C++ C++;结构位字段不为';无法正确解析数据
我正在尝试使用压缩结构从VLAN头提取字段: 我创建了这个结构:C++ C++;结构位字段不为';无法正确解析数据,c++,struct,network-programming,bit-fields,C++,Struct,Network Programming,Bit Fields,我正在尝试使用压缩结构从VLAN头提取字段: 我创建了这个结构: #pragma pack(push, 1) struct vlan_header { uint16_t PCP : 3, DEI : 1, ID : 12; }; #pragma pack(pop) 当我获取uint8\t数组并尝试从中提取字段时: uint8_t* data; vlan_header* vlanHeader; data = new uint8_t[2]
#pragma pack(push, 1)
struct vlan_header
{
uint16_t PCP : 3,
DEI : 1,
ID : 12;
};
#pragma pack(pop)
当我获取uint8\t
数组并尝试从中提取字段时:
uint8_t* data;
vlan_header* vlanHeader;
data = new uint8_t[2];
data[0] = 0;
data[1] = 0x14; // data is 00 14
// That means PCP is 0, DEI is 0 and vlan id is 20
vlanHeader = (vlan_header*)data;
std::cout << "PCP: " << vlanHeader->PCP << std::endl;
std::cout << "DEI: " << vlanHeader->DEI << std::endl;
std::cout << "ID: " << vlanHeader->ID << std::endl;
delete[] data;
显然,我们看到vlan id是320而不是20,这不是我的意思。我假设问题是endian(我的机器是little endian),我不知道如何优雅地解决这个问题
可能位字段不是作业的正确工具?您的id值是0x140而不是0x14,请记住位字段部分打包到类型中。您有16位可用。 如果希望它是0x14,则需要
data[0] = 0x40;
data[1] = 1;
OP问:
我假设问题是endian(我的机器是little endian),我不知道如何优雅地解决这个问题
也许位字段不是适合该作业的工具
尽管在处理位字段或联合时,考虑机器的端位始终是一个很好的考虑因素,而且不应该忘记。然而,在你目前的情况下,我不认为endian是任何问题的原因或关注点。至于问题的第二部分,则视乎具体需要而定。如果要写入的代码专门针对特定的体系结构/操作系统/平台,并且不可能是可移植的,那么如果正确构造了位字段,那么使用位字段应该没有问题。即使您决定移植到其他机器,您仍然可以使用位字段,但您必须更加小心,并且可能必须使用预处理器指令或控制开关&case语句编写更多代码,以使代码在一台机器上使用,并在另一台机器上做一件事
在使用位字段时,我认为混合类型时要考虑endian
struct Bitfield {
unsigned a : 10,
b : 10,
c : 16;
int x : 10,
y : 10,
z : 16;
};
像上面这样的事情可能需要考虑endian
通过查看您的位域结构,我看到的是对位域内位对齐与结构本身对齐的误解 当前结构为: 您正在将对齐方式打包为尽可能小的
1字节
,因此此结构中的边界对于每个边界为8位
。这没什么大不了的,也不言自明。然后使用一种类型的uint16\u t
,它是typedef
的unsigned short
,大小为2字节或16位。无符号短
的值范围为[065535]
然后在结构中,您将位字段成员PCP
,DEI
和ID
分别设置为具有位数:3
,1
,12
。我向结构添加了注释以显示此模式
现在,正如在主函数中声明指向uint8\u t
类型的指针一样,您可以创建上述结构的实例,然后为指针创建数组大小为[2]
的动态内存。这里的uint8\u t
是一个typedef
用于无符号字符的typedef
,该字符的大小为1字节或8位
,由于您有2位
,因此总共有2字节或16位
。好的,所以内存的总大小在位字段结构和数据[]
数组之间匹配
然后通过索引并使用十六进制值设置指针数组来填充指针数组。然后通过将数组中的值转换为该类型,将数组中的值分配给位字段。然而,我认为您假设的是data[0]
应该适用于位字段的第一个2
成员,而data[1]
应该适用于最后一个ID
值。但情况并非如此:
这里发生的是,在代码的这一部分:
上面所说的并不是你认为它应该做的
我将制作一个图表,只是给你们展示一些例子:但是它太大了,无法在这里显示;所以我能做的就是提供一点代码在你的机器上运行,生成一个日志文件供你查看模式
#include <iostream>
#include <fstream>
#pragma pack(push, 1)
struct vlan_header {
// uint16_t = 2bytes: - 16bits to work with
uint16_t PCP : 3, // bit(s) 0-2
DEI : 1, // bit(s) 3
ID : 12; // bit(s) 4-15
};
#pragma pack(pop)
int main() {
uint8_t* data; // sizeof(uint8_t) = 1byte - 8bits
vlan_header* vlanHeader;
data = new uint8_t[2];
std::ofstream log;
log.open( "results.txt" );
for ( unsigned i = 0; i < 256; i++ ) {
for ( unsigned j = 0; j < 256; j++ ) {
data[0] = j;
data[1] = i;
std::cout << "data[0] = " << static_cast<unsigned>(data[0]) << " ";
std::cout << "data[1] = " << static_cast<unsigned>(data[1]) << " ";
log << "data[0] = " << static_cast<unsigned>(data[0]) << " ";
log << "data[1] = " << static_cast<unsigned>(data[1]) << " ";
vlanHeader = reinterpret_cast<vlan_header*>(data);
std::cout << "PCP: " << std::hex << vlanHeader->PCP << " ";
