Comment by gpderetta

8 hours ago

> Unaligned access? Packed structs.

Packed structs are dangerous. You can do unaligned accesses through a packed type, but once you take the address of your misaligned int field, then you are back into UB territory. Very annoying in C++ when you try to pass the a misaligned field through what happens to be generic code that takes a const reference, as it will trigger a compiler warning. Unary operator+ is your friend.

1 comment

gpderetta

matheusmoreira 8 hours ago

> but once you take the address of your misaligned int field

Gotta work with the structure directly by taking the address of the packed structure itself.

  struct uu64 {
      u64 value;
  } __attribute__((packed));

  struct uu64 unaligned;
  struct uu64 *address = &unaligned;

  address->value; // this works

  u64 *broken = &address->value; // this doesn't

Taking the address of the field inside the structure essentially casts away the alignment information that was explicitly added to stop the compiler from screwing things up. So it should not be done.

Mercifully, both gcc and clang emit address-of-packed-member warnings if it's done. So the packed structures are effectively turning silently broken nonsense code into sensible warnings. Major win.