C：从字符数组复制到int

Question

  // Works
  int fnamesize=0;
  fnamesize=message[0]<<24;
  fnamesize+=message[1]<<16;
  fnamesize+=message[2]<<8;
  fnamesize+=message[3];

  // Doesn't work
  int fsize;
  memcpy(&fsize,message,sizeof(int));

Can someone explain why the second one doesn't work? The memory I'm copying from, message is a char * . When I try to test the values of fnamesize and fsize, like printf("fsize is %d,fnamesize is %d",fsize,fnamesize); , the fsize gives an unexpected value, but fnamesize gives the value I expect.

Thoughts?

Answer 1

That's because of endianess , which means the layout of bytes in an int.

In windows, the second way will give you an int that has the opposite byte order, like this:

fsize=message[3]<<24;
fsize+=message[2]<<16;
fsize+=message[1]<<8;
fsize+=message[0];

Answer 2

尝试在有效的代码中更改数组索引的顺序，将结果与无效的代码进行比较，然后查找术语“大端”和“小端”。