Bug 8383 – 64-bit ABI: unions not the same size as in C in some situations

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.

With github head for D2, this gives sizeof a: 32 when compiled with -m64 on Linuxx86_64. It only gives sizeof: a == 20 when compiled with -m32. This is true even on 1.073, released five months before this bug report was made. So I cannot reproduce this at all. Did the original poster fail to use the -m64 flag? It could of course have been an even older bug which was fixed before the start of 2012.

Hm, strange, I was sure I could repro this using both DMD and LDC at some point (and yes, building with -m64). In any case, thanks for taking the time to look at this.