xref: /csrg-svn/sys/kern/kern_descrip.c (revision 54346)

/*
 * Copyright (c) 1982, 1986, 1989, 1991 Regents of the University of California.
 * All rights reserved.
 *
 * %sccs.include.redist.c%
 *
 *	@(#)kern_descrip.c	7.35 (Berkeley) 06/23/92
 */

#include "param.h"
#include "systm.h"
#include "filedesc.h"
#include "kernel.h"
#include "vnode.h"
#include "proc.h"
#include "file.h"
#include "socket.h"
#include "socketvar.h"
#include "stat.h"
#include "ioctl.h"
#include "fcntl.h"
#include "malloc.h"
#include "syslog.h"
#include "resourcevar.h"

/*
 * Descriptor management.
 */
struct file *filehead;	/* head of list of open files */
int nfiles;		/* actual number of open files */

/*
 * System calls on descriptors.
 */
/* ARGSUSED */
getdtablesize(p, uap, retval)
	struct proc *p;
	struct args *uap;
	int *retval;
{

	*retval = p->p_rlimit[RLIMIT_OFILE].rlim_cur;
	return (0);
}

/*
 * Duplicate a file descriptor.
 */
/* ARGSUSED */
dup(p, uap, retval)
	struct proc *p;
	struct args {
		int	i;
	} *uap;
	int *retval;
{
	register struct filedesc *fdp = p->p_fd;
	struct file *fp;
	int fd, error;

	/*
	 * XXX Compatibility
	 */
	if (uap->i &~ 077) { uap->i &= 077; return (dup2(p, uap, retval)); }

	if ((unsigned)uap->i >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[uap->i]) == NULL)
		return (EBADF);
	if (error = fdalloc(p, 0, &fd))
		return (error);
	fdp->fd_ofiles[fd] = fp;
	fdp->fd_ofileflags[fd] = fdp->fd_ofileflags[uap->i] &~ UF_EXCLOSE;
	fp->f_count++;
	if (fd > fdp->fd_lastfile)
		fdp->fd_lastfile = fd;
	*retval = fd;
	return (0);
}

/*
 * Duplicate a file descriptor to a particular value.
 */
/* ARGSUSED */
dup2(p, uap, retval)
	struct proc *p;
	struct args {
		u_int	from;
		u_int	to;
	} *uap;
	int *retval;
{
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	register u_int old = uap->from, new = uap->to;
	int i, error;

	if (old >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[old]) == NULL ||
	    new >= p->p_rlimit[RLIMIT_OFILE].rlim_cur)
		return (EBADF);
	*retval = new;
	if (old == new)
		return (0);
	if (new >= fdp->fd_nfiles) {
		if (error = fdalloc(p, new, &i))
			return (error);
		if (new != i)
			panic("dup2: fdalloc");
	} else if (fdp->fd_ofiles[new]) {
		if (fdp->fd_ofileflags[new] & UF_MAPPED)
			(void) munmapfd(p, new);
		/*
		 * dup2() must succeed even if the close has an error.
		 */
		(void) closef(fdp->fd_ofiles[new], p);
	}
	fdp->fd_ofiles[new] = fp;
	fdp->fd_ofileflags[new] = fdp->fd_ofileflags[old] &~ UF_EXCLOSE;
	fp->f_count++;
	if (new > fdp->fd_lastfile)
		fdp->fd_lastfile = new;
	return (0);
}

/*
 * The file control system call.
 */
/* ARGSUSED */
fcntl(p, uap, retval)
	struct proc *p;
	register struct args {
		int	fd;
		int	cmd;
		int	arg;
	} *uap;
	int *retval;
{
	USES_VOP_ADVLOCK;
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	register char *pop;
	struct vnode *vp;
	int i, tmp, error, flg = F_POSIX;
	struct flock fl;

	if ((unsigned)uap->fd >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[uap->fd]) == NULL)
		return (EBADF);
	pop = &fdp->fd_ofileflags[uap->fd];
	switch(uap->cmd) {
	case F_DUPFD:
		if ((unsigned)uap->arg >= p->p_rlimit[RLIMIT_OFILE].rlim_cur)
			return (EINVAL);
		if (error = fdalloc(p, uap->arg, &i))
			return (error);
		fdp->fd_ofiles[i] = fp;
		fdp->fd_ofileflags[i] = *pop &~ UF_EXCLOSE;
		fp->f_count++;
		if (i > fdp->fd_lastfile)
			fdp->fd_lastfile = i;
		*retval = i;
		return (0);

