Question

My code used to work in the past, but now the struct size suddenly is 16 bytes. It used to be 13 bytes. I recently upgraded from Xcode 4.2 to Xcode 4.3.1 (4E1019).

#pragma pack(1)
struct ChunkStruct {
    uint32_t width;
    uint32_t height;
    uint8_t bit_depth;
    uint8_t color_type;
    uint8_t compression;
    uint8_t filter;
    uint8_t interlace;
};
#pragma pack()
STATIC_ASSERT(expected_13bytes, sizeof(struct ChunkStruct) == 13);

I have tried unsuccesfully using

#pragma pack(push, 1)
/* struct ChunkStruct { ... }; */
#pragma pack(pop)

I have also tried the following, but no luck

struct ChunkStruct {
    uint32_t width;
    uint32_t height;
    uint8_t bit_depth;
    uint8_t color_type;
    uint8_t compression;
    uint8_t filter;
    uint8_t interlace;
} __attribute__ ((aligned (1)));

How to pack structs with Xcode 4.3.1 ?

Answer 1

Xcode uses the gcc and clang compilers which both use __attribute__((packed)) to designate struct packing.

struct foo {
  uint8_t bar;
  uint8_t baz;
} __attribute__((packed));

Using __attribute__((aligned(1))) tells the compiler to begin each struct element on the next byte boundary but doesn’t tell it how much space it can put at the end. This means that the compiler is allowed to round the struct up to a multiple of the machine word size for better use in arrays and similar. __attribute__((packed)) tells the compiler to not use any padding at all, even at the end of the struct.