NAME
namei, NDINIT, NDFREE, NDHASGIANT - pathname translation and lookup
operations
SYNOPSIS
#include <sys/param.h>
#include <sys/proc.h>
#include <sys/namei.h>
int
namei(struct nameidata *ndp);
void
NDINIT(struct nameidata *ndp, u_long op, u_long flags,
enum uio_seg segflg, const char *namep, struct thread *td);
void
NDFREE(struct nameidata *ndp, const uint flags);
int
NDHASGIANT(struct nameidata *ndp);
DESCRIPTION
The namei facility allows the client to perform pathname translation and
lookup operations. The namei functions will increment the reference
count for the vnode in question. The reference count has to be
decremented after use of the vnode, by using either vrele(9) or vput(9),
depending on whether the LOCKLEAF flag was specified or not. If the
Giant lock is required, namei will acquire it if the caller indicates it
is MPSAFE, in which case the caller must later release Giant based on the
results of NDHASGIANT().
The NDINIT() function is used to initialize namei components. It takes
the following arguments:
ndp The struct nameidata to initialize.
op The operation which namei() will perform. The following
operations are valid: LOOKUP, CREATE, DELETE, and RENAME. The
latter three are just setup for those effects; just calling
namei() will not result in VOP_RENAME() being called.
flags Operation flags. Several of these can be effective at the same
time.
segflg UIO segment indicator. This indicates if the name of the object
is in userspace (UIO_USERSPACE) or in the kernel address space
(UIO_SYSSPACE).
namep Pointer to the component’s pathname buffer (the file or directory
name that will be looked up).
td The thread context to use for namei operations and locks.
NAMEI OPERATION FLAGS
The namei() function takes the following set of “operation flags” that
influence its operation:
LOCKLEAF Lock vnode on return. This is a full lock of the vnode; the
VOP_UNLOCK(9) should be used to release the lock (or vput(9)
which is equivalent to calling VOP_UNLOCK(9) followed by
vrele(9), all in one).
LOCKPARENT This flag lets the namei() function return the parent
(directory) vnode, ni_dvp in locked state, unless it is
identical to ni_vp, in which case ni_dvp is not locked per se
(but may be locked due to LOCKLEAF). If a lock is enforced,
it should be released using vput(9) or VOP_UNLOCK(9) and
vrele(9).
WANTPARENT This flag allows the namei() function to return the parent
(directory) vnode in an unlocked state. The parent vnode
must be released separately by using vrele(9).
MPSAFE With this flag set, namei() will conditionally acquire Giant
if it is required by a traversed file system. MPSAFE callers
should pass the results of NDHASGIANT() to VFS_UNLOCK_GIANT
in order to conditionally release Giant if necessary.
NOCACHE Avoid namei() creating this entry in the namecache if it is
not already present. Normally, namei() will add entries to
the name cache if they are not already there.
FOLLOW With this flag, namei() will follow the symbolic link if the
last part of the path supplied is a symbolic link (i.e., it
will return a vnode for whatever the link points at, instead
for the link itself).
NOOBJ Do not call vfs_object_create() for the returned vnode, even
though it meets required criteria for VM support.
NOFOLLOW Do not follow symbolic links (pseudo). This flag is not
looked for by the actual code, which looks for FOLLOW.
NOFOLLOW is used to indicate to the source code reader that
symlinks are intentionally not followed.
SAVENAME Do not free the pathname buffer at the end of the namei()
invocation; instead, free it later in NDFREE() so that the
caller may access the pathname buffer. See below for
details.
SAVESTART Retain an additional reference to the parent directory; do
not free the pathname buffer. See below for details.
ALLOCATED ELEMENTS
The nameidata structure is composed of the following fields:
ni_startdir In the normal case, this is either the current directory
or the root. It is the current directory if the name
passed in does not start with ‘/’ and we have not gone
through any symlinks with an absolute path, and the root
otherwise.
In this case, it is only used by lookup(), and should
not be considered valid after a call to namei(). If
SAVESTART is set, this is set to the same as ni_dvp,
with an extra vref(9). To block NDFREE() from releasing
ni_startdir, the NDF_NO_STARTDIR_RELE can be set.
ni_dvp Vnode pointer to directory of the object on which lookup
is performed. This is available on successful return if
LOCKPARENT or WANTPARENT is set. It is locked if
LOCKPARENT is set. Freeing this in NDFREE() can be
inhibited by NDF_NO_DVP_RELE, NDF_NO_DVP_PUT, or
NDF_NO_DVP_UNLOCK (with the obvious effects).
ni_vp Vnode pointer to the resulting object, NULL otherwise.
The v_usecount field of this vnode is incremented. If
LOCKLEAF is set, it is also locked.
Freeing this in NDFREE() can be inhibited by
NDF_NO_VP_RELE, NDF_NO_VP_PUT, or NDF_NO_VP_UNLOCK (with
the obvious effects).
ni_cnd.cn_pnbuf The pathname buffer contains the location of the file or
directory that will be used by the namei operations. It
is managed by the uma(9) zone allocation interface. If
the SAVESTART or SAVENAME flag is set, then the pathname
buffer is available after calling the namei() function.
To only deallocate resources used by the pathname
buffer, ni_cnd.cn_pnbuf, then NDF_ONLY_PNBUF flag can be
passed to the NDFREE() function. To keep the pathname
buffer intact, the NDF_NO_FREE_PNBUF flag can be passed
to the NDFREE() function.
FILES
src/sys/kern/vfs_lookup.c
SEE ALSO
uio(9), uma(9), VFS(9), VFS_UNLOCK_GIANT(9), vnode(9), vput(9), vref(9)
AUTHORS
This manual page was written by Eivind Eklund 〈eivind@FreeBSD.org〉 and
later significantly revised by Hiten M. Pandya 〈hmp@FreeBSD.org〉.
BUGS
The LOCKPARENT flag does not always result in the parent vnode being
locked. This results in complications when the LOCKPARENT is used. In
order to solve this for the cases where both LOCKPARENT and LOCKLEAF are
used, it is necessary to resort to recursive locking.
Non-MPSAFE file systems exist, requiring callers to conditionally unlock
Giant.