	case F_GETFD:
		*retval = *pop & 1;
		return (0);

	case F_SETFD:
		*pop = (*pop &~ 1) | (uap->arg & 1);
		return (0);

	case F_GETFL:
		*retval = OFLAGS(fp->f_flag);
		return (0);

	case F_SETFL:
		fp->f_flag &= ~FCNTLFLAGS;
		fp->f_flag |= FFLAGS(uap->arg) & FCNTLFLAGS;
		tmp = fp->f_flag & FNONBLOCK;
		error = (*fp->f_ops->fo_ioctl)(fp, FIONBIO, (caddr_t)&tmp, p);
		if (error)
			return (error);
		tmp = fp->f_flag & FASYNC;
		error = (*fp->f_ops->fo_ioctl)(fp, FIOASYNC, (caddr_t)&tmp, p);
		if (!error)
			return (0);
		fp->f_flag &= ~FNONBLOCK;
		tmp = 0;
		(void) (*fp->f_ops->fo_ioctl)(fp, FIONBIO, (caddr_t)&tmp, p);
		return (error);

	case F_GETOWN:
		if (fp->f_type == DTYPE_SOCKET) {
			*retval = ((struct socket *)fp->f_data)->so_pgid;
			return (0);
		}
		error = (*fp->f_ops->fo_ioctl)
			(fp, (int)TIOCGPGRP, (caddr_t)retval, p);
		*retval = -*retval;
		return (error);

	case F_SETOWN:
		if (fp->f_type == DTYPE_SOCKET) {
			((struct socket *)fp->f_data)->so_pgid = uap->arg;
			return (0);
		}
		if (uap->arg <= 0) {
			uap->arg = -uap->arg;
		} else {
			struct proc *p1 = pfind(uap->arg);
			if (p1 == 0)
				return (ESRCH);
			uap->arg = p1->p_pgrp->pg_id;
		}
		return ((*fp->f_ops->fo_ioctl)
			(fp, (int)TIOCSPGRP, (caddr_t)&uap->arg, p));

	case F_SETLKW:
		flg |= F_WAIT;
		/* Fall into F_SETLK */

	case F_SETLK:
		if (fp->f_type != DTYPE_VNODE)
			return (EBADF);
		vp = (struct vnode *)fp->f_data;
		/* Copy in the lock structure */
		error = copyin((caddr_t)uap->arg, (caddr_t)&fl, sizeof (fl));
		if (error)
			return (error);
		if (fl.l_whence == SEEK_CUR)
			fl.l_start += fp->f_offset;
		switch (fl.l_type) {

		case F_RDLCK:
			if ((fp->f_flag & FREAD) == 0)
				return (EBADF);
			p->p_flag |= SADVLCK;
			return (VOP_ADVLOCK(vp, (caddr_t)p, F_SETLK, &fl, flg));

		case F_WRLCK:
			if ((fp->f_flag & FWRITE) == 0)
				return (EBADF);
			p->p_flag |= SADVLCK;
			return (VOP_ADVLOCK(vp, (caddr_t)p, F_SETLK, &fl, flg));

		case F_UNLCK:
			return (VOP_ADVLOCK(vp, (caddr_t)p, F_UNLCK, &fl,
				F_POSIX));

		default:
			return (EINVAL);
		}

	case F_GETLK:
		if (fp->f_type != DTYPE_VNODE)
			return (EBADF);
		vp = (struct vnode *)fp->f_data;
		/* Copy in the lock structure */
		error = copyin((caddr_t)uap->arg, (caddr_t)&fl, sizeof (fl));
		if (error)
			return (error);
		if (fl.l_whence == SEEK_CUR)
			fl.l_start += fp->f_offset;
		if (error = VOP_ADVLOCK(vp, (caddr_t)p, F_GETLK, &fl, F_POSIX))
			return (error);
		return (copyout((caddr_t)&fl, (caddr_t)uap->arg, sizeof (fl)));

	default:
		return (EINVAL);
	}
	/* NOTREACHED */
}

/*
 * Close a file descriptor.
 */
/* ARGSUSED */
close(p, uap, retval)
	struct proc *p;
	struct args {
		int	fd;
	} *uap;
	int *retval;
{
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	register int fd = uap->fd;
	register u_char *pf;

	if ((unsigned)fd >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[fd]) == NULL)
		return (EBADF);
	pf = (u_char *)&fdp->fd_ofileflags[fd];
	if (*pf & UF_MAPPED)
		(void) munmapfd(p, fd);
	fdp->fd_ofiles[fd] = NULL;
	while (fdp->fd_lastfile > 0 && fdp->fd_ofiles[fdp->fd_lastfile] == NULL)
		fdp->fd_lastfile--;
	if (fd < fdp->fd_freefile)
		fdp->fd_freefile = fd;
	*pf = 0;
	return (closef(fp, p));
}

