NAME
namei
, lookup
,
relookup
, NDINIT
—
pathname lookup
SYNOPSIS
#include
<sys/param.h>
#include <sys/namei.h>
int
namei
(struct
nameidata *ndp);
int
lookup
(struct
nameidata *ndp);
int
relookup
(struct
vnode *dvp, struct vnode
**vpp, struct
componentname *cnp);
void
NDINIT
(struct
nameidata *ndp, u_long
op, u_long flags,
enum uio_seg segflg,
const char *namep,
struct proc *p);
DESCRIPTION
The namei
interface is used to convert
pathnames to file system vnodes. The name of the interface is actually a
contraction of the words
name and
inode
for name-to-inode conversion, in the days before the
vfs(9) interface was implemented.
The arguments passed to the functions are encapsulated in the nameidata structure. It has the following structure:
struct nameidata { /* * Arguments to namei/lookup. */ const char *ni_dirp; /* pathname pointer */ enum uio_seg ni_segflg; /* location of pathname */ /* * Arguments to lookup. */ struct vnode *ni_startdir; /* starting directory */ struct vnode *ni_rootdir; /* logical root directory */ /* * Results: returned from/manipulated by lookup */ struct vnode *ni_vp; /* vnode of result */ struct vnode *ni_dvp; /* vnode of intermediate dir */ /* * Shared between namei and lookup/commit routines. */ size_t ni_pathlen; /* remaining chars in path */ const char *ni_next; /* next location in pathname */ u_long ni_loopcnt; /* count of symlinks encountered */ /* * Lookup parameters */ struct componentname ni_cnd; };
The namei
interface
accesses vnode operations by passing arguments in the partially initialised
componentname
structure
ni_cnd.
This structure describes the subset of information from the nameidata
structure that is passed through to the vnode operations. See
VOP_LOOKUP(9) for more information. The details of the
componentname structure are not absolutely necessary since the members are
initialised by the helper macro
NDINIT
().
It is useful to know the operations and flags as specified in
VOP_LOOKUP(9).
The namei
interface
overloads
ni_cnd.cn_flags
with some additional flags. These flags should be specific to the
namei
interface and ignored by vnode operations.
However, due to the historic close relationship between the
namei
interface and the vnode operations, these
flags are sometimes used (and set) by vnode operations, particularly
VOP_LOOKUP
().
The additional flags are:
- NOCROSSMOUNT
- do not cross mount points
- RDONLY
- lookup with read-only semantics
- HASBUF
- caller has allocated pathname buffer ni_cnd.cn_pnbuf
- SAVENAME
- save pathname buffer
- SAVESTART
- save starting directory
- ISDOTDOT
- current pathname component is ..
- MAKEENTRY
- add entry to the name cache
- ISLASTCN
- this is last component of pathname
- ISSYMLINK
- symlink needs interpretation
- REQUIREDIR
- must be a directory
- PDIRUNLOCK
VOP_LOOKUP
() unlocked parent dir
If the caller of
namei
() sets
the SAVENAME flag, then it must free the buffer. If
VOP_LOOKUP
() sets the flag, then the buffer must be
freed by either the commit routine or the
VOP_ABORT
()
routine. The SAVESTART flag is set only by the callers of
namei
(). It implies SAVENAME plus the addition of
saving the parent directory that contains the name in
ni_startdir. It allows repeated calls to
lookup
() for the name being sought. The caller is
responsible for releasing the buffer and for invoking
vrele
()
on ni_startdir.
All access to the namei
interface must be
in process context. Pathname lookups cannot be done in interrupt
context.
FUNCTIONS
namei
(ndp)- Convert a pathname into a pointer to a vnode. The pathname is specified by
ndp->ni_dirp and is of length
ndp->ni_pathlen. The
ndp->segflg
flags defines whether the name in ndp->ni_dirp is an
address in kernel space (UIO_SYSSPACE) or an address in user space
(UIO_USERSPACE). The vnode for the pathname is referenced and returned in
ndp->ni_vp.
If ndp->ni_cnd.cn_flags has the FOLLOW flag set then symbolic links are followed when they occur at the end of the name translation process. Symbolic links are always followed for all other pathname components other than the last.
If the LOCKLEAF flag is set, a locked vnode is returned.
lookup
(ndp)- Search for a pathname. This is a very central and rather complicated
routine.
The pathname is specified by ndp->ni_dirp and is of length ndp->ni_pathlen. The starting directory is taken from ndp->ni_startdir. The pathname is descended until done, or a symbolic link is encountered.
The semantics of
lookup
() are altered by the operation specified by ndp->ni_cnd.cn_nameiop. When CREATE, RENAME, or DELETE is specified, information usable in creating, renaming, or deleting a directory entry may be calculated.If ndp->ci_cnd.cn_flags has LOCKPARENT set, the parent directory is returned locked in ndp->ni_dvp. If WANTPARENT is set, the parent directory is returned unlocked. Otherwise the parent directory is not returned. If the target of the pathname exists and LOCKLEAF is set, the target is returned locked in ndp->ni_vp, otherwise it is returned unlocked.
relookup
(dvp, vpp, cnp)- Reacquire a path name component in a directory. This is a quicker way to lookup a pathname component when the parent directory is known. The unlocked parent directory vnode is specified by dvp and the pathname component by cnp. The vnode of the pathname is returned in the address specified by vpp.
NDINIT
(ndp, op, flags, segflg, namep, p)- Initialise a nameidata structure pointed to by ndp
for use by the
namei
interface. It saves having to deal with the componentname structure inside ndp. The operation and flags are specified by op and flags respectively. These are the values to which ndp->ni_cnd.cn_nameiop and ndp->ni_cnd.cn_flags are respectively set. The segment flags which defines whether the pathname is in kernel address space or user address space is specified by segflg. The argument namep is a pointer to the pathname that ndp->ni_dirp is set to and p is the calling process.
CODE REFERENCES
The name lookup subsystem is implemented within the file sys/kern/vfs_lookup.c.
SEE ALSO
BUGS
It is unfortunate that much of the namei
interface makes assumptions on the underlying vnode operations. These
assumptions are an artefact of the introduction of the vfs interface to
split a file system interface which was historically designed as a tightly
coupled module.