VOP_RDWR(9) MidnightBSD 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 system. 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.

MidnightBSD 0.3 July 24, 1996 MidnightBSD 0.3