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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 directory.

     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 stored.

     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 situation.

     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

     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), VOP_RENAME(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.