我有一个TCP服务器套接字，它接收一个16字节的请求消息。请求消息将具有多个字段，并且基于我需要进行不同操作的字段值。
当我尝试将缓冲区复制到struct时，我可以看到丢失的数据。我尝试了所有可能的方法，但无法弄清楚我是否需要进行结构填充。
我的结构看起来像，
struct stRequestMsg {
uint16_t startTag;
uint32_t messageSize;
uint16_t messageID;
uint16_t sequenceNumber;
uint16_t messageType;
uint32_t reserved;
};
我所做的只是，
char buff[1024]
result = static_cast<int>(recv(sockDesc, buff, sizeof(stRequestMsg), 0));
if (0 < result) {
printf("\n Actual value on buffer");
for (int i = 0; i < result; i++)
{
printf("\n buff[%d] = 0x%x", i,buff[i]);
}
reqMessage = *(stRequestMsg *)buff;
printf("\n RESULT of reqMessage = *(stRequestMsg *)buff;");
printf("\nstartTag : 0x%x", reqMessage.startTag);
printf("\nmessageSize : 0x%x", reqMessage.messageSize);
printf("\nmessageID : 0x%x", reqMessage.messageID);
printf("\nsequenceNumber : 0x%x", reqMessage.sequenceNumber);
printf("\nmessageType : 0x%x", reqMessage.messageType);
printf("\nreserved : 0x%x", reqMessage.reserved);
stRequestMsg hdr;
std::copy(&hdr, &hdr + 1, reinterpret_cast<stRequestMsg*>(buff));
printf("\n RESULT of std::copy(&hdr, &hdr + 1, reinterpret_cast<stRequestMsg*>(buff));");
printf("\nstartTag : 0x%x", reqMessage.startTag);
printf("\nmessageSize : 0x%x", reqMessage.messageSize);
printf("\nmessageID : 0x%x", reqMessage.messageID);
printf("\nsequenceNumber : 0x%x", reqMessage.sequenceNumber);
printf("\nmessageType : 0x%x", reqMessage.messageType);
printf("\nreserved : 0x%x", reqMessage.reserved);
memcpy(&reqMessage, buff, sizeof(stRequestMsg));
printf("\n RESULT of std::copy(&hdr, &hdr + 1, reinterpret_cast<stRequestMsg*>(buff));");
printf("\nstartTag : 0x%x", reqMessage.startTag);
printf("\nmessageSize : 0x%x", reqMessage.messageSize);
printf("\nmessageID : 0x%x", reqMessage.messageID);
printf("\nsequenceNumber : 0x%x", reqMessage.sequenceNumber);
printf("\nmessageType : 0x%x", reqMessage.messageType);
printf("\nreserved : 0x%x", reqMessage.reserved);
reqMessage = *reinterpret_cast<stRequestMsg*>(buff);
printf("\n RESULT of reqMessage = *reinterpret_cast<stRequestMsg*>(buff);");
printf("\nstartTag : 0x%x", reqMessage.startTag);
printf("\nmessageSize : 0x%x", reqMessage.messageSize);
printf("\nmessageID : 0x%x", reqMessage.messageID);
printf("\nsequenceNumber : 0x%x", reqMessage.sequenceNumber);
printf("\nmessageType : 0x%x", reqMessage.messageType);
printf("\nreserved : 0x%x", reqMessage.reserved);
}
我可以看到buff上收到的值是正确的，但是当我试图将整个缓冲区映射到struct时，我可以看到丢失的数据。我确信解决方案可能很简单，但我不知道问题出在哪里。
输出看起来像，
Actual value on buffer
buff[0] = 0x50
buff[1] = 0x0
buff[2] = 0x1e
buff[3] = 0x0
buff[4] = 0x0
buff[5] = 0x0
buff[6] = 0x31
buff[7] = 0x0
buff[8] = 0x1
buff[9] = 0x0
buff[10] = 0x2
buff[11] = 0x0
buff[12] = 0x1
buff[13] = 0x0
buff[14] = 0x0
buff[15] = 0x0
RESULT of reqMessage = *(stRequestMsg *)buff;
startTag : 0x50
messageSize : 0x310000
messageID : 0x1
sequenceNumber : 0x2
messageType : 0x1
reserved : 0x8a5c6da
RESULT of std::copy(&hdr, &hdr + 1, reinterpret_cast<stRequestMsg*>(buff));
startTag : 0x50
messageSize : 0x310000
messageID : 0x1
sequenceNumber : 0x2
messageType : 0x1
reserved : 0x8a5c6da
RESULT of memcpy(&reqMessage, buff, sizeof(stRequestMsg));
startTag : 0xf7ec
messageSize : 0x1e0050
messageID : 0x0
sequenceNumber : 0x31
messageType : 0x1
reserved : 0x1
RESULT of reqMessage = *reinterpret_cast<stRequestMsg*>(buff);
startTag : 0xf7ec
messageSize : 0x1e0050
messageID : 0x0
sequenceNumber : 0x31
messageType : 0x1
reserved : 0x1
但我的期望是
startTag = 0x5000;
messageSize = 0x1E000000;
messageID = 0x3100;
sequenceNumber = 0x100;
messageType = 0x200;
reserved = 0x1000000;
编辑：我尝试更改messageSize的变量大小并保留给uint16_t，并通过memcpy我收到的所有内容都正确无误。
答案 0 :(得分：6)
50 00 1E 00 00 00 31 00 01 00 02 00 01 00 00 00
如果我们相信您的第一个输出，那么这就是缓冲区中的内容。
struct stRequestMsg {
uint16_t startTag;
uint32_t messageSize;
uint16_t messageID;
uint16_t sequenceNumber;
uint16_t messageType;
uint32_t reserved;
};
您似乎假设此结构定义如此映射到内存中：
50 00|1E 00 00 00|31 00|01 00|02 00|01 00 00 00
start|messagesize|msgID|seqNm|msgTp|reserved...
