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VOP_RDWR (9) | Kernel routines | Unix Manual Pages | :man

NAME

VOP_READ, VOP_WRITE - read or write a file

CONTENTS

Synopsis
Description
Locks
Return Values
Pseudocode
Errors
See Also
Authors

SYNOPSIS


.In sys/param.h
.In sys/vnode.h
.In 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


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