C语言中的fread如何工作? [英] how fread works in C?

问题描述

我有一个文本文件，其内容为:

I have text file with content as:

12345678901222344567

然后我使用此代码读取内容:

Then I used this code to read the content:

FILE * pFile;
  int c;
  char buffer [256];
  pFile = fopen ("myfile.txt","r");
  int a[50] = {0};
  fread(a, sizeof(a[0]), 5, pFile);
  fclose(pFile);
  for (c = 0; c < 5; c++)
  {
    printf("%d\n", a[c]);
  }

我得到了结果:

我无法解释自己为什么得到这样的结果.

I cannot explain myself why I got such results.

推荐答案

文本文件与二进制文件

您在应该用于二进制文件的文本文件上误用了fread函数. 文本文件与

You misuse the fread function on a text file which is supposed to be used on a binary file. A text file is different with a binary file.

假设，如果您保存带有四个 ASCII字符的txt "1"，"2" ，"3"，"4"，实际上存储在磁盘上的不是1，2，3，4.而是它们的ACSII代码:49，50，51，52. 因此，当您将sizeof(a[0])字节读入a[0]时，将在内存中读回四个字节，如:

Let's say, if you save txt with four ASCII characters "1", "2", "3", "4", what's actually stored on disk is not 1, 2, 3, 4. Instead, it's their ACSII codes: 49, 50, 51, 52. So when you read sizeof(a[0]) bytes into a[0], four bytes are read back in memory like:

Memory Address    Byte In Memory        Little-endian
  0x12345678        00110001     <----least significant byte
  0x12345679        00110010
  0x12345680        00110011
  0x12345681        00110100     <----most significant byte

(请注意，该地址只是伪造的，用于说明目的)

(Note that address is just faked for illustration)

字节序

关于如何将这四个字节转换为整数，这全都与 Endianness .简而言之，如果您的平台是little-endian(例如Intel x86处理器)，则最低内存地址(0x12345678)上的第一个字节(00110001)将是形成整数的最低有效字节.

As to how these four bytes are converted into an integer, it all has something to do with Endianness. In short, if your platform is little-endian(e.g., Intel x86 processor), the first byte(00110001) on the lowest memory address(0x12345678) will be the least significant byte to form the integer.

最终，整数以二进制形式为00110100001100110011001000110001，即以十进制形式为875770417.

Eventually, the integer is 00110100001100110011001000110001 in binary, that's 875770417 in decimal.

这篇关于C语言中的fread如何工作?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！