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NAME

       name_to_handle_at,  open_by_handle_at  -  obtain handle for a pathname and open file via a
       handle

SYNOPSIS

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int name_to_handle_at(int dirfd, const char *pathname,
                             struct file_handle *handle,
                             int *mount_id, int flags);

       int open_by_handle_at(int mount_fd, struct file_handle *handle,
                             int flags);

DESCRIPTION

       The name_to_handle_at() and open_by_handle_at() system calls split  the  functionality  of
       openat(2) into two parts: name_to_handle_at() returns an opaque handle that corresponds to
       a specified file; open_by_handle_at() opens the file corresponding to a handle returned by
       a previous call to name_to_handle_at() and returns an open file descriptor.

   name_to_handle_at()
       The  name_to_handle_at() system call returns a file handle and a mount ID corresponding to
       the file specified by the dirfd and pathname arguments.  The file handle is  returned  via
       the argument handle, which is a pointer to a structure of the following form:

           struct file_handle {
               unsigned int  handle_bytes;   /* Size of f_handle [in, out] */
               int           handle_type;    /* Handle type [out] */
               unsigned char f_handle[0];    /* File identifier (sized by
                                                caller) [out] */
           };

       It  is  the  caller's responsibility to allocate the structure with a size large enough to
       hold the handle returned in f_handle.  Before the call, the handle_bytes field  should  be
       initialized  to  contain  the  allocated  size for f_handle.  (The constant MAX_HANDLE_SZ,
       defined in <fcntl.h>, specifies the maximum expected size for a file handle.  It is not  a
       guaranteed  upper  limit  as  future filesystems may require more space.)  Upon successful
       return, the handle_bytes field is updated to contain the number of bytes actually  written
       to f_handle.

       The  caller  can discover the required size for the file_handle structure by making a call
       in which handle->handle_bytes is zero; in  this  case,  the  call  fails  with  the  error
       EOVERFLOW  and  handle->handle_bytes  is set to indicate the required size; the caller can
       then use this information to allocate a structure of the correct size (see EXAMPLE below).
       Some  care  is needed here as EOVERFLOW can also indicate that no file handle is available
       for this particular name in a filesystem which does normally support  file-handle  lookup.
       This  case can be detected when the EOVERFLOW error is returned without handle_bytes being
       increased.

       Other than the use of the handle_bytes field, the  caller  should  treat  the  file_handle
       structure as an opaque data type: the handle_type and f_handle fields are needed only by a
       subsequent call to open_by_handle_at().

       The flags argument  is  a  bit  mask  constructed  by  ORing  together  zero  or  more  of
       AT_EMPTY_PATH and AT_SYMLINK_FOLLOW, described below.

       Together,  the  pathname and dirfd arguments identify the file for which a handle is to be
       obtained.  There are four distinct cases:

       *  If pathname is a nonempty string containing an absolute  pathname,  then  a  handle  is
          returned for the file referred to by that pathname.  In this case, dirfd is ignored.

       *  If  pathname  is  a  nonempty  string  containing a relative pathname and dirfd has the
          special value AT_FDCWD, then pathname is interpreted relative to  the  current  working
          directory of the caller, and a handle is returned for the file to which it refers.

       *  If  pathname  is  a  nonempty string containing a relative pathname and dirfd is a file
          descriptor referring to a directory, then  pathname  is  interpreted  relative  to  the
          directory  referred  to  by  dirfd,  and  a handle is returned for the file to which it
          refers.  (See openat(2) for an explanation of  why  "directory  file  descriptors"  are
          useful.)

       *  If  pathname is an empty string and flags specifies the value AT_EMPTY_PATH, then dirfd
          can be an open file descriptor referring to any type of file, or AT_FDCWD, meaning  the
          current working directory, and a handle is returned for the file to which it refers.

       The  mount_id  argument returns an identifier for the filesystem mount that corresponds to
       pathname.   This  corresponds  to  the  first   field   in   one   of   the   records   in
       /proc/self/mountinfo.   Opening  the  pathname  in the fifth field of that record yields a
       file descriptor for the mount point; that file descriptor can be used in a subsequent call
       to  open_by_handle_at().   mount_id  is returned both for a successful call and for a call
       that results in the error EOVERFLOW.

