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Man Pages
VOP_LOOKUP(9) FreeBSD Kernel Developer's Manual VOP_LOOKUP(9)

VOP_LOOKUP
lookup a component of a pathname

#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/namei.h>
int
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. It 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 */ 
	int     cn_lkflags;     /* Lock flags LK_EXCLUSIVE or LK_SHARED */ 
	/* 
	 * 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 */ 
};
    
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.
notfound:
If creating or renaming and at end of pathname, return EJUSTRETURN, leaving info on available slots else return ENOENT.
found:
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 and exit, regardless of error condition. 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.

[]
The vnode dvp does not represent a directory.
[]
The component dvp was not found in this directory.
[]
Access for the specified operation is denied.
[]
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), VOP_SYMLINK(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.
August 8, 2018 FreeBSD 12.0-RELEASE

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