• dd0s (17/17) Jul 01 2015 i have the following struct, and i expect it to have 30 bytes
• Adam D. Ruppe (16/18) Jul 01 2015 That's expected because scope_id takes 4 bytes, so port has to be
• anonymous (12/29) Jul 01 2015 Disclaimer: My understanding of all things alignment is limited.
• dd0s (1/1) Jul 01 2015 thank you both, indeed missed that truncation step

"dd0s" <oggs gmx.at> writes:

i have the following struct, and i expect it to have 30 bytes
but sizeof tells me it has 32 bytes. dmd seems to still use 4byte 
alignment altough i specified to align 2bytes.

struct netadr_t {
	align(2):
     int    type; // 0
     int    scope_id; // 4
     short  port; // 8 // <-- this is 4 bytes instead of 2
     int    sock;   // 10

     union {
     ubyte[4]    ip; // 14
     ubyte[10] ipx;
     ubyte[16] ip6;
     }
}

since i'm interfacing with a c library the struct layout has to 
be equal :(

"Adam D. Ruppe" <destructionator gmail.com> writes:

On Wednesday, 1 July 2015 at 20:01:08 UTC, dd0s wrote:
     int    scope_id; // 4
     short  port; // 8 // <-- this is 4 bytes instead of 2

That's expected because scope_id takes 4 bytes, so port has to be 
at 8 so it doesn't overlap the preceding int.


The extra two bytes you see in sizeof are probably at the *end* 
of the struct, not between the members.

To get rid of them, out align(2) on the outside too:

align(2)
struct yourthing {
   align(2):
     members
}


That will trim the size. The align inside the struct packs the 
members, but still pads the end (which means an array of these 
structs will be aligned on the word boundary). The align on the 
outside removes the padding at the end, meaning an an array of 
the structs would be packed too.

"anonymous" <anonymous example.com> writes:

On Wednesday, 1 July 2015 at 20:01:08 UTC, dd0s wrote:
 i have the following struct, and i expect it to have 30 bytes
 but sizeof tells me it has 32 bytes. dmd seems to still use 
 4byte alignment altough i specified to align 2bytes.

 struct netadr_t {
 	align(2):
     int    type; // 0
     int    scope_id; // 4
     short  port; // 8 // <-- this is 4 bytes instead of 2
     int    sock;   // 10

     union {
     ubyte[4]    ip; // 14
     ubyte[10] ipx;
     ubyte[16] ip6;
     }
 }

 since i'm interfacing with a c library the struct layout has to 
 be equal :(

Disclaimer: My understanding of all things alignment is limited.

`pragma(msg, netadr_t.sock.offsetof);` prints "10LU", so port 
seems to really only take 2 bytes. The struct itself has padding 
at the end. You can eliminate that with an `align(1)` on the 
struct:
----
align(1) struct netadr_t {
     align(2):
     ...
}
----

"dd0s" <oggs gmx.at> writes:

thank you both, indeed missed that truncation step