freopen.c

标准c库代码,可以应用于各个系统提供了大量的基本函数

/*
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that the above copyright notice and this paragraph are
 * duplicated in all such forms and that any documentation,
 * advertising materials, and other materials related to such
 * distribution and use acknowledge that the software was developed
 * by the University of California, Berkeley.  The name of the
 * University may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

/*
FUNCTION
<<freopen>>---open a file using an existing file descriptor

INDEX
	freopen

ANSI_SYNOPSIS
	#include <stdio.h>
	FILE *freopen(const char *<[file]>, const char *<[mode]>,
		      FILE *<[fp]>);

TRAD_SYNOPSIS
	#include <stdio.h>
	FILE *freopen(<[file]>, <[mode]>, <[fp]>)
	char *<[file]>;
	char *<[mode]>;
	FILE *<[fp]>;

DESCRIPTION
Use this variant of <<fopen>> if you wish to specify a particular file
descriptor <[fp]> (notably <<stdin>>, <<stdout>>, or <<stderr>>) for
the file.

If <[fp]> was associated with another file or stream, <<freopen>>
closes that other file or stream (but ignores any errors while closing
it).

<[file]> and <[mode]> are used just as in <<fopen>>.

RETURNS
If successful, the result is the same as the argument <[fp]>.  If the
file cannot be opened as specified, the result is <<NULL>>.

PORTABILITY
ANSI C requires <<freopen>>.

Supporting OS subroutines required: <<close>>, <<fstat>>, <<isatty>>,
<<lseek>>, <<open>>, <<read>>, <<sbrk>>, <<write>>.
*/

#include <time.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include "local.h"

/*
 * Re-direct an existing, open (probably) file to some other file.
 */

FILE *
_DEFUN (freopen, (file, mode, fp),
	_CONST char *file _AND
	_CONST char *mode _AND
	register FILE *fp)
{
  register int f;
  int isopen, flags, oflags, e;
  struct _reent *ptr;

  CHECK_INIT (fp);
  ptr = fp->_data;

  if ((flags = __sflags (ptr, mode, &oflags)) == 0)
    {
      (void) fclose (fp);
      return NULL;
    }

  /*
   * Remember whether the stream was open to begin with, and
   * which file descriptor (if any) was associated with it.
   * If it was attached to a descriptor, defer closing it,
   * so that, e.g., freopen("/dev/stdin", "r", stdin) works.
   * This is unnecessary if it was not a Unix file.
   */

  if (fp->_flags == 0)
    {
      fp->_flags = __SEOF;	/* hold on to it */
      isopen = 0;
    }
  else
    {
      if (fp->_flags & __SWR)
	(void) fflush (fp);
      /* if close is NULL, closing is a no-op, hence pointless */
      isopen = fp->_close != NULL;
      if (fp->_file < 0 && isopen)
	{
	  (void) (*fp->_close) (fp->_cookie);
	  isopen = 0;
	}
    }

  /*
   * Now get a new descriptor to refer to the new file.
   */

  f = _open_r (ptr, (char *) file, oflags, 0666);
  if (f < 0 && isopen)
    {
      /*
       * May have used up all descriptors, so close the old
       * and try again.
       */
      (void) (*fp->_close) (fp->_cookie);
      isopen = 0;
      f = _open_r (ptr, (char *) file, oflags, 0666);
    }
  e = ptr->_errno;

  /*
   * Finish closing fp.  Even if the open succeeded above,
   * we cannot keep fp->_base: it may be the wrong size.
   * This loses the effect of any setbuffer calls,
   * but stdio has always done this before.
   */

  if (isopen)
    (void) (*fp->_close) (fp->_cookie);
  if (fp->_flags & __SMBF)
    _free_r (ptr, (char *) fp->_bf._base);
  fp->_w = 0;
  fp->_r = 0;
  fp->_p = NULL;
  fp->_bf._base = NULL;
  fp->_bf._size = 0;
  fp->_lbfsize = 0;
  if (HASUB (fp))
    FREEUB (fp);
  fp->_ub._size = 0;
  if (HASLB (fp))
    FREELB (fp);
  fp->_lb._size = 0;

  if (f < 0)
    {				/* did not get it after all */
      fp->_flags = 0;		/* set it free */
      ptr->_errno = e;		/* restore in case _close clobbered */
      return NULL;
    }

  fp->_flags = flags;
  fp->_file = f;
  fp->_cookie = (_PTR) fp;
  fp->_read = __sread;
  fp->_write = __swrite;
  fp->_seek = __sseek;
  fp->_close = __sclose;
  return fp;
}