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

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

       #define _GNU_SOURCE
       #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);

       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_han‐
       dle_at()	 opens the file corresponding to a handle returned by a previ‐
       ous call to name_to_handle_at() and returns an open file descriptor.

       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 argu‐
       ments.  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 possible size for a file handle.)	 Upon success‐
       ful 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).

       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_han‐

       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  han‐
	  dle 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(3)
	  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 direc‐
	  tory, 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

       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).

       The  open_by_handle_at() system call opens the file referred to by han‐
       dle, 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, direc‐
       tory,  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

       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.

       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

       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.

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

	      The filesystem does not support decoding of a pathname to a file

	      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.

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

       These system calls are nonstandard Linux extensions.

       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 han‐
       dles, 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	 func‐
       tionality  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 con‐
       verting 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  persis‐
       tent  identifier for the corresponding mounted filesystem.  However, an
       application can use the information in the mountinfo record that corre‐
       sponds 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().

       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_han‐
       dle_at()	 to  open the file using that handle.  If an optional command-
       line argument is supplied, then the mount_fd argument for  open_by_han‐
       dle_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 per‐

       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_han‐
       dle_at() recognizes that the original file referred to by the file han‐
       dle no longer exists.

	   $ stat --printf="%i\n" cecilia.txt	  # Display inode number
	   $ rm cecilia.txt
	   $ echo 'Can you please think about it?' > cecilia.txt
	   $ stat --printf="%i\n" cecilia.txt	  # Check inode number
	   $ 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)

       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]);

	   pathname = argv[1];

	   /* Allocate file_handle structure */

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

	   /* 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");

	   /* 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)

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

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

	   /* 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]);


   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)

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

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

	       if (mi_mount_id == mount_id)
		   found = 1;


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

	   return open(mount_path, O_RDONLY);

       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]);

	   /* 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");

	   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)

	   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);
	       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)

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

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

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


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

       The  libblkid  and  libmount  documentation  in	the  latest util-linux
       release at ⟨⟩

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-03-24		  OPEN_BY_HANDLE_AT(2)

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