(Original discussion: http://forum.dlang.org/thread/fckwpddiwxonabqaflqt@forum.dlang.org#post-pskjgieddhpntzaokohj:40forum.dlang.org)

    align(1) struct A
    {
    align(1):
        int qtim;
        int bid;
        int ofr;
        int bidsiz;
        int ofrsiz;
        short mode;
        char[1] ex;
        char[4] mmid;
    }

    align(1) struct B
    {
        int qtim;
        int bid;
        int ofr;
        int bidsiz;
        int ofrsiz;
        short mode;
        char[1] ex;
        char[4] mmid;
    }

I would expect `B` to have a gap between `ex` and `mmid`. AFAIK the outer `align(1)` only applies to the struct in its entirety, not to the individual fields. However for both DMD git and LDC 0.14.0-alpha1 (based on DMD 2.065), `A` and `B` have the same size.

After some thinking, I believe this is because arrays inherit the alignment of their element types. Is this correct? If yes, where is this documented? I had expected `char[4]` to be aligned at a 4-byte boundary.

On Thursday, 7 August 2014 at 17:22:15 UTC, Marc Schütz wrote:
> (Original discussion: http://forum.dlang.org/thread/fckwpddiwxonabqaflqt@forum.dlang.org#post-pskjgieddhpntzaokohj:40forum.dlang.org)

> I would expect `B` to have a gap between `ex` and `mmid`. AFAIK the outer `align(1)` only applies to the struct in its entirety, not to the individual fields. However for both DMD git and LDC 0.14.0-alpha1 (based on DMD 2.065), `A` and `B` have the same size.

> After some thinking, I believe this is because arrays inherit the alignment of their element types. Is this correct? If yes, where is this documented? I had expected `char[4]` to be aligned at a 4-byte boundary.

 I'm not sure about all the latest compiler changes, but let's try and answer some of this.

 TDPL pg. 268-269 explains this (although could be out of date with recent changes). I'll copy what's relevant..

7.1.11.1 The align Attribute

If you want to override the compiler's choice of alignment, which influences the padding inserted. You could use an align modifier... etc..

class A {
  char a;
  align(1) int b;
  char c;
}

 With the specification above the fields of A are laid out without gaps between them.

 You may use align with an entire class definition:

align(1) struct S {
  char a;
  int b;
  char c
}

 ...
 Align is not suppose to be used with pointers and references...



 Back to the question. Most fields will be aligned on 4-byte boundaries, or 8-byte depending on if it's 32/64 bit. This is mostly for performance reasons, but also with addresses it may affect the GC. Overriding the compiler is mostly going to be more useful when working against C/C++ structures where they are also forcibly aligned for space.

 So assuming we have the above struct.

 S[2] s;

 It's probably going to be aligned on 4's for the first one; But it doesn't have to be. But the others? I'm not so sure... The inner alignment and padding is 4 per, so if the struct S has a size of 12, then it's still going to be aligned by 4's by default... i think? Maybe the alignment has to do if it's inserted into another object. On the stack it probably ignores the alignment attribute...

struct B {
  char a;
  S s;  //infers align(1) by it's definition?
}

 Well regardless, unless you're overriding both, you're probably going to get some form of alignments of 4, be it for arrays or for speed... I hope this isn't confusing.

Still watching this, but the Dconf 2014 bare metal presentation gets into it a bit...

 http://youtu.be/qErXPomAWYI?t=37m20s

yeah, chars (and bytes, and so on) are not aligned. i.e.

align(1) struct B {
  int qtim;
  int bid;
  int ofr;
  int bidsiz;
  int ofrsiz;
  short mode;
  char ex;
  byte mmid;
  char z;
}

has sizeof == 25. not sure if specs mentions this, but they should.

p.s. seems that aligning works only on ints. i.e. on types which has sizeof >= default platform align.

On Friday, 8 August 2014 at 18:20:41 UTC, ketmar wrote:
> yeah, chars (and bytes, and so on) are not aligned. i.e.
>
> align(1) struct B {
>   int qtim;
>   int bid;
>   int ofr;
>   int bidsiz;
>   int ofrsiz;
>   short mode;
>   char ex;
>   byte mmid;
>   char z;
> }
>
> has sizeof == 25. not sure if specs mentions this, but they should.

It's not surprising that `char` and `byte` behave like this, because `byte.alignof == 1`. But it's not obvious that this also applies to arrays of them. I had expected them to be treated as opaque objects of a certain size, and therefore have an alignment that corresponds to their size.

    pragma(msg, byte.alignof);
    pragma(msg, (byte[1]).alignof);
    pragma(msg, (byte[2]).alignof);
    pragma(msg, (byte[3]).alignof);
    pragma(msg, (byte[4]).alignof);
    pragma(msg, (byte[5]).alignof);
    pragma(msg, (byte[6]).alignof);
    struct S {
        byte a;
        byte b;
        byte c;
        byte d;
    }
    struct T {
        byte a;
        short b;
        int c;
    }
    pragma(msg, S.alignof);
    pragma(msg, T.alignof);

This outputs "1" for all types except `T`, which has 4. So this even applies to structs, not only arrays. Which may make sense, because each element will always be accessed with correct alignment. However, accessing the aggregate as a whole might result in unaligned reads/writes.