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我正在尝试从以小端格式原始存储的数据文件中读取整数。一旦我得到相应的字节，我最初通过将数字乘以它们的权重（下面的算术方法）来计算整数值，但由于某种原因，该值总是在最高有效字节上偏离一个单位。

其他方法似乎有效，但我想知道为什么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);
}

输出是：

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

如您所见，使用简单的移位或乘法会给出错误的答案。为什么？

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2 回答 2

3

我已经这样做了一百次了。改变：

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

至

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

或者，更好的是

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

请注意，char可以超过 8 位（我在一个 32 位字符大小的系统上工作）

于 2013-09-04T02:44:02.963 回答
0

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};
于 2013-09-04T04:07:19.360 回答