Wrong data copied to struct from void pointer

Question

Ok so I've run out of things to try and fix this.

I've got a struct as follows:

typedef struct
{
   u8int  NodeType;
   u32int Group;
   u32int Direction;
   u16int ID;

} tsTargetNode;


void vTrackNode(const uint8 *pu8Val, uint8 u8Len, void *pvCbData)
{
   /* Copy data */
   memcpy(pvCbData, pu8Val, u8Len);

   /* Doing this gives the wrong data */
   //tsTargetNode *node = (tsTargetNode *) pvCbData;  

   /* Doing this gives the right data */
   memcpy(&tsTargetNode.NodeType, pvCbData, 1);
   memcpy(&tsTargetNode.Group, pvCbData+1, 4);
   memcpy(&tsTargetNode.Direction, pvCbData+1+4, 4);
   memcpy(&tsTargetNode.ID, pvCbData+1+4+4, 2);
}

For example: Data passed *pu8Val = 0x AA BB CC DD EE FF 11 22 33 44 55 66 77 88 99 00

using pointer, the data I retrieved is:

NodeType  = 0xAA
Group     = 0xFF112233
Direction = 0x44556677
ID        = 0x8899 

but manually memcpy, the data I retrieved is correct

NodeType  = 0xAA  
Group     = 0xBBCCDDEE
Direction = 0xFF112233
ID        = 0x4455

it seems like the first set it tries to copy 4 bytes into the NodeType.

Am I doing the pointer copy wrong? or could this be a device / HW problem?

Answer 1

The compiler is free to add padding between the members of a structure, so that each member is aligned to whatever is natural for your platform - here, it looks like 32 bits. Try printing sizeof(tsTargetNode) , and dumping that many bytes from an initialized tsTargetNode to see how the fields are laid out.

You haven't said what compiler you're using, but there will be a compiler-specific way to disable padding for your structure. If you do that, its memory layout will match the buffer.

Answer 2

You need to tell your compiler to "pack" the struct:

typedef struct __attribute__((__packed__))
{
   u8int  NodeType;
   u32int Group;
   u32int Direction;
   u16int ID;

} tsTargetNode;

https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Type-Attributes.html