【C语言】Traps in C Bitfield

Quan略略略

已于 2024-03-15 15:23:59 修改

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文章标签： c语言算法开发语言

于 2024-03-15 15:08:46 首次发布

本文链接：https://blog.youkuaiyun.com/tq384998430/article/details/136740033

版权

本文介绍了在C语言中尝试向位字段写入超过其位数的整数时，会导致溢出并丢失部分数据的现象。示例代码展示了如何因2写入一个1位的bitfielda而丢失最低有效位（LSB）。

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Trying to write an overlength value to a bitfield:

#include <stdio.h>

struct test
{
    char a:1;
    char b:1;
} test1;

int main() {
    printf("%d\n", sizeof(struct test));
    printf("%.2x\n", *(char *)&test1);
    test1.a = 2;
    test1.b = 1;
    printf("%.2x\n", *(char *)&test1);
    return 0;
}

The result is:

/tmp/fo0ZfcmKnf.c: In function 'main':
/tmp/fo0ZfcmKnf.c:14:11: warning: overflow in conversion from 'int' to 'signed char:1' changes value from '2' to '0' [-Woverflow]
   14 | test1.a = 2;
      |           ^
/tmp/fo0ZfcmKnf.o
1
00
02

The write operation to bitfield "a" is lost as the LSB of number "2" is 0b. Therefore, when writing an oversized number to a bitfield, only the equivalent length of bits of the bitfield will be written. Just like writing 0xFFF to an unsigned char.