#if defined(COMPAT_43) || defined(COMPAT_SUNOS)
/*
 * Return status information about a file descriptor.
 */
/* ARGSUSED */
ofstat(p, uap, retval)
	struct proc *p;
	register struct args {
		int	fd;
		struct	ostat *sb;
	} *uap;
	int *retval;
{
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	struct stat ub;
	struct ostat oub;
	int error;

	if ((unsigned)uap->fd >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[uap->fd]) == NULL)
		return (EBADF);
	switch (fp->f_type) {

	case DTYPE_VNODE:
		error = vn_stat((struct vnode *)fp->f_data, &ub, p);
		break;

	case DTYPE_SOCKET:
		error = soo_stat((struct socket *)fp->f_data, &ub);
		break;

	default:
		panic("ofstat");
		/*NOTREACHED*/
	}
	cvtstat(&ub, &oub);
	if (error == 0)
		error = copyout((caddr_t)&oub, (caddr_t)uap->sb, sizeof (oub));
	return (error);
}
#endif /* COMPAT_43 || COMPAT_SUNOS */

/*
 * Return status information about a file descriptor.
 */
/* ARGSUSED */
fstat(p, uap, retval)
	struct proc *p;
	register struct args {
		int	fd;
		struct	stat *sb;
	} *uap;
	int *retval;
{
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	struct stat ub;
	int error;

	if ((unsigned)uap->fd >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[uap->fd]) == NULL)
		return (EBADF);
	switch (fp->f_type) {

	case DTYPE_VNODE:
		error = vn_stat((struct vnode *)fp->f_data, &ub, p);
		break;

	case DTYPE_SOCKET:
		error = soo_stat((struct socket *)fp->f_data, &ub);
		break;

	default:
		panic("fstat");
		/*NOTREACHED*/
	}
	if (error == 0)
		error = copyout((caddr_t)&ub, (caddr_t)uap->sb, sizeof (ub));
	return (error);
}

/*
 * Allocate a file descriptor for the process.
 */
int fdexpand;

fdalloc(p, want, result)
	struct proc *p;
	int want;
	int *result;
{
	register struct filedesc *fdp = p->p_fd;
	register int i;
	int lim, last, nfiles;
	struct file **newofile;
	char *newofileflags;

	/*
	 * Search for a free descriptor starting at the higher
	 * of want or fd_freefile.  If that fails, consider
	 * expanding the ofile array.
	 */
	lim = p->p_rlimit[RLIMIT_OFILE].rlim_cur;
	for (;;) {
		last = min(fdp->fd_nfiles, lim);
		if ((i = want) < fdp->fd_freefile)
			i = fdp->fd_freefile;
		for (; i < last; i++) {
			if (fdp->fd_ofiles[i] == NULL) {
				fdp->fd_ofileflags[i] = 0;
				if (i > fdp->fd_lastfile)
					fdp->fd_lastfile = i;
				if (want <= fdp->fd_freefile)
					fdp->fd_freefile = i;
				*result = i;
				return (0);
			}
		}

		/*
		 * No space in current array.  Expand?
		 */
		if (fdp->fd_nfiles >= lim)
			return (EMFILE);
		if (fdp->fd_nfiles < NDEXTENT)
			nfiles = NDEXTENT;
		else
			nfiles = 2 * fdp->fd_nfiles;
		MALLOC(newofile, struct file **, nfiles * OFILESIZE,
		    M_FILEDESC, M_WAITOK);
		newofileflags = (char *) &newofile[nfiles];
		/*
		 * Copy the existing ofile and ofileflags arrays
		 * and zero the new portion of each array.
		 */
		bcopy(fdp->fd_ofiles, newofile,
			(i = sizeof(struct file *) * fdp->fd_nfiles));
		bzero((char *)newofile + i, nfiles * sizeof(struct file *) - i);
		bcopy(fdp->fd_ofileflags, newofileflags,
			(i = sizeof(char) * fdp->fd_nfiles));
		bzero(newofileflags + i, nfiles * sizeof(char) - i);
		if (fdp->fd_nfiles > NDFILE)
			FREE(fdp->fd_ofiles, M_FILEDESC);
		fdp->fd_ofiles = newofile;
		fdp->fd_ofileflags = newofileflags;
		fdp->fd_nfiles = nfiles;
		fdexpand++;
	}
}

