c - 字节级长度说明

Question

我有一个协议，它需要一个长达 32 位的长度字段，并且必须在运行时生成它来描述给定数据包中有多少字节。

下面的代码有点难看，但我想知道是否可以将其重构为更高效或更易于理解。问题是代码只会生成足够的字节来描述数据包的长度，所以小于 255 字节 = 1 字节长度，小于 65535 = 2 字节长度等等......

{
    extern char byte_stream[];
    int bytes = offset_in_packet;
    int n = length_of_packet;
   /* Under 4 billion, so this can be represented in 32 bits. */
    int t;
   /* 32-bit number used for temporary storage. */

    /* These are the bytes we will break up n into. */
    unsigned char first, second, third, fourth;

    t = n & 0xFF000000;
    /* We have used AND to "mask out" the first byte of the number. */
    /* The only bits which can be on in t are the first 8 bits. */
    first = t >> 24;
    if (t)  {
        printf("byte 1: 0x%02x\n",first );
        byte_stream[bytes] = first; bytes++;
        write_zeros = 1;
    }
    /* Now we shift t so that it is between 0 and 255. This is the first, highest byte of n. */
    t = n & 0x00FF0000;
    second = t >> 16;
    if (t || write_zeros) {
        printf("byte 2: 0x%02x\n", second );
        byte_stream[bytes] = second; bytes++;
        write_zeros = 1;
    }

    t = n & 0x0000FF00;
    third = t >> 8;
    if ( t || write_zeros) {
        printf("byte 3: 0x%02x\n", third );
        byte_stream[bytes] = third; bytes++;
        write_zeros = 1;
    }

    t = n & 0x000000FF;
    fourth = t;
    if (t || write_zeros) {
        printf("byte 4: 0x%02x\n", fourth);
        byte_stream[bytes] = fourth; bytes++;
    }
}

Answer 1

你真的应该为你的长度使用一个固定宽度的字段。

• 当接收端的程序必须读取数据包的长度字段时，它如何知道长度停止在哪里？
• 如果数据包的长度可能达到 4 GB，那么 1-3 字节的开销真的很重要吗？
• 你看到你的代码已经变得多么复杂了吗？

Answer 2

试试这个循环：

{
    extern char byte_stream[];
    int bytes = offset_in_packet;
    int n = length_of_packet; /* Under 4 billion, so this can be represented in 32 bits. */
    int t; /* 32-bit number used for temporary storage. */
    int i;

    unsigned char curByte;

    for (i = 0; i < 4; i++) {
        t = n & (0xFF000000 >> (i * 16));

        curByte = t >> (24 - (i * 8));
        if (t || write_zeros)  {
            printf("byte %d: 0x%02x\n", i, curByte );
            byte_stream[bytes] = curByte;
                            bytes++;
            write_zeros = 1;
        }

    }

}

Answer 3

我不确定我是否理解你的问题。你到底想计算什么？如果我理解正确，您正在尝试查找最重要的非零字节。
你可能最好使用这样的循环：

int i;  
int write_zeros = 0;  
for (i = 3; i >=0 ; --i) {  
    t = (n >> (8 * i)) & 0xff;  
    if (t || write_zeros) {  
        write_zeros = 1;  
        printf ("byte %d : 0x%02x\n", 4-i, t);  
        byte_stream[bytes++] = t;
    }  
}

Answer 4

实际上，您只进行了四次计算，因此在这里可读性似乎比效率更重要。我使这样的东西更具可读性的方法是

1. 将通用代码提取到函数中
2. 将类似的计算放在一起，使模式更明显
3. 摆脱中间变量 print_zeroes 并明确输出字节的情况，即使它们为零（即前面的字节非零）

我已将随机代码块更改为一个函数并更改了一些变量（下划线在降价预览屏幕中给我带来了麻烦）。我还假设正在传入字节，并且传入它的任何人都会向我们传递一个指针，以便我们可以修改它。

这是代码：

/* append byte b to stream, increment index */
/* really needs to check length of stream before appending */
void output( int i, unsigned char b, char stream[], int *index )
{
    printf("byte %d: 0x%02x\n", i, b);
    stream[(*index)++] = b;
}


void answer( char bytestream[], unsigned int *bytes, unsigned int n)
{
    /* mask out four bytes from word n */
    first  = (n & 0xFF000000) >> 24;
    second = (n & 0x00FF0000) >> 16;
    third  = (n & 0x0000FF00) >>  8;
    fourth = (n & 0x000000FF) >>  0;

    /* conditionally output each byte starting with the */
    /* first non-zero byte */
    if (first) 
       output( 1, first, bytestream, bytes);

    if (first || second) 
       output( 2, second, bytestream, bytes);

    if (first || second || third) 
       output( 3, third, bytestream, bytes);

    if (first || second || third || fourth) 
       output( 4, fourth, bytestream, bytes);
 }

对最后四个 if 语句的修改将更加高效，也许更容易理解：

    if (n>0x00FFFFFF) 
       output( 1, first, bytestream, bytes);

    if (n>0x0000FFFF) 
       output( 2, second, bytestream, bytes);

    if (n>0x000000FF)  
       output( 3, third, bytestream, bytes);

    if (1) 
       output( 4, fourth, bytestream, bytes);

但是，我同意压缩此字段会使接收状态机过于复杂。但是如果你不能改变协议，这段代码就更容易阅读了。