std::cout << "DEI: " << std::hex << vlanHeader->DEI << " ";
std::cout << "ID: " << std::hex << vlanHeader->ID << std::endl;
log << "PCP: " << std::hex << vlanHeader->PCP << " ";
log << "DEI: " << std::hex << vlanHeader->DEI << " ";
log << "ID: " << std::hex << vlanHeader->ID << std::endl;
}
}
log.close();
delete[] data;
std::cout << "\nPress any key and enter to quit." << std::endl;
char q;
std::cin >> q;
return 0;
}
内存中位字段的情况是,第一个字节或8位
同时消耗PCP
、DEI
以及ID
的第一个4位
,我想这就是您感到困惑的地方。正如SoronelHaetir
在其简短回答中所述,如果您希望3
位字段的值为十进制{0,0,20}
,则需要将数据数组设置为数据[0]=0x40
和数据[1]=0x01
。当数据[0]
中的位不能再包含比分配的位量所能支持的足够高的值时,该位将溢出到其他位字段成员中
这基本上意味着PCP
具有3
可用位
,其最大组合位数为2^3=8
,因此PCP
可以存储[0,7]
中的值。由于DEI
只有1位
,这就充当了一个单位布尔标志,它只能存储[0,1]
的值,最后ID
有12位
可用,其中第一个4
来自数据[0]
,最后一个8
都来自数据[1]
这将为您提供组合数字2^12=4096
这些数字的值范围为[04095]
,十六进制中的最大值为FFF
。这些都可以在日志或结果文件中看到
我还将显示数据[]
数组与的对齐方式
#pragma pack(push, 1)
struct vlan_header {
// uint16_t = 2bytes: - 16bits to work with
uint16_t PCP : 3, // bit(s) 0-2
DEI : 1, // bit(s) 3
ID : 12; // bit(s) 4-15
};
#pragma pack(pop)
data[0] = 0;
data[1] = 0x14; // data is 00 14
#include <iostream>
#include <fstream>
#pragma pack(push, 1)
struct vlan_header {
// uint16_t = 2bytes: - 16bits to work with
uint16_t PCP : 3, // bit(s) 0-2
DEI : 1, // bit(s) 3
ID : 12; // bit(s) 4-15
};
#pragma pack(pop)
int main() {
uint8_t* data; // sizeof(uint8_t) = 1byte - 8bits
vlan_header* vlanHeader;
data = new uint8_t[2];
std::ofstream log;
log.open( "results.txt" );
for ( unsigned i = 0; i < 256; i++ ) {
for ( unsigned j = 0; j < 256; j++ ) {
data[0] = j;
data[1] = i;
std::cout << "data[0] = " << static_cast<unsigned>(data[0]) << " ";
std::cout << "data[1] = " << static_cast<unsigned>(data[1]) << " ";
log << "data[0] = " << static_cast<unsigned>(data[0]) << " ";
log << "data[1] = " << static_cast<unsigned>(data[1]) << " ";
vlanHeader = reinterpret_cast<vlan_header*>(data);
std::cout << "PCP: " << std::hex << vlanHeader->PCP << " ";
std::cout << "DEI: " << std::hex << vlanHeader->DEI << " ";
std::cout << "ID: " << std::hex << vlanHeader->ID << std::endl;
log << "PCP: " << std::hex << vlanHeader->PCP << " ";
log << "DEI: " << std::hex << vlanHeader->DEI << " ";
log << "ID: " << std::hex << vlanHeader->ID << std::endl;
}
}
log.close();
delete[] data;
std::cout << "\nPress any key and enter to quit." << std::endl;
char q;
std::cin >> q;
return 0;
}
// Values are represented in hex
// For field member PCP: remember that 3 bits can only hold a max value of 7
// 8-bits 8-bits 3-bits 1-bit 12-bits
// data[0] data[1] PCP DEI ID
0x00 0x00 0 0 0
0x01 0x00 1 0 0
0x02 0x00 2 0 0
0x03 0x00 3 0 0
0x04 0x00 4 0 0
0x05 0x00 5 0 0
0x06 0x00 6 0 0
0x07 0x00 7 0 0 // PCP at max value since 3 bits only has 2^3 digit combinations
0x08 0x00 0 1 0
0x09 0x00 1 1 0
0x0a 0x00 2 1 0
0x0b 0x00 3 1 0
0x0c 0x00 4 1 0
0x0d 0x00 5 1 0
0x0e 0x00 6 1 0
0x0f 0x00 7 1 0 // the next iteration is where the bit carries into ID
0x10 0x00 0 0 1
// And this pattern repeats through out until ID has max value.
