具有位域的结构的内存布局 [英] Memory layout of struct having bitfields
问题描述
我有这个 C 结构:(代表一个 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];
};
我给它赋值,然后像这样以 16 位字打印它的内存布局:
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("
");
}
}
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;
}
我得到这个输出:
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:
输出部分正确;在某处,字节和半字节似乎可以互换.这里有字节序问题吗?位域不适合这个目的吗?我真的不知道.有什么帮助吗?提前致谢!
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
不,位域不适用于此目的.布局依赖于编译器.
No, bitfields are not good for this purpose. The layout is compiler-dependant.
在要控制结果布局的数据中使用位域通常不是一个好主意,除非您有(特定于编译器的)方法,例如 #pragma
s,可以这样做.
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 #pragma
s, to do so.
最好的方法可能是在没有位域的情况下实现这一点,即通过自己执行所需的按位运算.这很烦人,但比以某种方式挖掘解决此问题的方法要容易得多.此外,它与平台无关.
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.
将标头定义为一个 16 位字的数组,然后您就可以很容易地计算校验和.
Define the header as just an array of 16-bit words, and then you can compute the checksum easily enough.
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