VOP_RDWR(9)	       FreeBSD 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 file sys-
     tem.  When attempting a read, the high 16 bits are used to provide a
     read-ahead hint (in units of file system blocks) that the file system
     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 file system;

	 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 file system;
	 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) {

		 /* call private routine to truncate file. */
		 your_truncate(vp, osize, ioflag & IO_SYNC, cred, uio->uio_td);
		 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
     [EFBIG]		An attempt was made to write a file that exceeds the

			process's file size limit or the maximum file size.

     [ENOSPC]		The file system is full.

     [EPERM]		An append-only flag is set on the file, but the caller

			is attempting to write before the current end of file.

SEE ALSO
     uiomove(9), vnode(9)

AUTHORS
     This manual page was written by Doug Rabson.

FreeBSD 7.2			 July 24, 1996			   FreeBSD 7.2

NAME - SYNOPSIS - DESCRIPTION - LOCKS - RETURN VALUES - PSEUDOCODE - 
ERRORS - SEE ALSO - AUTHORS -