First Byte | Second Byte
data[0] | data[1]
data[n]: ([0][0][0]) ([0])-([0][0][0][0] | [0][0][0][0]-[0][0][0][0])
|
PCP DEI ID |
bitfield: ([0][0][0]) ([0])-([0][0][0][0] | [0][0][0][0]-[0][0][0][0])
Byte 1 Byte 2
data[0] = 0x40 data[1] = 0x01
[0][1][0][0] [0][0][0][0] | [0][0][0][0] [0][0][0][1]
Byte1 = Byte 2 =
============================|==========================
PCP DEI ID
0x00 0x00
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x01 0x00
[0][0][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x02 0x00
[0][1][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x03 0x00
[0][1][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x04 0x00
[1][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x05 | 0x00
[1][0][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x06 0x00
[1][1][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x07 0x00
[1][1][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x08 0x00
[0][0][0] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x09 0x00
[0][0][1] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0A 0x00
[0][1][0] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0B 0x00
[0][1][1] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0C 0x00
[1][0][0] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0D 0x00
[1][0][1] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0E 0x00
[1][1][0] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
0x0F 0x00
[1][1][1] [1] [0][0][0][0] | [0][0][0][0] [0][0][0][0]
// When we increment the hex value from 0x0F to 0x10 with a decimal value of 16
// this is where the overflow into the ID member happens and as of right now
// PCP has a max value of 7 and DEI has a max value of 1 where all bits are full.
// Watch what happens on the next iteration. Also note that we never gave any values
// to data[1] or byte 2 we only gave values to byte 1. This next value will
// populate a result into the bitfield's member ID.
0x10 0x00
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][1]
// then for the next iteration it'll be like this and so on...
0x11 0x00
[0][0][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][1]
0x12 0x00
[0][1][1] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][1]
// while this pattern continues we seen that `0x10` gave us a bit at the right end
// of member ID so lets look at values 0x20, 0x30 & 0x40 in the first byte
// if 0x10 =
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][0][1]
// then 0x20 should be
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][1][0]
// and 0x30 should be
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][0][1][1]
// finally 0x40 should be
[0][0][0] [0] [0][0][0][0] | [0][0][0][0] [0][1][0][0]
// This is all without touching byte.
// Remember we want both PCP & DEI to have values of 0 but we
// need a value of 0x16 or 20 in decimal in ID. Because of this
// overflow of bits due to the nature of bit fields, we can not
// just set the bytes directly with regular hex values as normal
// because member PCP only has 3 bits, member DEI has only 1, and
// the rest belong to ID. In order to get to the value we want
// we would have to iterate 0x40 all the way up to 0xFF before we would
// ever use byte 2 making it have a value of 0x01
// Another words: 0xFF 0x00 comes before 0x00 0x01 in this sequence
// bit patterns, but since we have the value of 0x40 already in the first
// byte of data[n] giving us a bit pattern of
[0][0][0] [0] [0][0][0] | [0][0][0][0] [0][1][0][0]
// what does making byte 2 with a value of 0x01 do to this pattern?
// It does this:
[0][0][0] [0] [0][0][0] | [0][0][0][1] [0][1][0][0]
// Okay so data[0] = 0x40 and data[1] = 0x01 so how does this
// give us the values of {0,0,20} or {0x00,0x00,0x14} ?
// Let's see from the far left going right the first 3 bits
// are PCP and all bits are 0 giving it a value of 0
// Next is the single bit for DEI which has a value of 0.
// Finally the next 12 bits are for ID and when we look at this 12 bit
// pattern we have [0][0][0][0] | [0][0][0][1] [0][1][0][0]
// Let's ignore the left 4 since they are all 0s or padding at this moment
// So we can see that [0][0][0][1] [0][1][0][0] = 0x14 in hex with a
// a decimal value of 20.
#include <iostream>
#include <bitset>
class VLANHeader
{
private:
// 000 0 000000000000
// PCP DEI ID
std::bitset<16> bin;
public:
VLANHeader(uint8_t byte1, uint8_t byte2) : bin(byte1 << 8 | byte2) {}
unsigned long getPCP() const { return (bin >> 13).to_ulong(); }
unsigned long getDEI() const { return ((bin >> 12) & std::bitset<16>(0x1)).to_ulong(); }
unsigned long getID() const { return (bin & std::bitset<16>(0xFFF)).to_ulong(); }
};
int main()
{
VLANHeader vh(0x00, 0x14);
std::cout << "PCP: " << vh.getPCP() << std::endl;
std::cout << "DEI: " << vh.getDEI() << std::endl;
std::cout << "ID: " << vh.getID() << std::endl;
system("pause");
return 0;
}