/*
 * Check to see whether n user file descriptors
 * are available to the process p.
 */
fdavail(p, n)
	struct proc *p;
	register int n;
{
	register struct filedesc *fdp = p->p_fd;
	register struct file **fpp;
	register int i;

	if ((i = p->p_rlimit[RLIMIT_OFILE].rlim_cur - fdp->fd_nfiles) > 0 &&
	    (n -= i) <= 0)
		return (1);
	fpp = &fdp->fd_ofiles[fdp->fd_freefile];
	for (i = fdp->fd_nfiles - fdp->fd_freefile; --i >= 0; fpp++)
		if (*fpp == NULL && --n <= 0)
			return (1);
	return (0);
}

/*
 * Create a new open file structure and allocate
 * a file decriptor for the process that refers to it.
 */
falloc(p, resultfp, resultfd)
	register struct proc *p;
	struct file **resultfp;
	int *resultfd;
{
	register struct file *fp, *fq, **fpp;
	int error, i;

	if (error = fdalloc(p, 0, &i))
		return (error);
	if (nfiles >= maxfiles) {
		tablefull("file");
		return (ENFILE);
	}
	/*
	 * Allocate a new file descriptor.
	 * If the process has file descriptor zero open, add to the list
	 * of open files at that point, otherwise put it at the front of
	 * the list of open files.
	 */
	nfiles++;
	MALLOC(fp, struct file *, sizeof(struct file), M_FILE, M_WAITOK);
	if (fq = p->p_fd->fd_ofiles[0])
		fpp = &fq->f_filef;
	else
		fpp = &filehead;
	p->p_fd->fd_ofiles[i] = fp;
	if (fq = *fpp)
		fq->f_fileb = &fp->f_filef;
	fp->f_filef = fq;
	fp->f_fileb = fpp;
	*fpp = fp;
	fp->f_count = 1;
	fp->f_msgcount = 0;
	fp->f_offset = 0;
	fp->f_cred = p->p_ucred;
	crhold(fp->f_cred);
	if (resultfp)
		*resultfp = fp;
	if (resultfd)
		*resultfd = i;
	return (0);
}

/*
 * Free a file descriptor.
 */
ffree(fp)
	register struct file *fp;
{
	register struct file *fq;

	if (fq = fp->f_filef)
		fq->f_fileb = fp->f_fileb;
	*fp->f_fileb = fq;
	crfree(fp->f_cred);
#ifdef DIAGNOSTIC
	fp->f_filef = NULL;
	fp->f_fileb = NULL;
	fp->f_count = 0;
#endif
	nfiles--;
	FREE(fp, M_FILE);
}

/*
 * Copy a filedesc structure.
 */
struct filedesc *
fdcopy(p)
	struct proc *p;
{
	register struct filedesc *newfdp, *fdp = p->p_fd;
	register struct file **fpp;
	register int i;

	MALLOC(newfdp, struct filedesc *, sizeof(struct filedesc0),
	    M_FILEDESC, M_WAITOK);
	bcopy(fdp, newfdp, sizeof(struct filedesc));
	VREF(newfdp->fd_cdir);
	if (newfdp->fd_rdir)
		VREF(newfdp->fd_rdir);
	newfdp->fd_refcnt = 1;

