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/*

 * 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

<<fseek>>---set file position

INDEX

        fseek

ANSI_SYNOPSIS

        #include <stdio.h>

        int fseek(FILE *<[fp]>, long <[offset]>, int <[whence]>)

TRAD_SYNOPSIS

        #include <stdio.h>

        int fseek(<[fp]>, <[offset]>, <[whence]>)

        FILE *<[fp]>;

        long <[offset]>;

        int <[whence]>;

DESCRIPTION

Objects of type <<FILE>> can have a ``position'' that records how much

of the file your program has already read.  Many of the <<stdio>> functions

depend on this position, and many change it as a side effect.

You can use <<fseek>> to set the position for the file identified by

<[fp]>.  The value of <[offset]> determines the new position, in one

of three ways selected by the value of <[whence]> (defined as macros

in `<<stdio.h>>'):

<<SEEK_SET>>---<[offset]> is the absolute file position (an offset

from the beginning of the file) desired.  <[offset]> must be positive.

<<SEEK_CUR>>---<[offset]> is relative to the current file position.

<[offset]> can meaningfully be either positive or negative.

<<SEEK_END>>---<[offset]> is relative to the current end of file.

<[offset]> can meaningfully be either positive (to increase the size

of the file) or negative.

See <<ftell>> to determine the current file position.

RETURNS

<<fseek>> returns <<0>> when successful.  If <<fseek>> fails, the

result is <<EOF>>.  The reason for failure is indicated in <<errno>>:

either <<ESPIPE>> (the stream identified by <[fp]> doesn't support

repositioning) or <<EINVAL>> (invalid file position).

PORTABILITY

ANSI C requires <<fseek>>.

Supporting OS subroutines required: <<close>>, <<fstat>>, <<isatty>>,

<<lseek>>, <<read>>, <<sbrk>>, <<write>>.

*/

#include <stdio.h>

#include <time.h>

#include <fcntl.h>

#include <stdlib.h>

#include <errno.h>

#include <sys/stat.h>

#include "local.h"

#define POS_ERR (-(fpos_t)1)

/*

 * Seek the given file to the given offset.

 * `Whence' must be one of the three SEEK_* macros.

 */

int

fseek (fp, offset, whence)

     register FILE *fp;

     long offset;

     int whence;

{

  struct _reent *ptr;

  fpos_t _EXFUN ((*seekfn), (void *, fpos_t, int));

  fpos_t target, curoff;

  size_t n;

  struct stat st;

  int havepos;

  /* Make sure stdio is set up.  */

  CHECK_INIT (fp);

  ptr = fp->_data;

  /* If we've been doing some writing, and we're in append mode

     then we don't really know where the filepos is.  */

  if (fp->_flags & __SAPP && fp->_flags & __SWR)

    {

      /* So flush the buffer and seek to the end.  */

      fflush (fp);

    }

  /* Have to be able to seek.  */

  if ((seekfn = fp->_seek) == NULL)

    {

      ptr->_errno = ESPIPE;     /* ??? */

      return EOF;

    }

  /*

   * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.

   * After this, whence is either SEEK_SET or SEEK_END.

   */

  switch (whence)

    {

    case SEEK_CUR:

      /*

       * In order to seek relative to the current stream offset,

       * we have to first find the current stream offset a la

       * ftell (see ftell for details).

       */

      fflush(fp);   /* may adjust seek offset on append stream */

      if (fp->_flags & __SOFF)

        curoff = fp->_offset;

      else

        {

          curoff = (*seekfn) (fp->_cookie, (fpos_t) 0, SEEK_CUR);

          if (curoff == -1L)

            return EOF;

        }

      if (fp->_flags & __SRD)

        {

          curoff -= fp->_r;

          if (HASUB (fp))

            curoff -= fp->_ur;

        }

      else if (fp->_flags & __SWR && fp->_p != NULL)

        curoff += fp->_p - fp->_bf._base;

      offset += curoff;

      whence = SEEK_SET;

      havepos = 1;

      break;

    case SEEK_SET:

    case SEEK_END:

      havepos = 0;

      break;

    default:

      ptr->_errno = EINVAL;

      return (EOF);

    }

  /*

   * Can only optimise if:

   *    reading (and not reading-and-writing);

   *    not unbuffered; and

   *    this is a `regular' Unix file (and hence seekfn==__sseek).