       By default, name_to_handle_at() does not dereference pathname if it is  a  symbolic  link,
       and  thus  returns  a  handle  for  the link itself.  If AT_SYMLINK_FOLLOW is specified in
       flags, pathname is dereferenced if it is a symbolic link  (so  that  the  call  returns  a
       handle for the file referred to by the link).

       name_to_handle_at()  does  not trigger a mount when the final component of the pathname is
       an automount point.  When a filesystem supports both file handles and automount points,  a
       name_to_handle_at()  call  on  an automount point will return with error EOVERFLOW without
       having increased handle_bytes.  This can happen since Linux 4.13 with NFS when accessing a
       directory  which  is  on a separate filesystem on the server.  In this case, the automount
       can be triggered by adding a "/" to the end of the pathname.

   open_by_handle_at()
       The open_by_handle_at() system call opens the file referred to by handle,  a  file  handle
       returned by a previous call to name_to_handle_at().

       The  mount_fd argument is a file descriptor for any object (file, directory, etc.)  in the
       mounted filesystem with respect to which handle should be interpreted.  The special  value
       AT_FDCWD can be specified, meaning the current working directory of the caller.

       The  flags  argument  is  as for open(2).  If handle refers to a symbolic link, the caller
       must specify the O_PATH flag, and the symbolic link is not  dereferenced;  the  O_NOFOLLOW
       flag, if specified, is ignored.

       The caller must have the CAP_DAC_READ_SEARCH capability to invoke open_by_handle_at().

RETURN VALUE

       On  success,  name_to_handle_at() returns 0, and open_by_handle_at() returns a nonnegative
       file descriptor.

       In the event of an error, both system calls return -1 and set errno to indicate the  cause
       of the error.

ERRORS

       name_to_handle_at() and open_by_handle_at() can fail for the same errors as openat(2).  In
       addition, they can fail with the errors noted below.

       name_to_handle_at() can fail with the following errors:

       EFAULT pathname, mount_id, or handle points outside your accessible address space.

       EINVAL flags includes an invalid bit value.

       EINVAL handle->handle_bytes is greater than MAX_HANDLE_SZ.

       ENOENT pathname is an empty string, but AT_EMPTY_PATH was not specified in flags.

       ENOTDIR
              The file descriptor supplied in dirfd does not refer to a directory, and it is  not
              the case that both flags includes AT_EMPTY_PATH and pathname is an empty string.

       EOPNOTSUPP
              The filesystem does not support decoding of a pathname to a file handle.

       EOVERFLOW
              The handle->handle_bytes value passed into the call was too small.  When this error
              occurs, handle->handle_bytes is updated to  indicate  the  required  size  for  the
              handle.

       open_by_handle_at() can fail with the following errors:

       EBADF  mount_fd is not an open file descriptor.

       EFAULT handle points outside your accessible address space.

       EINVAL handle->handle_bytes is greater than MAX_HANDLE_SZ or is equal to zero.

       ELOOP  handle refers to a symbolic link, but O_PATH was not specified in flags.

       EPERM  The caller does not have the CAP_DAC_READ_SEARCH capability.

       ESTALE The specified handle is not valid.  This error will occur if, for example, the file
              has been deleted.

VERSIONS

       These system calls first appeared in Linux 2.6.39.  Library support is provided  in  glibc
       since version 2.14.

CONFORMING TO

       These system calls are nonstandard Linux extensions.

       FreeBSD has a broadly similar pair of system calls in the form of getfh() and openfh().

NOTES

       A  file handle can be generated in one process using name_to_handle_at() and later used in
       a different process that calls open_by_handle_at().

       Some filesystem don't support the translation of pathnames to file handles,  for  example,
       /proc, /sys, and various network filesystems.

       A  file handle may become invalid ("stale") if a file is deleted, or for other filesystem-
       specific  reasons.    Invalid   handles   are   notified   by   an   ESTALE   error   from
       open_by_handle_at().

       These  system calls are designed for use by user-space file servers.  For example, a user-
       space NFS server might generate a file handle and pass it to an NFS client.   Later,  when
       the client wants to open the file, it could pass the handle back to the server.  This sort
       of functionality allows a user-space file server to operate in a  stateless  fashion  with
       respect to the files it serves.

       If  pathname  refers to a symbolic link and flags does not specify AT_SYMLINK_FOLLOW, then
       name_to_handle_at() returns a handle for the link  (rather  than  the  file  to  which  it
       refers).   The  process  receiving the handle can later perform operations on the symbolic
       link by converting the handle to a file  descriptor  using  open_by_handle_at()  with  the
       O_PATH  flag,  and  then passing the file descriptor as the dirfd argument in system calls
       such as readlinkat(2) and fchownat(2).

