Question

I’m trying to pack data in a c++ struct.

My struct has this layout:

struct structName
{
  int16_t member1;
  int32_t member2;
  uint32_t member3;
  uint32_t member4;
  uint32_t member5;
  etc
}__attribute__((packed));

Using offsetof($structname, $membername) I get back the correct offsets of the data (0,2,6,10,14 . . .), but when I access the data by member-name I get the data at 4 byte offsets (0,4,8,12,16 . . .) as if the struct wasn’t packed.

Is

} __attribute__((packed));

the correct way to make a struct packed?
.
.

Answer 1

Update: mydogisbox wrote:

For the record, __attribute__((packed)), #pramga pack(1) and #pragma pack(push, 1) all worked.

__attribute__((packed)) is a gcc extension, which is supported.

The clang documentation says it also supports #pragma pack(...) directive:

clang has some experimental support for extensions from Microsoft
Visual C++; to enable it, use the -fms-extensions command-line option.
This is the default for Windows targets. Note that the support is
incomplete; enabling Microsoft extensions will silently drop certain
constructs (including __declspec and Microsoft-style asm statements).

clang supports the Microsoft #pragma pack feature for controlling record layout.

source: http://clang.llvm.org/docs/UsersManual.html

Just say:

#pragma pack(1)
struct my_struct {
    int16_t x;
    // etc.
};

to see if it works (compile with -fms-extensions if not using Windows).

Note the above are all non-standard extensions, and the new C++11 standard has a new alignas keyword: http://en.cppreference.com/w/cpp/language/alignas

struct alignas(1) my_struct {
    int16_t x;
    // etc.
};

but its support is still a bit sketchy.