[Issue 19516] New: Alignment of members & size of structs inconsistent with C

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19516

          Issue ID: 19516
           Summary: Alignment of members & size of structs inconsistent
                    with C
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bdh_bugzilla at outlook.com

According to the spec [1], D structs are supposed to have the same memory
layout as their C counterparts. However, writing some bindings for a C library
I found the following inconsistency.

-- D code (see [2])
struct s {
    union {
        double d;
        char[20] cs;
    };
    char b;
}
pragma(msg, s.b.offsetof); // Expecting 24 but getting 20
pragma(msg, s.sizeof); // Expecting 32 but getting 24

void main(){}
--

In C (using clang 7 or gcc 8.1 [3]) the offset of b is 20, but 24 when using
dmd/ldc [2]. The mismatching sizeof output seems to be a result of that.


-- C code (see [3])
#include <stdio.h>

struct S {
    union {
        double d;
        char cs[20];
    };
    char b;
};

int main(){
    struct S s;
    // Yields 24, unlike the 20 in D
    printf("b at: %lu\n", (void*)&s.b-(void*)&s);

    // Yields size 32, unlike the 24 in D
    printf("size of s: %lu\n", sizeof(s));
}
--

[1] https://dlang.org/spec/struct.html#struct_layout
[2] https://dpaste.dzfl.pl/ff55c816a940
[3] https://www.jdoodle.com/a/Sgc

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