Linux C程序中函数strcpy的奇怪分段错误

Question

Here is my source code:

char *
cpy_strcpy (dest, src)
     char *dest;
     const char *src;
{
  char c;
  char *s = (char *) src;
  const ptrdiff_t off = dest - s - 1;

  do
    {
       //c = *s++
       //s[off] = c;
       s[off] = *s;
    }
  while (*s++ != '\0');
  //while( c != '\0' );
  return dest;
}

I got this when i using gdb debug it:

(gdb) s
26    while (*s++ != '\0');
(gdb) 
27    return dest;
(gdb) 
28  }
(gdb) 

Program received signal SIGSEGV, Segmentation fault.
0x000000000040050a in cpy_strcpy (dest=can't compute CFA for this frame
) at strcpy.c:28
28  }

This source code was copied from glibc but i changed some line(with comment //)

I really can't figure out what is wrong with my new code. anyone can help me?

Thanks in advance!

Answer 1

s[off] = *s is wrong in the first loop.

s[off] points the address s + off , which in the first loop is:

s + off = s + (dest - s - 1) = dest - 1

The address (dest - 1) is out of the left bound of char array.

The original code is right, because c = *s++ can firstly assign c with s, then increase s, which makes s[off] in s[off] = c just points to dest , not dest - 1 .