8 is 1 byte, uint16 is 2 bytes, and uint32 is 4 bytes.  The size of struct multipart_header should be 16 bytes but it reports 20 bytes in the debugger.  I can see what look like two shorts in the memory window, one after packetType and one after totalParts.  Is Keil padding this structure to a 32-bit boundary?  This is a problem for me because I am filling up this structure and then memcpy-ing it in to a buffer as a header.  I am getting four garbage bytes into the buffer.  How can I avoid this?  Must I move each field into the buffer separately?

Thanks!

[Turbo J]

tmloos said:

Is Keil padding this structure to a 32-bit boundary?

Yes, and you usually want that even on a Cortex-M4F for performance reasons.

One might be tempted to use "packed" struct (see compiler documentation), but in some cases you also need to fiddle with endian conversions, which means custom (de-)serialisation code.