[dart:ffi] Structure padding and packing in C

[mannprerak2]

For a struct marked with __attribute__((packed)) -

typedef struct __attribute__((packed))
{
    char a;
    int b;
} A;

The layout is as follows (Generated using clang -Xclang -fdump-record-layouts test.c)-

*** Dumping AST Record Layout
         0 | A
         0 |   char a
         1 |   int b
           | [sizeof=5, align=1]

Layout without __attribute__((packed))

*** Dumping AST Record Layout
         0 | A
         0 |   char a
         4 |   int b
           | [sizeof=8, align=4]

Dart will automatically add relevant padding(so that the int b starts at the 4-byte boundary) If I declare a class A extends ffi.Struct.

There should someway to specify to dart VM how to pack a Struct.

[mannprerak2]

Duplicate of #38158