Packing multiple integers into a 64 bit integer

Question

I want to pack the following numbers into a 64 bit int64_t field in the following order:

int8_t num1
int8_t num2
int32_t num3
int16_t num4

So, the 64 bits should be in the following layout:

[  num1(8)  |  num2(8)  |       num3(32)       |   num4(16)  ]

I'm not able to wrap my head around the bit packing logic, i.e. I want those numbers to be packed into a single int64_t field, similar to this question.

Any help is greatly appreciated.

*"I'm not able to wrap my head around the bit packing logic."* is not a question. What exactly is your problem? — Baum mit Augen, Nov 10 '17 at 13:40
I want those numbers to be packed into a single int64_t field similar to: https://stackoverflow.com/questions/2810280/how-to-store-a-64-bit-integer-in-two-32-bit-integers-and-convert-back-again — jeffreyveon, Nov 10 '17 at 13:42
That edit does not make it any better. Have you tried something? What went wrong? Please provide some *specific* problem statement. — Baum mit Augen, Nov 10 '17 at 13:43
One thing I've seen is to make a union of an int64 and a struct containing the individual fields. Just be careful about ordering and endianness. — 0x5453, Nov 10 '17 at 13:46
@arminb And that's UB, too. Are you folks done guessing already? :/ — Baum mit Augen, Nov 10 '17 at 13:48
You keep saying it's UB, but what part is undefined? (genuinely curious) — byxor, Nov 10 '17 at 13:50
@byxor You cannot type-pun through unions and signed integer overflow is UB. — Baum mit Augen, Nov 10 '17 at 13:50
@BaummitAugen Type-punning through unions seems OK in C, but not C++, which is why the OP needs to choose one language to ask about. — underscore_d, Nov 10 '17 at 13:51
Do those integers all actually need to be signed? Having to worry about sign extension complicates life ... — Useless, Nov 10 '17 at 13:59
Telling us what you want is not a question nor a problem description. What have you tried so far and where does it fail? — Gerhardh, Nov 10 '17 at 14:01
@0x5453 you forgot to mention padding in your "take care of.." list. ;) — Gerhardh, Nov 10 '17 at 14:02
Oh, and I forgot to ask: do you need to unpack these fields as well, or is it a one-way operation? — Useless, Nov 10 '17 at 14:09

Jabberwocky · Accepted Answer · 2017-11-10T15:33:43.113

4

You probably want this:

  int8_t num1;
  int8_t num2;
  int32_t num3;
  int16_t num4;
  ...
  uint64_t number = ((uint64_t)num1 << (16 + 32 + 8)) | ((uint64_t)num2 << (16 + 32)) | ((uint64_t)num3 << 16) | (uint64_t)num4;

From this you should be able to figure out how to do the inverse conversion. If not, post another question.

edited Nov 10 '17 at 15:33

answered Nov 10 '17 at 13:42

Jabberwocky

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Packing multiple integers into a 64 bit integer

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