将struct的成员声明为uint32_t时额外的字节

Extra bytes when declaring a member of a struct as uint32_t

本文关键字：字节 uint32 struct 成员 声明      更新时间：2023-10-16

当使用stint .h库的uint32_t类型时，我有一个问题。如果我运行以下代码(在Ubuntu linux 11.10 x86_64, g++版本4.6.1):

#include "stdint.h"
#include <iostream>
using std::cout;
typedef struct{
    // api identifier
    uint8_t api_id;
    uint8_t frame_id;
    uint32_t dest_addr_64_h;
    uint32_t dest_addr_64_l;
    uint16_t dest_addr_16;
    uint8_t broadcast_radius;
    uint8_t options;
    // packet fragmentation
    uint16_t order_index;
    uint16_t total_packets;
    uint8_t rf_data[];
} xbee_tx_a;
typedef struct{
    // api identifier
    uint8_t api_id;
    uint8_t frame_id;
    uint16_t dest_addr_64_h;
    uint16_t dest_addr_64_l;
    uint16_t dest_addr_16;
    uint8_t broadcast_radius;
    uint8_t options;
    // packet fragmentation
    uint16_t order_index;
    uint16_t total_packets;
    uint8_t rf_data[];
} xbee_tx_b;

int main(int argc, char**argv){
   xbee_tx_a a;
   cout<<"size of xbee_tx_a "<<sizeof (xbee_tx_a)<<std::endl;
   cout<<"size of xbee_tx_a.api_id "<<sizeof (a.api_id)<<std::endl;
   cout<<"size of xbee_tx_a.frame_id "<<sizeof (a.frame_id)<<std::endl;
   cout<<"size of xbee_tx_a.dest_addr_64_h "<<sizeof (a.dest_addr_64_h)<<std::endl;
   cout<<"size of xbee_tx_a.dest_addr_64_l "<<sizeof (a.dest_addr_64_l)<<std::endl;
   cout<<"size of xbee_tx_a.dest_addr_16 "<<sizeof (a.dest_addr_16)<<std::endl;
   cout<<"size of xbee_tx_a.broadcast_radius "<<sizeof (a.broadcast_radius)<<std::endl;
   cout<<"size of xbee_tx_a.options "<<sizeof (a.options)<<std::endl;
   cout<<"size of xbee_tx_a.order_index "<<sizeof (a.order_index)<<std::endl;
   cout<<"size of xbee_tx_a.total_packets "<<sizeof (a.total_packets)<<std::endl;
   cout<<"size of xbee_tx_a.rf_data "<<sizeof (a.rf_data)<<std::endl;
   cout<<"----------------------------------------------------------n";
   xbee_tx_b b;
   cout<<"size of xbee_tx_b "<<sizeof (xbee_tx_b)<<std::endl;
   cout<<"size of xbee_tx_b.api_id "<<sizeof (b.api_id)<<std::endl;
   cout<<"size of xbee_tx_b.frame_id "<<sizeof (b.frame_id)<<std::endl;
   cout<<"size of xbee_tx_b.dest_addr_64_h "<<sizeof (b.dest_addr_64_h)<<std::endl;
   cout<<"size of xbee_tx_b.dest_addr_64_l "<<sizeof (b.dest_addr_64_l)<<std::endl;
   cout<<"size of xbee_tx_b.dest_addr_16 "<<sizeof (b.dest_addr_16)<<std::endl;
   cout<<"size of xbee_tx_b.broadcast_radius "<<sizeof (b.broadcast_radius)<<std::endl;
   cout<<"size of xbee_tx_b.options "<<sizeof (b.options)<<std::endl;
   cout<<"size of xbee_tx_b.order_index "<<sizeof (b.order_index)<<std::endl;
   cout<<"size of xbee_tx_b.total_packets "<<sizeof (b.total_packets)<<std::endl;
   cout<<"size of xbee_tx_b.rf_data "<<sizeof (b.rf_data)<<std::endl;
}

则得到以下输出:

size of xbee_tx_a 20
size of xbee_tx_a.api_id 1
size of xbee_tx_a.frame_id 1
size of xbee_tx_a.dest_addr_64_h 4
size of xbee_tx_a.dest_addr_64_l 4
size of xbee_tx_a.dest_addr_16 2
size of xbee_tx_a.broadcast_radius 1
size of xbee_tx_a.options 1
size of xbee_tx_a.order_index 2
size of xbee_tx_a.total_packets 2
size of xbee_tx_a.rf_data 0
----------------------------------------------------------
size of xbee_tx_b 14
size of xbee_tx_b.api_id 1
size of xbee_tx_b.frame_id 1
size of xbee_tx_b.dest_addr_64_h 2
size of xbee_tx_b.dest_addr_64_l 2
size of xbee_tx_b.dest_addr_16 2
size of xbee_tx_b.broadcast_radius 1
size of xbee_tx_b.options 1
size of xbee_tx_b.order_index 2
size of xbee_tx_b.total_packets 2
size of xbee_tx_b.rf_data 0

我所做的是打印出结构体的总大小和结构体中每个成员的大小。

对于xbee_tx_b，成员的大小加起来等于结构体(14)的大小

在xbee_tx_a的情况下，成员的大小加起来是18字节…但是结构体的大小是20字节!

xbee_tx_a和xbee_tx_b的唯一区别在于dest_addr_64_X成员的类型。它们是xbee_tx_a中的uint32_t和xbee_tx_b中的uint16_t。为什么当我使用uint32_t时，结构的大小大于其成员大小的总和?那两个额外的字节是从哪里来的?

谢谢!

结构被填充为4字节1的整数倍，以便它们是对齐的。http://en.wikipedia.org/wiki/Data_structure_alignment Data_structure_padding

参见:

  • 为什么struct的sizeof等于每个成员的sizeof之和?
  • 结构填充和包装
  • 使用结构体填充

1正如@Mooing Duck评论的那样，这并不总是为真:

它并不总是4字节的倍数，它根据成员的不同(略有不同)。另一方面，99%的情况下它是4字节的倍数。

这是因为对齐。在你的平台上，uint32_t需要4字节对齐。为了实现这一点，dest_addr_64_h必须在它前面有两个字节的填充，因为在两个uint8_t成员之后的位置是2的倍数，而不是4。

您可以使用宏offsetof()来确定成员在结构体中的确切位置，以查看这是否为真。

您可以尝试让编译器将成员更紧密地打包在一起，或者您可以重新排列成员，以便不需要填充。

不同平台的数据类型有不同的对齐要求。额外的字节用于将结构的一个成员对齐到特定的大小和/或位置。如果您需要更精确的控制，您可以使用__attribute__或#pragma pack

指定此对齐方式。

您需要向编译器声明打包结构

我相信这将工作在GCC

struct test
    {
            unsigned char  field1;
            unsigned short field2;
            unsigned long  field3;
    } __attribute__((__packed__));

在MS中，它将使用打包的pragma

http://www.cplusplus.com/forum/general/14659/

#pragma pack(push, 1) // exact fit - no padding
struct MyStruct
{
  char b; 
  int a; 
  int array[2];
};
#pragma pack(pop) //back to whatever the previous packing mode was
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