c-根据字节数计算一个整数会得到奇怪的错误结果

Question

我正试图从以小端序格式原始存储的数据文件中读取一个整数。一旦我得到了相应的字节，我最初通过将数字乘以其权重来计算整数值（下面的算术方法），但由于某种原因，该值在最高有效字节上一直相差一个单位。

其他方法似乎有效，但我想知道为什么在使用以下代码时结果是错误的。

#include <stdio.h>
#include <stdint.h>
void main(){
  //Two bytes that form a 16-bit integer (Little endian)
  char b[2] = {0xBD, 0x74};
  //This gives a correct answer (Shift + Extra masking)
  uint16_t n_correct;
  n_correct = (b[0] & 0xFF) + ((b[1]<<8) & 0xFF00);
  //This should give a correct answer but doesn't (Shifting method)
  uint16_t n_incorrect;
  n_incorrect = b[0] + (b[1]<<8);
  //This should also give a correct answer but doesn't (Aritmetic)
  uint16_t n_arith;
  n_arith = b[0] + (b[1]*256);
  //This works, on little endian machines. Dirty but works. (Hack)
  uint16_t n_hack;
  uint8_t* n_ptr = (uint8_t*)&n_hack;
  n_ptr[0] = b[0];
  n_ptr[1] = b[1];
  printf("Shifting method: %X == %X%X?n", n_incorrect, b[1]&0xFF, b[0]&0xFF);
  printf("Shift + Masking: %X == %X%X?n", n_correct, b[1]&0xFF, b[0]&0xFF);
  printf("     Arithmetic: %X == %X%X?n", n_arith, b[1]&0xFF, b[0]&0xFF);
  printf("           Hack: %X == %X%X?n", n_hack, b[1]&0xFF, b[0]&0xFF);
}

输出为：

Shifting method: 73BD == 74BD?
Shift + Masking: 74BD == 74BD?
     Arithmetic: 73BD == 74BD?
           Hack: 74BD == 74BD?

正如你所看到的，使用简单的移位或乘法会给出错误的答案。为什么？

Answer 1

我已经做了一百次了。更改：

char b[2] = {0xBD, 0x74};

至

unsigned char b[2] = {0xBD, 0x74};

或者，更好的

uint8_t b[2] = {0xBD, 0x74};

注意，char可以超过8位（我在一个32位字符大小的系统上工作）

Answer 2

要进一步探索这个问题，请尝试以下值：

 char b[2] = {0xBD, 0x01};
 char b[2] = {0xBD, 0x00};