namei(9) - NetBSD Manual Pages

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


NAME
namei, lookup, relookup, NDINIT -- pathname lookup
SYNOPSIS
#include <sys/namei.h> #include <sys/uio.h> #include <sys/vnode.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);
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) inter- face 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 { /* * Arguments to lookup. */ u_long cn_nameiop; /* namei operation */ u_long cn_flags; /* flags to namei */ kauth_cred_t cn_cred; /* credentials */ /* * Shared between lookup and commit routines. */ char *cn_pnbuf; /* pathname buffer */ const char *cn_nameptr; /* pointer to looked up name */ long cn_namelen; /* length of looked up component */ u_long cn_hash; /* hash value of looked up name */ long cn_consume; /* chars to consume in lookup() */ } 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 vnodeops(9) for more infor- mation. 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 vnodeops(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 ISWHITEOUT found whiteout DOWHITEOUT do whiteouts REQUIREDIR must be a directory CREATEDIR trailing slashes are ok PARAMASK mask of parameter descriptors 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 returned in ndp->ni_vp. The par- ent directory is returned locked in ndp->ni_dvp iff LOCKPARENT is specified. 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 trans- lation process. Symbolic links are always followed for all other pathname components other than the last. lookup(ndp) Search for a pathname. This is a very central and rather com- plicated 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 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 is a directory. This is a quicker way to lookup a pathname component when the parent directory is known. The locked parent directory vnode is speci- fied 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) Initialise a nameidata structure pointed to by ndp for use by the namei interface. It saves having to deal with the compo- nentname 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 segflag. The argument namep is a pointer to the pathname that ndp->ni_dirp is set to. This routine stores the credentials of the calling thread (curlwp) in ndp. In the rare case that another set of creden- tials is required for the namei operation, ndp->ni_cnd.cn_cred must be set manually.
CODE REFERENCES
This section describes places within the NetBSD source tree where actual code implementing or using the name lookup subsystem can be found. All pathnames are relative to /usr/src. The name lookup subsystem is implemented within the file sys/kern/vfs_lookup.c.
SEE ALSO
intro(9), namecache(9), vfs(9), vnode(9), vnodeops(9)
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. NetBSD 5.0 December 28, 2007 NetBSD 5.0
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