casting 115bit data to union of bitfields gets erroneous results

Question

I'm trying to cast a 115bit data to a union of bitfields and getting a wrong result.

Setup

I have two types of data:

Configuration: data[62:0]
               addr[113:107]
               type[114]

RawBits:       lowBits[63:0]
               highBits[115:64]

So I defined the following bitfields and union:

typedef struct RawBits {
  unsigned long int lowBits:64;
  unsigned long int highBits:51;
  unsigned long int reserved :13;
} __attribute__((packed))rawBits;

typedef struct Configuration {
  unsigned long int data : 62;
  unsigned long int addr : 7;
  unsigned long int type : 1;
  unsigned long int reserved : 58;
} __attribute__((packed))Configuration ;

typedef union Instruction {
  RawBits bits;
  Configuration configuration;
} __attribute__((packed))Instruction;

As data I used:

uint8_t configuration_test[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xfc,
                                 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0xfc, 0x00,

And to convert the buffer to the union type I used simple casting:

Instruction *instruction = (Instruction *)configuration_test;

Expected result

instruction->bits->lowBits  = 0xfffffffffffffffc
instruction->bits->highBits = 0x00000000000001fc
instruction->bits->reserved = 0x0000000000000000

instruction->configuration->data     = 0x3fffffffffffffff
instruction->configuration->addr     = 0x000000000000007f
instruction->configuration->type     = 0x0000000000000000
instruction->configuration->reserved = 0x0000000000000000

Real result

instruction->bits->lowBits  = 0xfcffffffffffffff
instruction->bits->highBits = 0x0004010000000000
instruction->bits->reserved = 0x000000000000001f

instruction->configuration->data     = 0x3cffffffffffffff
instruction->configuration->addr     = 0x0000000000000003
instruction->configuration->type     = 0x0000000000000000
instruction->configuration->reserved = 0x0003f00400000000

The compiler is free to represent bitfields in any way it chooses. The moment you chose to type pun that array as a `Instruction` is the moment your program's behavior became undefined... or at least implementation-specific. — paddy, Mar 31 '21 at 06:28
@paddy i need the bitfield to represent hw registers, so it need to be tightly packed — Elad Chamilevsky, Mar 31 '21 at 06:30
A bitfield can't "straddle" integers – `Configuration` is larger than you think. Use bitmasks and shifting. — molbdnilo, Mar 31 '21 at 06:37
Looks like a little-endian byte order took place. Bitfields use the hardware byte order, not the network byte order. — 273K, Mar 31 '21 at 06:37

casting 115bit data to union of bitfields gets erroneous results

Setup

Expected result

Real result

0 Answers0