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 5.0.1                   December 28, 2007                  NetBSD 5.0.1

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