   Obtaining a persistent filesystem ID
       The mount IDs in /proc/self/mountinfo can be  reused  as  filesystems  are  unmounted  and
       mounted.   Therefore,  the  mount ID returned by name_to_handle_at() (in *mount_id) should
       not be treated as a  persistent  identifier  for  the  corresponding  mounted  filesystem.
       However,  an  application can use the information in the mountinfo record that corresponds
       to the mount ID to derive a persistent identifier.

       For example, one can use the device name in the fifth field of  the  mountinfo  record  to
       search for the corresponding device UUID via the symbolic links in /dev/disks/by-uuid.  (A
       more comfortable way of obtaining the UUID is  to  use  the  libblkid(3)  library.)   That
       process  can  then  be  reversed,  using  the  UUID  to  look up the device name, and then
       obtaining the corresponding mount point, in order to produce the mount_fd argument used by
       open_by_handle_at().

EXAMPLE

       The two programs below demonstrate the use of name_to_handle_at() and open_by_handle_at().
       The first program (t_name_to_handle_at.c) uses  name_to_handle_at()  to  obtain  the  file
       handle  and  mount  ID for the file specified in its command-line argument; the handle and
       mount ID are written to standard output.

       The second program (t_open_by_handle_at.c) reads a mount ID and file handle from  standard
       input.   The  program then employs open_by_handle_at() to open the file using that handle.
       If an  optional  command-line  argument  is  supplied,  then  the  mount_fd  argument  for
       open_by_handle_at()  is  obtained  by  opening  the  directory  named  in  that  argument.
       Otherwise, mount_fd is obtained by scanning /proc/self/mountinfo to find  a  record  whose
       mount  ID matches the mount ID read from standard input, and the mount directory specified
       in that record is opened.  (These programs do not deal with the fact that  mount  IDs  are
       not persistent.)

       The following shell session demonstrates the use of these two programs:

           $ echo 'Can you please think about it?' > cecilia.txt
           $ ./t_name_to_handle_at cecilia.txt > fh
           $ ./t_open_by_handle_at < fh
           open_by_handle_at: Operation not permitted
           $ sudo ./t_open_by_handle_at < fh      # Need CAP_SYS_ADMIN
           Read 31 bytes
           $ rm cecilia.txt

       Now  we  delete  and  (quickly) re-create the file so that it has the same content and (by
       chance) the same inode.  Nevertheless, open_by_handle_at() recognizes  that  the  original
       file referred to by the file handle no longer exists.

           $ stat --printf="%i\n" cecilia.txt     # Display inode number
           4072121
           $ rm cecilia.txt
           $ echo 'Can you please think about it?' > cecilia.txt
           $ stat --printf="%i\n" cecilia.txt     # Check inode number
           4072121
           $ sudo ./t_open_by_handle_at < fh
           open_by_handle_at: Stale NFS file handle

   Program source: t_name_to_handle_at.c

       #define _GNU_SOURCE
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <errno.h>
       #include <string.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       int
       main(int argc, char *argv[])
       {
           struct file_handle *fhp;
           int mount_id, fhsize, flags, dirfd, j;
           char *pathname;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s pathname\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           pathname = argv[1];

           /* Allocate file_handle structure */

           fhsize = sizeof(*fhp);
           fhp = malloc(fhsize);
           if (fhp == NULL)
               errExit("malloc");

           /* Make an initial call to name_to_handle_at() to discover
              the size required for file handle */

           dirfd = AT_FDCWD;           /* For name_to_handle_at() calls */
           flags = 0;                  /* For name_to_handle_at() calls */
           fhp->handle_bytes = 0;
           if (name_to_handle_at(dirfd, pathname, fhp,
                       &mount_id, flags) != -1 || errno != EOVERFLOW) {
               fprintf(stderr, "Unexpected result from name_to_handle_at()\n");
               exit(EXIT_FAILURE);
           }

           /* Reallocate file_handle structure with correct size */

           fhsize = sizeof(struct file_handle) + fhp->handle_bytes;
           fhp = realloc(fhp, fhsize);         /* Copies fhp->handle_bytes */
           if (fhp == NULL)
               errExit("realloc");

