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     VOP_LOOKUP — lookup a component of a pathname


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

     VOP_LOOKUP(struct vnode *dvp, struct vnode **vpp,
         struct componentname *cnp);


     This entry point looks up a single pathname component in a given

     Its arguments are:

     dvp  The locked vnode of the directory to search.

     vpp  The address of a variable where the resulting locked vnode should be

     cnp  The pathname component to be searched for.

     Cnp is a pointer to a componentname structure defined as follows:

     struct componentname {
              * Arguments to lookup.
             u_long  cn_nameiop;     /* namei operation */
             u_long  cn_flags;       /* flags to namei */
             struct  thread *cn_thread;      /* thread requesting lookup */
             struct  ucred *cn_cred; /* credentials */
              * Shared between lookup and commit routines.
             char    *cn_pnbuf;      /* pathname buffer */
             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() */

     Convert a component of a pathname into a pointer to a locked vnode.  This
     is a very central and rather complicated routine.  If the file system is
     not maintained in a strict tree hierarchy, this can result in a deadlock

     The cnp->cn_nameiop argument is LOOKUP, CREATE, RENAME, or DELETE
     depending on the intended use of the object.  When CREATE, RENAME, or
     DELETE is specified, information usable in creating, renaming, or
     deleting a directory entry may be calculated.

     Overall outline of VOP_LOOKUP:

           Check accessibility of directory.  Look for name in cache, if
           found, then return name.  Search for name in directory, goto to
           found or notfound as appropriate.


           If creating or renaming and at end of pathname, return EJUSTRETURN,
           leaving info on available slots else return ENOENT.


           If at end of path and deleting, return information to allow delete.
           If at end of path and renaming, lock target inode and return info
           to allow rename.  If not at end, add name to cache; if at end and
           neither creating nor deleting, add name to cache.


     The directory, dvp should be locked on entry.  If an error (note: the
     return value EJUSTRETURN is not considered an error) is detected, it will
     be returned locked.  Otherwise, it will be unlocked unless both
     LOCKPARENT and ISLASTCN are specified in cnp->cn_flags.  If an entry is
     found in the directory, it will be returned locked.


     Zero is returned with *vpp set to the locked vnode of the file if the
     component is found.  If the component being searched for is ".", then the
     vnode just has an extra reference added to it with vref(9).  The caller
     must take care to release the locks appropriately in this case.

     If the component is not found and the operation is CREATE or RENAME, the
     flag ISLASTCN is specified and the operation would succeed, the special
     return value EJUSTRETURN is returned.  Otherwise, an appropriate error
     code is returned.


     [ENOTDIR]          The vnode dvp does not represent a directory.

     [ENOENT]           The component dvp was not found in this directory.

     [EACCES]           Access for the specified operation is denied.

     [EJUSTRETURN]      A CREATE or RENAME operation would be successful.


     vnode(9), VOP_ACCESS(9), VOP_CREATE(9), VOP_MKDIR(9), VOP_MKNOD(9),


     The function VOP_LOOKUP appeared in 4.3BSD.


     This manual page was written by Doug Rabson, with some text from comments
     in ufs_lookup.c.