	/*
	 * If the number of open files fits in the internal arrays
	 * of the open file structure, use them, otherwise allocate
	 * additional memory for the number of descriptors currently
	 * in use.
	 */
	if (newfdp->fd_lastfile < NDFILE) {
		newfdp->fd_ofiles = ((struct filedesc0 *) newfdp)->fd_dfiles;
		newfdp->fd_ofileflags =
		    ((struct filedesc0 *) newfdp)->fd_dfileflags;
		i = NDFILE;
	} else {
		/*
		 * Compute the smallest multiple of NDEXTENT needed
		 * for the file descriptors currently in use,
		 * allowing the table to shrink.
		 */
		i = newfdp->fd_nfiles;
		while (i > 2 * NDEXTENT && i >= newfdp->fd_lastfile * 2)
			i /= 2;
		MALLOC(newfdp->fd_ofiles, struct file **, i * OFILESIZE,
		    M_FILEDESC, M_WAITOK);
		newfdp->fd_ofileflags = (char *) &newfdp->fd_ofiles[i];
	}
	newfdp->fd_nfiles = i;
	bcopy(fdp->fd_ofiles, newfdp->fd_ofiles, i * sizeof(struct file **));
	bcopy(fdp->fd_ofileflags, newfdp->fd_ofileflags, i * sizeof(char));
	fpp = newfdp->fd_ofiles;
	for (i = newfdp->fd_lastfile; i-- >= 0; fpp++)
		if (*fpp != NULL)
			(*fpp)->f_count++;
	return (newfdp);
}

/*
 * Release a filedesc structure.
 */
void
fdfree(p)
	struct proc *p;
{
	register struct filedesc *fdp = p->p_fd;
	struct file **fpp;
	register int i;

	if (--fdp->fd_refcnt > 0)
		return;
	fpp = fdp->fd_ofiles;
	for (i = fdp->fd_lastfile; i-- >= 0; fpp++)
		if (*fpp)
			(void) closef(*fpp, p);
	if (fdp->fd_nfiles > NDFILE)
		FREE(fdp->fd_ofiles, M_FILEDESC);
	vrele(fdp->fd_cdir);
	if (fdp->fd_rdir)
		vrele(fdp->fd_rdir);
	FREE(fdp, M_FILEDESC);
}

/*
 * Internal form of close.
 * Decrement reference count on file structure.
 * Note: p may be NULL when closing a file
 * that was being passed in a message.
 */
closef(fp, p)
	register struct file *fp;
	register struct proc *p;
{
	USES_VOP_ADVLOCK;
	struct vnode *vp;
	struct flock lf;
	int error;

	if (fp == NULL)
		return (0);
	/*
	 * POSIX record locking dictates that any close releases ALL
	 * locks owned by this process.  This is handled by setting
	 * a flag in the unlock to free ONLY locks obeying POSIX
	 * semantics, and not to free BSD-style file locks.
	 * If the descriptor was in a message, POSIX-style locks
	 * aren't passed with the descriptor.
	 */
	if (p && (p->p_flag & SADVLCK) && fp->f_type == DTYPE_VNODE) {
		lf.l_whence = SEEK_SET;
		lf.l_start = 0;
		lf.l_len = 0;
		lf.l_type = F_UNLCK;
		vp = (struct vnode *)fp->f_data;
		(void) VOP_ADVLOCK(vp, (caddr_t)p, F_UNLCK, &lf, F_POSIX);
	}
	if (--fp->f_count > 0)
		return (0);
	if (fp->f_count < 0)
		panic("closef: count < 0");
	if ((fp->f_flag & FHASLOCK) && fp->f_type == DTYPE_VNODE) {
		lf.l_whence = SEEK_SET;
		lf.l_start = 0;
		lf.l_len = 0;
		lf.l_type = F_UNLCK;
		vp = (struct vnode *)fp->f_data;
		(void) VOP_ADVLOCK(vp, (caddr_t)fp, F_UNLCK, &lf, F_FLOCK);
	}
	error = (*fp->f_ops->fo_close)(fp, p);
	ffree(fp);
	return (error);
}

/*
 * Apply an advisory lock on a file descriptor.
 *
 * Just attempt to get a record lock of the requested type on
 * the entire file (l_whence = SEEK_SET, l_start = 0, l_len = 0).
 */

/* ARGSUSED */
flock(p, uap, retval)
	struct proc *p;
	register struct args {
		int	fd;
		int	how;
	} *uap;
	int *retval;
{
	USES_VOP_ADVLOCK;
	register struct filedesc *fdp = p->p_fd;
	register struct file *fp;
	struct vnode *vp;
	struct flock lf;
	int error;