   * We must check __NBF first, because it is possible to have __NBF

   * and __SOPT both set.

   */

  if (fp->_bf._base == NULL)

    __smakebuf (fp);

  if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT))

    goto dumb;

  if ((fp->_flags & __SOPT) == 0)

    {

      if (seekfn != __sseek

          || fp->_file < 0

          || _fstat_r (ptr, fp->_file, &st)

          || (st.st_mode & S_IFMT) != S_IFREG)

        {

          fp->_flags |= __SNPT;

          goto dumb;

        }

#ifdef  HAVE_BLKSIZE

      fp->_blksize = st.st_blksize;

#else

      fp->_blksize = 1024;

#endif

      fp->_flags |= __SOPT;

    }

  /*

   * We are reading; we can try to optimise.

   * Figure out where we are going and where we are now.

   */

  if (whence == SEEK_SET)

    target = offset;

  else

    {

      if (_fstat_r (ptr, fp->_file, &st))

        goto dumb;

      target = st.st_size + offset;

    }

  if (!havepos)

    {

      if (fp->_flags & __SOFF)

        curoff = fp->_offset;

      else

        {

          curoff = (*seekfn) (fp->_cookie, 0L, SEEK_CUR);

          if (curoff == POS_ERR)

            goto dumb;

        }

      curoff -= fp->_r;

      if (HASUB (fp))

        curoff -= fp->_ur;

    }

  /*

   * Compute the number of bytes in the input buffer (pretending

   * that any ungetc() input has been discarded).  Adjust current

   * offset backwards by this count so that it represents the

   * file offset for the first byte in the current input buffer.

   */

  if (HASUB (fp))

    {

      curoff += fp->_r;       /* kill off ungetc */

      n = fp->_up - fp->_bf._base;

      curoff -= n;

      n += fp->_ur;

    }

  else

    {

      n = fp->_p - fp->_bf._base;

      curoff -= n;

      n += fp->_r;

    }

  /*

   * If the target offset is within the current buffer,

   * simply adjust the pointers, clear EOF, undo ungetc(),

   * and return.  (If the buffer was modified, we have to

   * skip this; see fgetline.c.)

   */

  if ((fp->_flags & __SMOD) == 0 &&

      target >= curoff && target < curoff + n)

    {

      register int o = target - curoff;

      fp->_p = fp->_bf._base + o;

      fp->_r = n - o;

      if (HASUB (fp))

        FREEUB (fp);

      fp->_flags &= ~__SEOF;

      return 0;

    }

  /*

   * The place we want to get to is not within the current buffer,

   * but we can still be kind to the kernel copyout mechanism.

   * By aligning the file offset to a block boundary, we can let

   * the kernel use the VM hardware to map pages instead of

   * copying bytes laboriously.  Using a block boundary also

   * ensures that we only read one block, rather than two.

   */

  curoff = target & ~(fp->_blksize - 1);

  if ((*seekfn) (fp->_cookie, curoff, SEEK_SET) == POS_ERR)

    goto dumb;

  fp->_r = 0;

  if (HASUB (fp))

    FREEUB (fp);

  fp->_flags &= ~__SEOF;

  n = target - curoff;

  if (n)

    {

      if (__srefill (fp) || fp->_r < n)

        goto dumb;

      fp->_p += n;

      fp->_r -= n;

    }

  return 0;

  /*

   * We get here if we cannot optimise the seek ... just

   * do it.  Allow the seek function to change fp->_bf._base.

   */

dumb:

  if (fflush (fp) || (*seekfn) (fp->_cookie, offset, whence) == POS_ERR)

    return EOF;

  /* success: clear EOF indicator and discard ungetc() data */

  if (HASUB (fp))

    FREEUB (fp);

  fp->_p = fp->_bf._base;

  fp->_r = 0;

  /* fp->_w = 0; *//* unnecessary (I think...) */

  fp->_flags &= ~__SEOF;

  return 0;

}