freebsd-skq/share/man/man9/VOP_RDWR.9

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.\" Copyright (c) 1996 Doug Rabson
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.Dd July 24, 1996
.Os
.Dt VOP_RDWR 9
.Sh NAME
.Nm VOP_READ ,
.Nm VOP_WRITE
.Nd read or write a file
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.In sys/uio.h
.Ft int
.Fn VOP_READ "struct vnode *vp" "struct uio *uio" "int ioflag" "struct ucred *cred"
.Ft int
.Fn VOP_WRITE "struct vnode *vp" "struct uio *uio" "int ioflag" "struct ucred *cred"
.Sh DESCRIPTION
These entry points read or write the contents of a file
.Pp
The arguments are:
.Bl -tag -width ioflag
.It Ar vp
the vnode of the file
.It Ar uio
the location of the data to be read or written
.It Ar ioflag
various flags
.It Ar cnp
the credentials of the caller
.El
.Pp
The
.Fa ioflag
argument is used to give directives and hints to the filesystem.
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:
.Bl -tag -width IO_NODELOCKED
.It Dv IO_UNIT
do I/O as atomic unit
.It Dv IO_APPEND
append write to end
.It Dv IO_SYNC
do I/O synchronously
.It Dv IO_NODELOCKED
underlying node already locked
.It Dv IO_NDELAY
.Dv FNDELAY
flag set in file table
.It Dv IO_VMIO
data already in VMIO space
.El
.Sh LOCKS
The file should be locked on entry and will still be locked on exit.
.Sh RETURN VALUES
Zero is returned on success, otherwise an error code is returned.
.Sh PSEUDOCODE
.Bd -literal
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;
}
.Ed
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EFBIG
An attempt was made to write a file that exceeds the process's file size
limit or the maximum file size.
.It Bq Er ENOSPC
The filesystem is full.
.It Bq Er EPERM
An append-only flag is set on the file, but the caller is attempting to
write before the current end of file.
.El
.Sh SEE ALSO
.Xr uiomove 9 ,
.Xr vnode 9
.Sh AUTHORS
This man page was written by
.An Doug Rabson .