Minix1.5/lib/other/seekdir.c

#include <lib.h>
/* seekdir -- reposition a directory stream	Author: D.A. Gwyn */

/*	last edit:	24-May-1987	D A Gwyn	*/

#include	<errno.h>
#include	<sys/types.h>
#include	<limits.h>
#include	<dirent.h>
#include	<unistd.h>

#define DULL (DIR *) 0
#define DE_NULL (struct dirent *) 0

typedef int bool;		/* Boolean data type */
#define	false	0
#define	true	1

void seekdir(dirp, loc)
register DIR *dirp;		/* stream from opendir() */
register off_t loc;		/* position from telldir() */
{
  register bool rewind;		/* "start over when stymied" flag */

  if (dirp == DULL || dirp->dd_buf == (char *)NULL || dirp->dd_magic != _DIR_MAGIC) {
	errno = EBADF;
	return;			/* invalid pointer */
  }

  /* A (struct dirent)'s d_off is an invented quantity on 4.nBSD
   * NFS-supporting systems, so it is not safe to lseek() to it. */

  /* Monotonicity of d_off is heavily exploited in the following. */

  /* This algorithm is tuned for modest directory sizes.  For huge
   * directories, it might be more efficient to read blocks until the
   * first d_off is too large, then back up one block, or even to use
   * binary search on the directory blocks.  I doubt that the extra
   * code for that would be worthwhile. */

  if (dirp->dd_loc >= dirp->dd_size	/* invalid index */
      || ((struct dirent *) & dirp->dd_buf[dirp->dd_loc])->d_off > loc
  /* Too far along in buffer */
	)
	dirp->dd_loc = 0;	/* reset to beginning of buffer */
  /* Else save time by starting at current dirp->dd_loc */

  for (rewind = true;;) {
	register struct dirent *dp;

	/* See whether the matching entry is in the current buffer. */

	if ((dirp->dd_loc < dirp->dd_size	/* valid index */
	     || readdir(dirp) != DE_NULL	/* next buffer read */
	     && (dirp->dd_loc = 0, true)	/* beginning of buffer set */
	     )
	    && (dp = (struct dirent *) & dirp->dd_buf[dirp->dd_loc])->d_off
	    <= loc		/* match possible in this buffer */
		) {
		for ( 	/* dp initialized above */ ;
		     (char *) dp < &dirp->dd_buf[dirp->dd_size];
		 dp = (struct dirent *) ((char *) dp + dp->d_reclen)
			)
			if (dp->d_off == loc) {	/* found it! */
				dirp->dd_loc =
					(char *) dp - dirp->dd_buf;
				return;
			}
		rewind = false;	/* no point in backing up later */
		dirp->dd_loc = dirp->dd_size;	/* set end of buffer */
	} else
	 /* Whole buffer past matching entry */ if (!rewind) {		/* no point in searching
								 * further */
		errno = EINVAL;
		return;		/* no entry at specified loc */
	} else {		/* rewind directory and start over */
		rewind = false;	/* but only once! */

		dirp->dd_loc = dirp->dd_size = 0;

		if (lseek(dirp->dd_fd, (off_t) 0, SEEK_SET) != 0
			)
			return;	/* errno already set (EBADF) */

		if (loc == 0) return;	/* save time */
	}
  }
}