在您的特定情况下，情况并非如此
50 00|1E 00|00 00 31 00|01 00|02 00|01 00|00 00|GG GG GG GG
start|xx xx|messagesize|msgID|seqNm|msgTp|xx xx|reserved...
其中GG表示垃圾（因为它位于缓冲区之外）。结构在内存中这样布局的原因是结构填充：结构的字段根据它们的对齐要求在内存中布局。对于int（可能是uint32_t），通常为4个字节，这意味着这样的成员只会以4的倍数偏移量开始。为此，编译器会插入填充。因此，您的结构实际上看起来像：
struct stRequestMsg {
uint16_t startTag;
char xxxxPad1[2];
uint32_t messageSize;
uint16_t messageID;
uint16_t sequenceNumber;
uint16_t messageType;
char xxxxPad2[2];
uint32_t reserved;
};
您遇到的另一个问题是字节顺序：您的系统似乎使用 little endian ，而来自网络的数据来自 big endian （也是网络字节顺序，所以这是一件好事）。
这就是startTag == 0x0050代替0x5000的原因。作为小端，您的系统假定第一个字节保持最低位，而不是最高位。
要做到这一点，你应该在序列化和反序列化上查找一些资源......
反序列化示例（C中的，但应该可以很容易地调整为C ++。抱歉，写作时没有注意标记，我看到printf并假设C：D < / em>的）：
#include <stdio.h>
#include <stdint.h>
struct stRequestMsg {
uint16_t startTag;
uint32_t messageSize;
uint16_t messageID;
uint16_t sequenceNumber;
uint16_t messageType;
uint32_t reserved;
};
void printIt(struct stRequestMsg m) {
printf("{\n"
" .startTag = %#x;\n"
" .messageSize = %#x;\n"
" .messageID = %#x;\n"
" .sequenceNumber = %#x;\n"
" .messageType = %#x;\n"
" .reserved = %#x;\n"
"}\n",
m.startTag, m.messageSize, m.messageID,
m.sequenceNumber, m.messageType, m.reserved);
}
uint16_t deserialize_uint16(char const * const b) {
return ((uint16_t) b[0] << 8u) |
((uint16_t) b[1]);
}
uint32_t deserialize_uint32(char const * const b) {
return ((uint16_t) b[0] << 24u) |
((uint16_t) b[1] << 16u) |
((uint16_t) b[2] << 8u) |
((uint16_t) b[3]);
}
struct stRequestMsg deserialize(char const * const b) {
struct stRequestMsg r;
r.startTag = deserialize_uint16(b);
r.messageSize = deserialize_uint32(b + 2);
r.messageID = deserialize_uint16(b + 6);
r.sequenceNumber = deserialize_uint16(b + 8);
r.messageType = deserialize_uint16(b + 10);
r.reserved = deserialize_uint32(b + 12);
return r;
}
int main(void) {
char buff[16];
buff[0] = 0x50; buff[1] = 0x00;
buff[2] = 0x1E; buff[3] = 0x00;
buff[4] = 0x00; buff[5] = 0x00;
buff[6] = 0x31; buff[7] = 0x00;
buff[8] = 0x10; buff[9] = 0x00;
buff[10] = 0x20; buff[11] = 0x00;
buff[12] = 0x10; buff[13] = 0x00;
buff[14] = 0x00; buff[15] = 0x00;
struct stRequestMsg msg = deserialize(&(buff[0]));
printIt(msg);
return 0;
}