其结构位域的内存布局 [英] Memory layout of struct having bitfields

问题描述

我有这样的C结构：（重新presenting一个IP数据报）

I have this C struct: (representing an IP datagram)

struct ip_dgram
{
    unsigned int ver   : 4;
    unsigned int hlen  : 4;
    unsigned int stype : 8;
    unsigned int tlen  : 16;
    unsigned int fid   : 16;
    unsigned int flags : 3;
    unsigned int foff  : 13;
    unsigned int ttl   : 8;
    unsigned int pcol  : 8;
    unsigned int chksm : 16;
    unsigned int src   : 32;
    unsigned int des   : 32;
    unsigned char opt[40];
};

我赋值给它，然后打印在16位字的内存布局是这样的：

I'm assigning values to it, and then printing its memory layout in 16-bit words like this:

//prints 16 bits at a time
void print_dgram(struct ip_dgram dgram)
{
    unsigned short int* ptr = (unsigned short int*)&dgram;
    int i,j;
    //print only 10 words
    for(i=0 ; i<10 ; i++)
    {
        for(j=15 ; j>=0 ; j--)
        {
            if( (*ptr) & (1<<j) ) printf("1");
            else printf("0");
            if(j%8==0)printf(" ");
        }
        ptr++;
        printf("\n");
    }
}

int main()
{
    struct ip_dgram dgram;

    dgram.ver   = 4;
    dgram.hlen  = 5;
    dgram.stype = 0;
    dgram.tlen  = 28;
    dgram.fid   = 1;
    dgram.flags = 0;
    dgram.foff  = 0;
    dgram.ttl   = 4;
    dgram.pcol  = 17;
    dgram.chksm = 0;
    dgram.src   = (unsigned int)htonl(inet_addr("10.12.14.5"));
    dgram.des   = (unsigned int)htonl(inet_addr("12.6.7.9"));

    print_dgram(dgram);

    return 0;
}

我得到这样的输出：

I get this output:

00000000 01010100 
00000000 00011100 
00000000 00000001 
00000000 00000000 
00010001 00000100 
00000000 00000000 
00001110 00000101 
00001010 00001100 
00000111 00001001 
00001100 00000110

但我希望这样的：

But I expect this:

输出是部分正确的;某处，字节和半字节似乎互换。是有一些问题的字节顺序在这里？有位字段用于此目的不是很好吗？我真的不知道。任何帮助吗？在此先感谢！

The output is partially correct; somewhere, the bytes and nibbles seem to be interchanged. Is there some endianness issue here? Are bit-fields not good for this purpose? I really don't know. Any help? Thanks in advance!

推荐答案

没有，位域是不利于这一目的。布局是编译器相关的。

No, bitfields are not good for this purpose. The layout is compiler-dependant.

这是一般不要使用要控制得到的布局，除非你有数据位域是一个好主意（编译器特定的）手段，如的#pragma S，这样做。

It's generally not a good idea to use bitfields for data where you want to control the resulting layout, unless you have (compiler-specific) means, such as #pragmas, to do so.

最好的办法可能是做自己所需要的位操作来实现这一点没有位域，即。这很烦人，但这样更容易比莫名其妙地挖了一个方法来解决这个问题。此外，它是平台无关的。

The best way is probably to implement this without bitfields, i.e. by doing the needed bitwise operations yourself. This is annoying, but way easier than somehow digging up a way to fix this. Also, it's platform-independent.

定义页眉刚才的16位字的数组，然后你就可以计算校验和很轻松了。

Define the header as just an array of 16-bit words, and then you can compute the checksum easily enough.

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