DragonFly On-Line Manual Pages

VOP_RDWR(9)           DragonFly Kernel Developer's Manual          VOP_RDWR(9)

NAME
     VOP_READ, VOP_WRITE -- read or write a file

SYNOPSIS
     #include <sys/param.h>
     #include <sys/vnode.h>
     #include <sys/uio.h>

     int
     VOP_READ(struct vnode *vp, struct uio *uio, int ioflag,
         struct ucred *cred);

     int
     VOP_WRITE(struct vnode *vp, struct uio *uio, int ioflag,
         struct ucred *cred);

DESCRIPTION
     These entry points read or write the contents of a file

     The arguments are:

     vp      the vnode of the file

     uio     the location of the data to be read or written

     ioflag  various flags

     cnp     the credentials of the caller

     The ioflag argument is used to give directives and hints to the filesys-
     tem.  When attempting a read, the high 16 bits are used to provide a
     read-ahead hint (in units of filesystem blocks) that the filesystem
     should attempt.  The low 16 bits are a bit mask which can contain the
     following flags:

     IO_UNIT        do I/O as atomic unit

     IO_APPEND      append write to end

     IO_SYNC        do I/O synchronously

     IO_NODELOCKED  underlying node already locked

     IO_NDELAY      FNDELAY flag set in file table

     IO_VMIO        data already in VMIO space

LOCKS
     The file should be locked on entry and will still be locked on exit.

RETURN VALUES
     Zero is returned on success, otherwise an error code is returned.

PSEUDOCODE
     int
     vop_read(struct vnode *vp, struct uio *uio, int ioflag, struct ucred *cred)
     {
         struct buf *bp;
         off_t bytesinfile;
         daddr_t lbn, nextlbn;
         long size, xfersize, blkoffset;
         int error;

         size = block size of filesystem;

         for (error = 0, bp = NULL; uio->uio_resid > 0; bp = NULL) {
             bytesinfile = size of file - uio->uio_offset;
             if (bytesinfile <= 0)
                 break;

             lbn = uio->uio_offset / size;
             blkoffset = uio->uio_offset - lbn * size;

             xfersize = size - blkoffset;
             if (uio->uio_resid < xfersize)
                 xfersize = uio->uio_resid;
             if (bytesinfile < xfersize)
                 xfersize = bytesinfile;

             error = bread(vp, lbn, size, NOCRED, &bp);
             if (error) {
                 brelse(bp);
                 bp = NULL;
                 break;
             }

             /*
              * We should only get non-zero b_resid when an I/O error
              * has occurred, which should cause us to break above.
              * However, if the short read did not cause an error,
              * then we want to ensure that we do not uiomove bad
              * or uninitialized data.
              */
             size -= bp->b_resid;
             if (size < xfersize) {
                 if (size == 0)
                     break;
                 xfersize = size;
             }

             error = uiomove((char *)bp->b_data + blkoffset, (int)xfersize, uio);
             if (error)
                 break;

             bqrelse(bp);
         }
         if (bp != NULL)
             bqrelse(bp);

         return error;
     }

     int
     vop_write(struct vnode *vp, struct uio *uio, int ioflag, struct ucred *cred)
     {
         struct buf *bp;
         off_t bytesinfile;
         daddr_t lbn, nextlbn;
         off_t osize;
         long size, resid, xfersize, blkoffset;
         int flags;
         int error;

         osize = size of file;
         size = block size of filesystem;
         resid = uio->uio_resid;
         if (ioflag & IO_SYNC)
             flags = B_SYNC;
         else
             flags = 0;

         for (error = 0; uio->uio_resid > 0;) {
             lbn = uio->uio_offset / size;
             blkoffset = uio->uio_offset - lbn * size;

             xfersize = size - blkoffset;
             if (uio->uio_resid < xfersize)
                 xfersize = uio->uio_resid;

             if (uio->uio_offset + xfersize > size of file)
                 vnode_pager_setsize(vp, uio->uio_offset + xfersize);

             if (size > xfersize)
                 flags |= B_CLRBUF;
             else
                 flags &= ~B_CLRBUF;

             error = find_block_in_file(vp, lbn, blkoffset + xfersize,
                                        cred, &bp, flags);
             if (error)
                 break;

             if (uio->uio_offset + xfersize > size of file)
                 set size of file to uio->uio_offset + xfersize;

             error = uiomove((char *)bp->b_data + blkoffset, (int) xfersize, uio);
             /* XXX ufs does not check the error here.  Why? */

             if (ioflag & IO_VMIO)
                 bp->b_flags |= B_RELBUF; /* ??? */

             if (ioflag & IO_SYNC)
                 bwrite(bp);
             else if (xfersize + blkoffset == size)
                 bawrite(bp);
             else
                 bdwrite(bp);

             if (error || xfersize == 0)
                 break;
         }

         if (error) {
             if (ioflag & IO_UNIT) {
                 VOP_TRUNCATE(vp, osize, ioflag & IO_SYNC, cred, uio->uio_procp);
                 uio->uio_offset -= resid - uio->uio_resid;
                 uio->uio_resid = resid;
             }
         } else if (resid > uio->uio_resid && (ioflag & IO_SYNC)) {
             struct timeval tv;
             error = VOP_UPDATE(vp, &tv, &tv, 1); /* XXX what does this do? */
         }

         return error;
     }

ERRORS
     [ENOSPC]           The filesystem is full.

SEE ALSO
     uiomove(9), vnode(9)

AUTHORS
     This man page was written by Doug Rabson.

DragonFly 3.5                    July 24, 1996                   DragonFly 3.5