Struct Byte Alignment Issue XCode 4.3

问题:

I'm trying to use struct to map header of a BitMap file. It seems that compiler is doing 4byte (32bit) alignment but I need 2Byte. I tried to change that via complier directive as below

#pragma pack(2)

and

__attribute__ ((aligned(xx)));

those two doesn't seem to have any effect. Is there another way to do this? I'm using XCode 4.3 on Mac OS X Lion. I Tested both Apple LLVM and Apple GCC compliers.

Here is the Struct type definition

typedef struct {
    int16_t bfType; 
    int32_t bfSize;
    int16_t bfReserved1;
    int16_t bfReserved2;
    int32_t bfOffBits;
    int32_t biSize;
    int32_t biWidth;
    int32_t biHeight;
    int16_t biPlanes;
    int16_t biBitCount;
    int32_t biComression;
    int32_t biSizeImage;
    int32_t biXPelsPerMeter;
    int32_t biYPelsPerMeter;
    int32_t biClrUsed;
    int32_t biClrImportant;
} THeader;

回答1:

Huh? works on my machine? Bear in mind that the pack pragma is possibly being overridden somewhere else?

#include <inttypes.h>
#include <stddef.h>

#pragma pack(push,2)

typedef struct {
int16_t bfType; 
int32_t bfSize;
int16_t bfReserved1;
int16_t bfReserved2;
int32_t bfOffBits;
int32_t biSize;
int32_t biWidth;
int32_t biHeight;
int16_t biPlanes;
int16_t biBitCount;
int32_t biComression;
int32_t biSizeImage;
int32_t biXPelsPerMeter;
int32_t biYPelsPerMeter;
int32_t biClrUsed;
int32_t biClrImportant;
} THeader;

#pragma pack(pop)

#include <stdio.h>

int main(void)
{
printf("%lu\n", offsetof(THeader, bfType));
printf("%lu\n", offsetof(THeader, bfSize));
printf("%lu\n", offsetof(THeader, bfReserved1));
printf("%lu\n", offsetof(THeader, bfReserved2));
return 0;
}


$ clang -o pack pack.c
$ ./pack
0
2
6
8