	if ((unsigned)uap->fd >= fdp->fd_nfiles ||
	    (fp = fdp->fd_ofiles[uap->fd]) == NULL)
		return (EBADF);
	if (fp->f_type != DTYPE_VNODE)
		return (EOPNOTSUPP);
	vp = (struct vnode *)fp->f_data;
	lf.l_whence = SEEK_SET;
	lf.l_start = 0;
	lf.l_len = 0;
	if (uap->how & LOCK_UN) {
		lf.l_type = F_UNLCK;
		fp->f_flag &= ~FHASLOCK;
		return (VOP_ADVLOCK(vp, (caddr_t)fp, F_UNLCK, &lf, F_FLOCK));
	}
	if (uap->how & LOCK_EX)
		lf.l_type = F_WRLCK;
	else if (uap->how & LOCK_SH)
		lf.l_type = F_RDLCK;
	else
		return (EBADF);
	fp->f_flag |= FHASLOCK;
	if (uap->how & LOCK_NB)
		return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf, F_FLOCK));
	return (VOP_ADVLOCK(vp, (caddr_t)fp, F_SETLK, &lf, F_FLOCK|F_WAIT));
}

/*
 * File Descriptor pseudo-device driver (/dev/fd/).
 *
 * Opening minor device N dup()s the file (if any) connected to file
 * descriptor N belonging to the calling process.  Note that this driver
 * consists of only the ``open()'' routine, because all subsequent
 * references to this file will be direct to the other driver.
 */
/* ARGSUSED */
fdopen(dev, mode, type, p)
	dev_t dev;
	int mode, type;
	struct proc *p;
{

	/*
	 * XXX Kludge: set curproc->p_dupfd to contain the value of the
	 * the file descriptor being sought for duplication. The error
	 * return ensures that the vnode for this device will be released
	 * by vn_open. Open will detect this special error and take the
	 * actions in dupfdopen below. Other callers of vn_open or VOP_OPEN
	 * will simply report the error.
	 */
	p->p_dupfd = minor(dev);
	return (ENODEV);
}

/*
 * Duplicate the specified descriptor to a free descriptor.
 */
dupfdopen(fdp, indx, dfd, mode, error)
	register struct filedesc *fdp;
	register int indx, dfd;
	int mode;
	int error;
{
	register struct file *wfp;
	struct file *fp;

	/*
	 * If the to-be-dup'd fd number is greater than the allowed number
	 * of file descriptors, or the fd to be dup'd has already been
	 * closed, reject.  Note, check for new == old is necessary as
	 * falloc could allocate an already closed to-be-dup'd descriptor
	 * as the new descriptor.
	 */
	fp = fdp->fd_ofiles[indx];
	if ((u_int)dfd >= fdp->fd_nfiles ||
	    (wfp = fdp->fd_ofiles[dfd]) == NULL || fp == wfp)
		return (EBADF);

	/*
	 * There are two cases of interest here.
	 *
	 * For ENODEV simply dup (dfd) to file descriptor
	 * (indx) and return.
	 *
	 * For ENXIO steal away the file structure from (dfd) and
	 * store it in (indx).  (dfd) is effectively closed by
	 * this operation.
	 *
	 * Any other error code is just returned.
	 */
	switch (error) {
	case ENODEV:
		/*
		 * Check that the mode the file is being opened for is a
		 * subset of the mode of the existing descriptor.
		 */
		if (((mode & (FREAD|FWRITE)) | wfp->f_flag) != wfp->f_flag)
			return (EACCES);
		fdp->fd_ofiles[indx] = wfp;
		fdp->fd_ofileflags[indx] = fdp->fd_ofileflags[dfd];
		wfp->f_count++;
		if (indx > fdp->fd_lastfile)
			fdp->fd_lastfile = indx;
		return (0);

	case ENXIO:
		/*
		 * Steal away the file pointer from dfd, and stuff it into indx.
		 */
		fdp->fd_ofiles[indx] = fdp->fd_ofiles[dfd];
		fdp->fd_ofiles[dfd] = NULL;
		fdp->fd_ofileflags[indx] = fdp->fd_ofileflags[dfd];
		fdp->fd_ofileflags[dfd] = 0;
		/*
		 * Complete the clean up of the filedesc structure by
		 * recomputing the various hints.
		 */
		if (indx > fdp->fd_lastfile)
			fdp->fd_lastfile = indx;
		else
			while (fdp->fd_lastfile > 0 &&
			       fdp->fd_ofiles[fdp->fd_lastfile] == NULL)
				fdp->fd_lastfile--;
			if (dfd < fdp->fd_freefile)
				fdp->fd_freefile = dfd;
		return (0);

	default:
		return (error);
	}
	/* NOTREACHED */
}