           /* Get file handle from pathname supplied on command line */

           if (name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags) == -1)
               errExit("name_to_handle_at");

           /* Write mount ID, file handle size, and file handle to stdout,
              for later reuse by t_open_by_handle_at.c */

           printf("%d\n", mount_id);
           printf("%d %d   ", fhp->handle_bytes, fhp->handle_type);
           for (j = 0; j < fhp->handle_bytes; j++)
               printf(" %02x", fhp->f_handle[j]);
           printf("\n");

           exit(EXIT_SUCCESS);
       }

   Program source: t_open_by_handle_at.c

       #define _GNU_SOURCE
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <string.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       /* Scan /proc/self/mountinfo to find the line whose mount ID matches
          'mount_id'. (An easier way to do this is to install and use the
          'libmount' library provided by the 'util-linux' project.)
          Open the corresponding mount path and return the resulting file
          descriptor. */

       static int
       open_mount_path_by_id(int mount_id)
       {
           char *linep;
           size_t lsize;
           char mount_path[PATH_MAX];
           int mi_mount_id, found;
           ssize_t nread;
           FILE *fp;

           fp = fopen("/proc/self/mountinfo", "r");
           if (fp == NULL)
               errExit("fopen");

           found = 0;
           linep = NULL;
           while (!found) {
               nread = getline(&linep, &lsize, fp);
               if (nread == -1)
                   break;

               nread = sscanf(linep, "%d %*d %*s %*s %s",
                              &mi_mount_id, mount_path);
               if (nread != 2) {
                   fprintf(stderr, "Bad sscanf()\n");
                   exit(EXIT_FAILURE);
               }

               if (mi_mount_id == mount_id)
                   found = 1;
           }
           free(linep);

           fclose(fp);

           if (!found) {
               fprintf(stderr, "Could not find mount point\n");
               exit(EXIT_FAILURE);
           }

           return open(mount_path, O_RDONLY);
       }

       int
       main(int argc, char *argv[])
       {
           struct file_handle *fhp;
           int mount_id, fd, mount_fd, handle_bytes, j;
           ssize_t nread;
           char buf[1000];
       #define LINE_SIZE 100
           char line1[LINE_SIZE], line2[LINE_SIZE];
           char *nextp;

           if ((argc > 1 && strcmp(argv[1], "--help") == 0) || argc > 2) {
               fprintf(stderr, "Usage: %s [mount-path]\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           /* Standard input contains mount ID and file handle information:

                Line 1: <mount_id>
                Line 2: <handle_bytes> <handle_type>   <bytes of handle in hex>
           */

           if ((fgets(line1, sizeof(line1), stdin) == NULL) ||
                  (fgets(line2, sizeof(line2), stdin) == NULL)) {
               fprintf(stderr, "Missing mount_id / file handle\n");
               exit(EXIT_FAILURE);
           }

           mount_id = atoi(line1);

           handle_bytes = strtoul(line2, &nextp, 0);

           /* Given handle_bytes, we can now allocate file_handle structure */

           fhp = malloc(sizeof(struct file_handle) + handle_bytes);
           if (fhp == NULL)
               errExit("malloc");

           fhp->handle_bytes = handle_bytes;

           fhp->handle_type = strtoul(nextp, &nextp, 0);

           for (j = 0; j < fhp->handle_bytes; j++)
               fhp->f_handle[j] = strtoul(nextp, &nextp, 16);

           /* Obtain file descriptor for mount point, either by opening
              the pathname specified on the command line, or by scanning
              /proc/self/mounts to find a mount that matches the 'mount_id'
              that we received from stdin. */

           if (argc > 1)
               mount_fd = open(argv[1], O_RDONLY);
           else
               mount_fd = open_mount_path_by_id(mount_id);

           if (mount_fd == -1)
               errExit("opening mount fd");

           /* Open file using handle and mount point */

           fd = open_by_handle_at(mount_fd, fhp, O_RDONLY);
           if (fd == -1)
               errExit("open_by_handle_at");

           /* Try reading a few bytes from the file */

           nread = read(fd, buf, sizeof(buf));
           if (nread == -1)
               errExit("read");

           printf("Read %zd bytes\n", nread);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       open(2), libblkid(3), blkid(8), findfs(8), mount(8)

       The   libblkid   and   libmount   documentation   in  the  latest  util-linux  release  at
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩

COLOPHON

       This page is part of release 5.02 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.