110 lines
2.9 KiB
C
110 lines
2.9 KiB
C
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/*
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seekdir -- reposition a directory stream
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last edit: 24-May-1987 D A Gwyn
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An unsuccessful seekdir() will in general alter the current
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directory position; beware.
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NOTE: 4.nBSD directory compaction makes seekdir() & telldir()
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practically impossible to do right. Avoid using them!
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*/
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#include <errno.h>
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#include <sys/errno.h>
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#include <sys/types.h>
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#include <dirent.h>
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extern off_t lseek(int d, int offset, int whence);
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#ifndef NULL
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#define NULL 0
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#endif
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#ifndef SEEK_SET
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#define SEEK_SET 0
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#endif
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typedef int bool; /* Boolean data type */
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#define false 0
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#define true 1
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void
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seekdir(register DIR *dirp, register off_t loc)
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/* loc == position from telldir() */
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{
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register bool rewind; /* "start over when stymied" flag */
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if ( dirp == NULL || dirp->dd_buf == NULL )
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{
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errno = EFAULT;
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return; /* invalid pointer */
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}
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/* A (struct dirent)'s d_off is an invented quantity on 4.nBSD
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NFS-supporting systems, so it is not safe to lseek() to it. */
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/* Monotonicity of d_off is heavily exploited in the following. */
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/* This algorithm is tuned for modest directory sizes. For
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huge directories, it might be more efficient to read blocks
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until the first d_off is too large, then back up one block,
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or even to use binary search on the directory blocks. I
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doubt that the extra code for that would be worthwhile. */
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if ( dirp->dd_loc >= dirp->dd_size /* invalid index */
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|| ((struct dirent *)&dirp->dd_buf[dirp->dd_loc])->d_off > loc
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/* too far along in buffer */
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)
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dirp->dd_loc = 0; /* reset to beginning of buffer */
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/* else save time by starting at current dirp->dd_loc */
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for ( rewind = true; ; )
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{
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register struct dirent *dp;
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/* See whether the matching entry is in the current buffer. */
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if ( (dirp->dd_loc < dirp->dd_size /* valid index */
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|| readdir( dirp ) != NULL /* next buffer read */
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&& (dirp->dd_loc = 0, true) /* beginning of buffer set */
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)
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&& (dp = (struct dirent *)&dirp->dd_buf[dirp->dd_loc])->d_off
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<= loc /* match possible in this buffer */
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) {
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for ( /* dp initialized above */ ;
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(char *)dp < &dirp->dd_buf[dirp->dd_size];
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dp = (struct dirent *)((char *)dp + dp->d_reclen)
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)
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if ( dp->d_off == loc )
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{ /* found it! */
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dirp->dd_loc =
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(char *)dp - dirp->dd_buf;
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return;
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}
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rewind = false; /* no point in backing up later */
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dirp->dd_loc = dirp->dd_size; /* set end of buffer */
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}
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else /* whole buffer past matching entry */
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if ( !rewind )
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{ /* no point in searching further */
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errno = EINVAL;
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return; /* no entry at specified loc */
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}
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else { /* rewind directory and start over */
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rewind = false; /* but only once! */
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dirp->dd_loc = dirp->dd_size = 0;
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if ( lseek( dirp->dd_fd, (off_t)0, SEEK_SET )
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!= 0
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)
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return; /* errno already set (EBADF) */
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if ( loc == 0 )
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return; /* save time */
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}
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}
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}
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