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VOP_RDWR(9)		 BSD 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 man page was written by Doug Rabson.

BSD				 July 24, 1996				   BSD

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

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