结构字节对齐问题x $ C $Ç4.3 [英] Struct Byte Alignment Issue XCode 4.3

问题描述

我试图用结构映射位图文件的标题。看来，编译器做4字节（32位）比对，但我需要2字节。我试图通过编译指令来改变这种状况，如下

 的#pragma包（2）
 

和

  __ attribute__（（排列（XX）））;
 

这两个似乎没有产生任何影响。是否有另一种方式做到这一点？ 我用X code 4.3的Mac OS X狮子。我测试了苹果LLVM和苹果GCC执行者。

下面是结构类型定义

  typedef结构{
    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双翼飞机;
    int16_t biBitCount;
    int32_t biComression;
    int32_t biSizeImage;
    int32_t biXPelsPerMeter;
    int32_t biYPelsPerMeter;
    int32_t biClrUsed;
    int32_t biClrImportant;
} THEADER;
 

解决方案

咦？作品在我的机器上？请记住，包编译可能是被覆盖在别的地方？

 的#include＆LT; inttypes.h＆GT;
＃包括＆LT; STDDEF.H＆GT;

的#pragma包（推，2）

typedef结构{
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双翼飞机;
int16_t biBitCount;
int32_t biComression;
int32_t biSizeImage;
int32_t biXPelsPerMeter;
int32_t biYPelsPerMeter;
int32_t biClrUsed;
int32_t biClrImportant;
} THEADER;

的#pragma包（POP）

＃包括＆LT; stdio.h中＆GT;

INT主要（无效）
{
的printf（％鲁\ N，对offsetof（THEADER，bfType））;
的printf（％鲁\ N，对offsetof（THEADER，bfSize））;
的printf（％鲁\ N，对offsetof（THEADER，bfReserved1））;
的printf（％鲁\ N，对offsetof（THEADER，bfReserved2））;
返回0;
}


$铛-o包pack.c
$ ./pack
0
2
6
8
 

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;

解决方案

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

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