如何更改 unsigned char 中的 4 位？

发布于 2024-10-13 00:46:09 字数 458 浏览 5 评论 0原文

unsigned char *adata = (unsigned char*)malloc(500*sizeof(unsigned char));
unsigned char *single_char = adata+100;

如何更改 single_char 中的前四位以表示 1..10 (int) 之间的值？

问题来自TCP头结构：

Data Offset: 4 bits 

The number of 32 bit words in the TCP Header.  This indicates where
the data begins.  The TCP header (even one including options) is an
integral number of 32 bits long.

通常它的值为4..5，字符值类似于0xA0。

原文

unsigned char *adata = (unsigned char*)malloc(500*sizeof(unsigned char));
unsigned char *single_char = adata+100;

How do I change first four bits in single_char to represent values between 1..10 (int)?

The question came from TCP header structure:

Data Offset: 4 bits 

The number of 32 bit words in the TCP Header.  This indicates where
the data begins.  The TCP header (even one including options) is an
integral number of 32 bits long.

Usually it has value of 4..5, the char value is like 0xA0.

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稚气少女 2024-10-20 00:46:09

这些假设您已将 *single_char 初始化为某个值。否则，咖啡馆发布的解决方案可以满足您的需要。

<代码>(*single_char) = ((*single_char) & 0xF0) | val;

(*single_char) & 11110000 -- 将低 4 位重置为 0
| val -- 将最后 4 位设置为值（假设 val < 16）

如果您想访问最后 4 位，可以使用
unsigned char v = (*single_char) & 0x0F;

如果你想访问高 4 位，你可以将掩码向上移动 4 位，即。

unsigned char v = (*single_char) & 0xF0;

并设置它们：

(*single_char) = ((*single_char) & 0x0F) | (val << 4);

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白鸥掠海 2024-10-20 00:46:09

这会将 *single_char 的高 4 位设置为数据偏移量，并清除低 4 位：

unsigned data_offset = 5; /* Or whatever */

if (data_offset < 0x10)
    *single_char = data_offset << 4;
else
    /* ERROR! */

This will set the high 4 bits of *single_char to the data offset, and clear the lower 4 bits:

unsigned data_offset = 5; /* Or whatever */

if (data_offset < 0x10)
    *single_char = data_offset << 4;
else
    /* ERROR! */

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潦草背影 2024-10-20 00:46:09

您可以使用按位运算符来访问各个位并根据您的要求进行修改。

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情释 2024-10-20 00:46:09

我知道这是一篇旧文章，但我不希望其他人阅读有关按位运算符的长文章来获得与这些类似的函数 -

//sets b as the first 4 bits of a(this is the one you asked for
void set_h_c(unsigned char *a, unsigned char b)
{
    (*a) = ((*a)&15) | (b<<4);
}

//sets b as the last 4 bits of a(extra)
void set_l_c(unsigned char *a, unsigned char b)
{
    (*a) = ((*a)&240) | b;
}

希望它对将来的人有所帮助

I know this is an old post but I don't want others to read long articles on bitwise operators to get a function similar to these -

//sets b as the first 4 bits of a(this is the one you asked for
void set_h_c(unsigned char *a, unsigned char b)
{
    (*a) = ((*a)&15) | (b<<4);
}

//sets b as the last 4 bits of a(extra)
void set_l_c(unsigned char *a, unsigned char b)
{
    (*a) = ((*a)&240) | b;
}

Hope it helps someone in the future

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