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NAMEI(9) Kernel Developer's Manual NAMEI(9)

namei, NDINIT, NDINIT_AT, NDFREE_PNBUF — pathname translation and lookup operations

#include <sys/param.h>
#include <sys/fcntl.h>
#include <sys/namei.h>

int
namei(struct nameidata *ndp);

void
NDINIT(struct nameidata *ndp, enum nameiop op, u_int64_t flags, enum uio_seg segflg, const char *namep);

void
NDINIT_AT(struct nameidata *ndp, enum nameiop op, u_int64_t flags, enum uio_seg segflg, const char *namep, int dirfd);

void
NDFREE_PNBUF(struct nameidata *ndp);

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.

The () macro is used to initialize namei components. It takes the following arguments:

ndp
A pointer to the struct nameidata to initialize.
op
The operation which () 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 () being called.
flags
Operation flags, described in the next section. 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).

The () macro is similar to NDINIT(), but takes one extra argument:

dirfd
File descriptor referencing a directory, or the special value AT_FDCWD meaning the calling thread's current working directory. Lookups will be performed relative to this directory.

The () macro is used to free the pathname buffer. It must be called exactly once for each successful () call. It takes the following argument:

ndp
A pointer to a struct nameidata that was used in a successful namei() call.

The namei() function takes the following set of “operation flags” that influence its operation:

An alias for NOCACHE.
Keep the positive-caching entry in cache. This flag is typically combined with NOCACHE to not cache a new entry, but keep existing one, or with MAKEENTRY.
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.
Lock vnode on return with LK_EXCLUSIVE unless LOCKSHARED is also set. 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).
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).
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).
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).
Makes the namei operation fail if the target exists. It requires that the LOCKPARENT flag is set and LOCKLEAF is not.
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).
For namei call initialized with NDINIT_AT(), allow the namep path to be empty. In this case, the dirfd file descriptor may reference a file of arbitrary type, not necessary a directory, and lookup returns the vnode for this file.
Lock vnode on return with LK_SHARED, if permitted by the file system that owns the vnode. The file system must explicitly permit this by setting MNTK_LOOKUP_SHARED in mp->mnt_kern_flag during mount and by calling () when allocating the vnode. If LOCKLEAF is specified but shared locking is not permitted, then the vnode will be returned with LK_EXCLUSIVE. 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).
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.
Requires that namei did not cross the dirfd directory. The flag is used to implement O_RESOLVE_BENEATH flag for openat(2).
The component is embedded in a namei lookup structure, and the () function can be used to obtain that structure. This can be useful in VOP_LOOKUP(9) implementations which need to obtain extra lookup metadata.

These flags are used for several purposes. Some of them affects the global namei operation, some provide information for processing of the specific path element, for instance, by the VOP_LOOKUP implementation of the involved filesystem.

Specifies that the lookup should act as if the final node is located on read-only mount. The flag is typically used by file servers, e.g. NFS, to handle read-only exports.
The namei was restarted with (). This is used internally for double-root lookups used by ABI subsystems, after the lookup with native root failed. The components are reset to the original values, and lookup is repeated with different root, once.
Ignore whiteouts, e.g. when checking if a directory is empty.
The result of lookup is whiteout.
Handle whiteouts, the instruction for the () filesystem methods.
The lookup is done for creating a new entry that will be directory. It allows the trailing slash in the path string.
The caller is the code that opens a file. This allows to weaken the lock mode of the return vnode, if the mount point indicates extended shared lock support.
Do not cross mount points during lookup.

For “..” lookup leading to mount root, returns the root vnode of the mount instead of the covered vnode of the filesystem where the mount was placed.

For other lookups passing over mount, do not jump into the mounted filesystem. This allows to descend into the file hierarchy otherwise shadowed by the mount point.

Do not perform MAC checks during lookup.
Audit the looked up vnode information, use the first slot for audit information.
Same as AUDITVNODE1 but use the second slot.
Do not perform capability checks. If the calling process is in capability mode, lookup is denied outright.
The lookup was for open and file will be opened for read.
The lookup was for open and file will be opened for write.
Leave ioctl caps for the caller. See the description of namei results.
Opening a named attribute (directory).
Do not perform check for allowed execution on the starting directory. It is used to implement the POSIX-required semantic for openat(2) lookups that must use the permissions from time the directory was opened, and not when used for lookup.
Looked-up entry is to be added to name cache.
Current component is symlink, and it needs the interpretation according to the FOLLOW or NOFOLLOW flags.
This is last component of pathname. It is handled specially, many flags augment its processing.
Current component name is “..”. Usually implies a need to specially handle the vnode locking for instantiation of the target vnode. The generic () function and its more specialized variant () might be useful.
Path ended in a slash.
Create a named attribute dir.

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 (), and should not be considered valid after a call to ().

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.
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.
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.

The struct namei member ni_resflags returns the following flags giving some details of the succesfull operation:

The path passed was absolute.
Restricted lookup result. Only relative lookups were done to resolve the path to vnode.
The EMPTYPATH flag was provided and used. In particular, the passed path was empty.

If the WANTIOCTLCAPS flag was specified, on return the ni_filecaps member of the struct namei contains the capabilities of the file descriptor used as the lookup starting point (dirfd).

If successful, namei() will return 0, otherwise it will return an error.

src/sys/kern/vfs_lookup.c
 

Assuming the path variable contains a pointer to userspace path string, the following example looks up the file named by it, and performs required error and resource handling:

	char *path;
	struct nameidata nd;
	int error;

	NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF | AUDITVNODE1, UIO_USERSPACE,
	    path);
	if ((error = namei(&nd)) != 0)
		return (error);
	NDFREE_PNBUF(&nd);
	... use nd.ni_vp vnode

Errors which namei() may return:

[]
A component of the specified pathname is not a directory when a directory is expected.
[]
A component of a pathname exceeded 255 characters, or an entire pathname exceeded 1023 characters.
[]
A component of the specified pathname does not exist, or the pathname is an empty string.
[]
An attempt is made to access a file in a way forbidden by its file access permissions.
[]
Too many symbolic links were encountered in translating the pathname.
[]
An attempt is made to open a directory with write mode specified.
[]
The last component of the pathname specified for a DELETE or RENAME operation is ‘.’.
[]
An attempt is made to modify a file or directory on a read-only file system.

uio(9), uma(9), VFS(9), vnode(9), vput(9), vref(9), vrele(9)

This manual page was written by Eivind Eklund <eivind@FreeBSD.org> and later significantly revised by Hiten M. Pandya <hmp@FreeBSD.org>.

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.

May 16, 2025 release-26.07