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path_resolution(2) - Unix/Linux path resolution - find the file referred to by a filename - man 2 path_resolution

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PATH_RESOLUTION(2)         Linux Programmer's Manual        PATH_RESOLUTION(2)



NAME
       Unix/Linux path resolution - find the file(1,n) referred to by a filename

DESCRIPTION
       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 current process. (A process
       inherits its root directory from its parent. Usually this will  be  the
       root  directory  of  the  file(1,n) hierarchy. A process may get a different
       root directory by use of the chroot(1,2)(2) system call. A process  may  get
       an  entirely  private  namespace in(1,8) 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(1,8)
       the current lookup directory.

       If the process does not have search permission on  the  current  lookup
       directory, an EACCES error(8,n) is returned ("Permission denied").

       If  the  component  is not found, an ENOENT error(8,n) is returned ("No such
       file(1,n) or directory").

       If the component is found, but is neither a directory  nor  a  symbolic
       link(1,2), an ENOTDIR error(8,n) is returned ("Not a directory").

       If the component is found and is a directory, we set(7,n,1 builtins) the current lookup
       directory to that directory, and go to the next component.

       If the component is found and is a symbolic link(1,2)  (symlink),  we  first
       resolve this symbolic link(1,2) (with the current lookup directory as start-
       ing lookup directory). Upon error(8,n), that  error(8,n)  is  returned.   If  the
       result  is not a directory, an ENOTDIR error(8,n) is returned.  If the reso-
       lution of the symlink is successful and returns a directory, we set(7,n,1 builtins) 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 symlinks followed. An ELOOP error(8,n)
       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(1,8) 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(8,n)  if(3,n)  the
       component  is not found - maybe we are just creating it. The details on
       the treatment of the final entry are described in(1,8) 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-
       tively.

       The  path  resolution process will assume that these entries have their
       conventional meanings, regardless of whether they are actually  present
       in(1,8) the physical filesystem.

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


   Mount points
       After  a  "mount(2,8)  dev  path" command, the pathname "path" refers to the
       root of the filesystem hierarchy on the device "dev", and no longer  to
       whatever it referred to earlier.

       One  can walk out of a mounted filesystem: "path/.." refers to the par-
       ent directy of "path", outside of the filesystem hierarchy on "dev".


   Trailing slashes
       If a pathname ends in(1,8) a '/', that forces resolution  of  the  preceding
       component  as  in(1,8)  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(1,2), then it depends
       on  the  system  call whether the file(1,n) referred to will be the symbolic
       link(1,2) or the result of path resolution on its  contents.   For  example,
       the  system  call  lstat(2)  will operate on the symlink, while stat(1,2)(2)
       operates on the file(1,n) 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(8,n) 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(1,8) this case.


   Permissions
       The permission bits of a file(1,n) consist of three groups  of  three  bits,
       cf.  chmod(1,2)(1)  and  stat(1,2)(2).  The first group of three is used when the
       effective user ID of the current process equals the  owner  ID  of  the
       file.  The  second group of three is used when the group ID of the file(1,n)
       either equals the effective group ID of the current process, or is  one
       of  the  supplementary group IDs of the current process (as set(7,n,1 builtins) by set-
       groups(2)).  When neither holds, the third group is used.

       Of the three bits used, the first bit determines read(2,n,1 builtins)  permission,  the
       second  write(1,2)  permission,  and  the last execute permission in(1,8) case of
       ordinary files, or search permission in(1,8) case of directories.

       Linux uses the fsuid instead of the effective  user  ID  in(1,8)  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(1,n)  system  user  ID".   The
       concept  was required for the implementation of a user space NFS server
       at a time(1,2,n) when processes could send(2,n) a signal(2,7) to a process with the same
       effective  user ID. It is obsolete now. Nobody should use setfsuid(2).)

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



   Capabilities
       If  the  permission bits of the file(1,n) deny whatever is asked, permission
       can still be granted by the appropriate capabilities.

       Traditional systems do not use capabilities and root  (user  ID  0)  is
       all-powerful. Such systems are presently (2.6.7) handled by giving root
       all capabilities except for CAP_SETPCAP. More precisely, at exec(3,n,1 builtins) time(1,2,n) a
       process gets(3,n) all capabilities except CAP_SETPCAP and the five capabili-
       ties  CAP_CHOWN,  CAP_DAC_OVERRIDE,  CAP_DAC_READ_SEARCH,   CAP_FOWNER,
       CAP_FSETID, in(1,8) case it has zero euid, and it gets(3,n) these last five capa-
       bilities in(1,8) case it has zero fsuid, while all other  processes  get  no
       capabilities.

       The  CAP_DAC_OVERRIDE capability overrides all permission checking, but
       will only grant execute permission when at least one of the three  exe-
       cute permission bits is set.

       The  CAP_DAC_READ_SEARCH  capability will grant read(2,n,1 builtins) and search permis-
       sion on directories, and read(2,n,1 builtins) permission on ordinary files.

       The CAP_SYS_ADMIN capability will (e.g.) allow a process to violate the
       limit  (visible in(1,8) /proc(5,n)/sys/fs/file-max) on the maximum number of open(2,3,n)
       files in(1,8) the system, where a process lacking that capability would  see
       an ENFILE error(8,n) return.




SEE ALSO
       capabilities(7)



Linux 2.6.7                       2004-06-21                PATH_RESOLUTION(2)

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