Question

I have text file with content as:

12345678901222344567

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]);
  }

and I got the results:

I cannot explain myself why I got such results.

Answer 1

Text file vs. Binary file

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.

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

(Note that address is just faked for illustration)

Endianness

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.

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