text: /*	$OpenBSD: fseek.c,v 1.11 2012/05/21 22:24:19 matthew Exp $ */
  /*-
   * Copyright (c) 1990, 1993
   *	The Regents of the University of California.  All rights reserved.
   *
   * This code is derived from software contributed to Berkeley by
   * Chris Torek.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   * 3. Neither the name of the University nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   * SUCH DAMAGE.
   */
  
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <errno.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
  fseeko(FILE *fp, off_t offset, int whence)
  {
  	fpos_t (*seekfn)(void *, fpos_t, int);
  	fpos_t target, curoff;
  	size_t n;
  	struct stat st;
  	int havepos;
  
  	/* make sure stdio is set up */
  	if (!__sdidinit)
  		__sinit();
  
  	/*
  	 * Have to be able to seek.
  	 */
  	if ((seekfn = fp->_seek) == NULL) {
  		errno = ESPIPE;			/* historic practice */
  		return (EOF);
  	}
  
  	/*
  	 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
  	 * After this, whence is either SEEK_SET or SEEK_END.
  	 */
  	FLOCKFILE(fp);
  	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).
  		 */
  		__sflush(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 == (fpos_t)-1) {
  				FUNLOCKFILE(fp);
  				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:
  		curoff = 0;		/* XXX just to keep gcc quiet */
  		havepos = 0;
  		break;
  
  	default:
  		FUNLOCKFILE(fp);
  		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(fp->_file, &st) ||
  		    (st.st_mode & S_IFMT) != S_IFREG) {
  			fp->_flags |= __SNPT;
  			goto dumb;
  		}
  		fp->_blksize = st.st_blksize;
  		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(fp->_file, &st))
  			goto dumb;
  		target = st.st_size + offset;
  	}
  
  	if (!havepos) {
  		if (fp->_flags & __SOFF)
  			curoff = fp->_offset;
  		else {
  			curoff = (*seekfn)(fp->_cookie, (fpos_t)0, 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 fgetln.c.)
  	 */
  	if ((fp->_flags & __SMOD) == 0 &&
  	    target >= curoff && target < curoff + n) {
  		int o = target - curoff;
  
  		fp->_p = fp->_bf._base + o;
  		fp->_r = n - o;
  		if (HASUB(fp))
  			FREEUB(fp);
  		fp->_flags &= ~__SEOF;
  		FUNLOCKFILE(fp);
  		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;
   	fp->_p = fp->_bf._base;
  	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;
  	}
  	FUNLOCKFILE(fp);
  	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 (__sflush(fp) ||
  	    (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) {
  		FUNLOCKFILE(fp);
  		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;
  	FUNLOCKFILE(fp);
  	return (0);
  }
  
  int
  fseek(FILE *fp, long offset, int whence)
  {
  	return (fseeko(fp, offset, whence));
  }
  