undefined | Better HN

0 pointsSomeone6y ago0 comments

For packed structures, C already has bit fields. Example (from https://en.cppreference.com/w/cpp/language/bit_field):

  struct S {
    // will usually occupy 2 bytes:
    // 3 bits: value of b1
    // 2 bits: unused
    // 6 bits: value of b2
    // 2 bits: value of b3
    // 3 bits: unused
    unsigned char b1 : 3, : 2, b2 : 6, b3 : 2;
  };

This is more aimed at large integers.

struct S { // will usually occupy 2 bytes: // 3 bits: value of b1 // 2 bits: unused // 6 bits: value of b2 // 2 bits: value of b3 // 3 bits: unused unsigned char b1 : 3, : 2, b2 : 6, b3 : 2; };

0 comments

5 comments · 3 top-level

gray_-_wolf6y ago· 2 in thread

Kinda important part mentioned in the link is

> Adjacent bit field members may be packed to share and straddle the individual bytes.

So it might not be packed. Ofc it usually is, at least on gcc and such.

hermitdev6y ago

I think the bigger issue in regards to networking is ordering, not the (lack of) packing. I don't think the ordering is guaranteed and probably depends on CPU architecture (e.g. big vs little endian), not necessarily compiler.

gdwatson6y ago

In practice, little-endian machines have the first bit field in the least-significant bits of the data type they fit into (usually but not always an int), and big-endian machines have the first bit field in the most-significant bits.

drblast6y ago

I'm pretty sure the exact layout is not guaranteed in this case, so while the above code may work in many cases it's not possible to represent all data structures like this, particularly if the bits are not byte or machine word aligned.

I don't think the standard even guarantees which bits in an integer the bitfields will be stored. That's important for network protocols.

zamadatix6y ago

"so it's not just limited to a couple of small sized bitflag style things."

The examples I gave are integers you may have to compare or manipulate not just bitfields to copy.

I also disagree this is aimed at large integers based on the examples of the syntax in the article commonly being <16 bits, sometimes <8, while the examples I cited had non power of 2 use cases up to 48 bits.

j / k navigate · click thread line to collapse