fseek.c

ReactOS是一些高手根据Windows XP的内核编写出的类XP。内核实现机理和API函数调用几乎相同。甚至可以兼容XP的程序。喜欢研究系统内核的人可以看一看。

/* Copyright (C) 1997 DJ Delorie, see COPYING.DJ for details */
/* Copyright (C) 1996 DJ Delorie, see COPYING.DJ for details */
/* Copyright (C) 1995 DJ Delorie, see COPYING.DJ for details */

#include <precomp.h>

/*
 * @implemented
 */
int fseek(FILE *f, long offset, int ptrname)
{
  long p = -1;			/* can't happen? */
  if ( f == NULL ) {
	__set_errno (EINVAL);
       return -1;
  }

  f->_flag &= ~_IOEOF;
  if (!OPEN4WRITING(f))
  {
    if (f->_base && !(f->_flag & _IONBF))
    {
      p = ftell(f);
      if (ptrname == SEEK_CUR)
      {
	offset += p;
	ptrname = SEEK_SET;
      }
      /* check if the target position is in the buffer and
        optimize seek by moving inside the buffer */
      if (ptrname == SEEK_SET && (f->_flag & (_IOUNGETC|_IOREAD|_IOWRT )) == 0
      && p-offset <= f->_ptr-f->_base && offset-p <= f->_cnt)
      {
        f->_ptr+=offset-p;
        f->_cnt+=p-offset;
        return 0;
      }
    }

    p = _lseek(_fileno(f), offset, ptrname);
    f->_cnt = 0;
    f->_ptr = f->_base;
    f->_flag &= ~_IOUNGETC;
  }
  else
  {
    p = fflush(f);
    return _lseek(_fileno(f), offset, ptrname) == -1 || p == EOF ?
      -1 : 0;
  }
  return p==-1 ? -1 : 0;
}