c - Struct packing and alignment with mingw

Question

I am emulating code from an embedded system (stm32, Keil μVision 5, MDK-ARM) on a PC (mingw32, 32bit arch). The alignment of the ARM compiler does not match my desktop mingw build:

// ARM Code (ARM compiler uses __packed)
typedef __packed struct _file
{
    uint8_t var1;
    uint16_t var2;
} FILE;

// PC mingw gcc code trying to emulate layout above.
typedef struct __attribute__((packed, aligned(1))) _file
{
    uint8_t var1;
    uint16_t var2;
} FILE;

In the source I do the following: file.var1 = 0x22; file.var2 = 0xAA55; which is then written to memory. When I read the memory on the embedded system it shows 0x22, 0x55, 0xAA. On the Windows machine it reads 0x22, 0xFF, 0x55, 0xAA, with padding at the 2^nd byte. How can I correct this behaviour?

Answer 1

I fixed it by myself -> Compiling with -mno-ms-bitfields helps! The code above is indeed correct. It is necessary to tell mingw to use gcc's bitfield organisation instead of the microsoft style. Though the code can be uncompileable with microsoft compilers then, I do not care at this point.