c - 简单的位操作失败

Question

我正在学习 C 中的位操作，并且编写了一个简单的程序。但是程序失败了。有人可以查看这段代码吗？基本上，我想提取一个 4 字节的“长”变量并将其重新组合到它的单个字节中，反之亦然。这是我的代码:

    printf("sizeof char= %d\n", sizeof(char));
    printf("sizeof unsigned char= %d\n", sizeof(unsigned char));
    printf("sizeof int= %d\n", sizeof(int));
    printf("sizeof long= %d\n", sizeof(long));
    printf("sizeof unsigned long long= %d\n", sizeof(unsigned long long));

    long val = 2;
    int k = 0;
    size_t len = sizeof(val);
    printf("val = %ld\n", val);
    printf("len = %d\n", len);

    char *ptr;
    ptr = (char *)malloc(sizeof(len));
    //converting 'val' to char array
    //val = b3b2b1b0 //where 'b is 1 byte. Since 'long' is made of 4 bytes, and char is 1 byte, extracting byte by byte of long into char
    //do{
        //val++;
    for(k = 0; k<len; k++){
        ptr[k] = ((val >> (k*len)) && 0xFF);
        printf("ptr[%d] = %02X\n", k,ptr[k]);
    }
    //}while(val < 12);

    //reassembling the bytes from char and converting them to long
    long xx = 0;
    int m = 0;
    for(m = 0; m< len; m++){
        xx = xx |(ptr[m]<<(m*8));
    }
    printf("xx= %ld\n", xx);

为什么我没有看到 xx 返回 2？此外，无论 'val' 的值如何，ptr[0] 似乎存储 1 :(请帮忙

提前致谢

Answer 1

ptr[k] = ((val >> (k*len)) && 0xFF);

应该是

ptr[k] = ((val >> (k*8)) & 0xFF);

&& 用于条件语句，& 用于按位与。此外，当您将值拆分为字符时，循环的每次迭代都希望移动与字节中一样多的位。这几乎总是 8，但也可以是其他值。头文件 limits.h 包含相关信息。