newlib-cygwin/newlib/libc/stdio/fseek.c

/*
 * 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;
}