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PATH_RESOLUTION(7)	   Linux Programmers Manual	   PATH_RESOLUTION(7)

       Unix/Linux path resolution - find the file referred to by a filename

       Some  Unix/Linux  system calls have as parameter one or more filenames.
       A filename (or pathname) is resolved as follows.

   Step 1: Start of the resolution process
       If the pathname starts with the	'/'  character,  the  starting	lookup
       directory  is  the  root  directory of the calling process.  (A process
       inherits its root directory from its parent.  Usually this will be  the
       root  directory	of  the file hierarchy.  A process may get a different
       root directory by use of the chroot(2) system call.  A process may  get
       an  entirely  private  namespace in case it  or one of its ancestors
       was started by an invocation of the clone(2) system call that  had  the
       CLONE_NEWNS flag set.)  This handles the '/' part of the pathname.

       If  the	pathname  does	not start with the '/' character, the starting
       lookup directory of the	resolution  process  is  the  current  working
       directory of the process.  (This is also inherited from the parent.  It
       can be changed by use of the chdir(2) system call.)

       Pathnames starting with a '/' character are called absolute  pathnames.
       Pathnames not starting with a '/' are called relative pathnames.

   Step 2: Walk along the path
       Set  the  current  lookup  directory  to the starting lookup directory.
       Now, for each non-final component of the pathname, where a component is
       a substring delimited by '/' characters, this component is looked up in
       the current lookup directory.

       If the process does not have search permission on  the  current	lookup
       directory, an EACCES error is returned ("Permission denied").

       If  the	component  is not found, an ENOENT error is returned ("No such
       file or directory").

       If the component is found, but is neither a directory  nor  a  symbolic
       link, an ENOTDIR error is returned ("Not a directory").

       If the component is found and is a directory, we set the current lookup
       directory to that directory, and go to the next component.

       If the component is found and is a symbolic link  (symlink),  we  first
       resolve this symbolic link (with the current lookup directory as start
       ing lookup directory).  Upon error, that error  is  returned.   If  the
       result  is not a directory, an ENOTDIR error is returned.  If the reso
       lution of the symlink is successful and returns a directory, we set the
       current	lookup	directory to that directory, and go to the next compo
       nent.  Note that the resolution process here  involves  recursion.   In
       order to protect the kernel against stack overflow, and also to protect
       against denial of service, there are limits on  the  maximum  recursion
       depth,  and on the maximum number of symbolic links followed.  An ELOOP
       error is returned when the maximum is exceeded  ("Too  many  levels  of
       symbolic links").

   Step 3: Find the final entry
       The  lookup  of the final component of the pathname goes just like that
       of all other components, as described in the previous  step,  with  two
       differences:  (i) the final component need not be a directory (at least
       as far as the path resolution process is concerned  it may have to  be
       a  directory,  or  a  non-directory, because of the requirements of the
       specific system call), and (ii) it is not necessarily an error  if  the
       component is not found  maybe we are just creating it.  The details on
       the treatment of the final entry are described in the manual  pages  of
       the specific system calls.

   . and ..
       By  convention,	every  directory  has  the entries "." and "..", which
       refer to the directory itself and  to  its  parent  directory,  respec

       The  path  resolution process will assume that these entries have their
       conventional meanings, regardless of whether they are actually  present
       in the physical file system.

       One cannot walk down past the root: "/.." is the same as "/".

   Mount points
       After  a  "mount  dev  path" command, the pathname "path" refers to the
       root of the file system hierarchy on the device "dev", and no longer to
       whatever it referred to earlier.

       One can walk out of a mounted file system: "path/.." refers to the par
       ent directory of "path", outside of the file system hierarchy on "dev".

   Trailing slashes
       If  a  pathname	ends in a '/', that forces resolution of the preceding
       component as in Step 2: it has to exist and  resolve  to  a  directory.
       Otherwise  a  trailing  '/'  is ignored.  (Or, equivalently, a pathname
       with a trailing '/' is equivalent to the pathname obtained by appending
       '.' to it.)

   Final symlink
       If the last component of a pathname is a symbolic link, then it depends
       on the system call whether the file referred to will  be  the  symbolic
       link  or  the  result of path resolution on its contents.  For example,
       the system call lstat(2) will operate on  the  symlink,	while  stat(2)
       operates on the file pointed to by the symlink.

   Length limit
       There  is  a  maximum  length  for pathnames.  If the pathname (or some
       intermediate pathname obtained while resolving symbolic links)  is  too
       long, an ENAMETOOLONG error is returned ("File name too long").

   Empty pathname
       In the original Unix, the empty pathname referred to the current direc
       tory.  Nowadays POSIX decrees  that  an	empty  pathname  must  not  be
       resolved successfully.  Linux returns ENOENT in this case.

       The  permission	bits  of a file consist of three groups of three bits,
       cf. chmod(1) and stat(2).  The first group of three is  used  when  the
       effective  user	ID  of	the calling process equals the owner ID of the
       file.  The second group of three is used when the group ID of the  file
       either  equals the effective group ID of the calling process, or is one
       of the supplementary group IDs of the calling process (as set  by  set
       groups(2)).  When neither holds, the third group is used.

       Of  the	three bits used, the first bit determines read permission, the
       second write permission, and the last execute  permission  in  case  of
       ordinary files, or search permission in case of directories.

       Linux  uses  the  fsuid	instead of the effective user ID in permission
       checks.	Ordinarily the fsuid will equal the effective user ID, but the
       fsuid can be changed by the system call setfsuid(2).

       (Here  "fsuid"  stands  for  something like "file system user ID".  The
       concept was required for the implementation of a user space NFS	server
       at a time when processes could send a signal to a process with the same
       effective user ID.  It  is  obsolete  now.   Nobody  should  use  setf

       Similarly, Linux uses the fsgid ("file system group ID") instead of the
       effective group ID.  See setfsgid(2).

   Bypassing permission checks: superuser and capabilities
       On a traditional Unix system, the superuser (root, user ID 0)  is  all-
       powerful,  and  bypasses  all  permissions  restrictions when accessing

       On Linux, superuser privileges are divided into capabilities (see capa
       bilities(7)).   Two  capabilities  are  relevant  for  file permissions
       checks: CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH.  (A process has these
       capabilities if its fsuid is 0.)

       The  CAP_DAC_OVERRIDE capability overrides all permission checking, but
       only grants execute permission when at least one of  the  files	three
       execute permission bits is set.

       The CAP_DAC_READ_SEARCH capability grants read and search permission on
       directories, and read permission on ordinary files.

       capabilities(7), credentials(7), symlink(7)

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2004-06-21		    PATH_RESOLUTION(7)

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