[Issue 8383] New: 64-bit ABI: unions not the same size as in C in some situations

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8383

           Summary: 64-bit ABI: unions not the same size as in C in some
                    situations
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: code at klickverbot.at


--- Comment #0 from klickverbot <code at klickverbot.at> 2012-07-13 02:55:33 PDT ---
Originally reported by "bll" at http://www.dsource.org/projects/ldc/ticket/475:

---
#include <stdio.h>
#include <stdlib.h>

union a {
  long double a1;
  char        ac[20];
};

int
main (int argc, char *argv[]) {
  union a a;
  printf ("C\n");
  printf ("sizeof a: %d\n", sizeof (a));
  printf ("sizeof a.a1: %d\n", sizeof (a.a1));
  printf ("sizeof a.ac: %d\n", sizeof (a.ac));
  printf ("offsetof a.ac: %d\n", (int) ((char *) &a.ac - (char *) &a.a1));
  return 0;
}
---
yields
---
C
sizeof a: 32
sizeof a.a1: 16
sizeof a.ac: 20
offsetof a.ac: 0
---

but

---
import std.stdio;

union ua {
  real     a1;
  char     ac[20];
};

int
main (string argv[]) {
  ua a;
  writefln ("D");
  writefln ("sizeof a: %d", a.sizeof);
  writefln ("sizeof a.a1: %d", a.a1.sizeof);
  writefln ("sizeof a.ac: %d", a.ac.sizeof);
  writefln ("offsetof a.ac: %d", a.ac.offsetof);
  return 0;
}
---
yields
---
D
sizeof a: 24
sizeof a.a1: 16
sizeof a.ac: 20
offsetof a.ac: 0
---

when compiling on Linux x86_64.

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