utimensat(2)



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

NAME
       utimensat, futimens - change file timestamps with nanosecond precision

SYNOPSIS
       #include <fcntl.h> /* Definition of AT_* constants */
       #include <sys/stat.h>

       int utimensat(int dirfd, const char *pathname,
                     const struct timespec times[2], int flags);

       int futimens(int fd, const struct timespec times[2]);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       utimensat():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE
       futimens():
           Since glibc 2.10:
                  _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
                  _GNU_SOURCE

DESCRIPTION
       utimensat()  and  futimens()  update  the  timestamps  of  a  file with
       nanosecond precision.  This contrasts with the historical utime(2)  and
       utimes(2),  which permit only second and microsecond precision, respec-
       tively, when setting file timestamps.

       With utimensat() the file is specified via the pathname given in  path-
       name.   With  futimens() the file whose timestamps are to be updated is
       specified via an open file descriptor, fd.

       For both calls, the new file timestamps  are  specified  in  the  array
       times:  times[0] specifies the new "last access time" (atime); times[1]
       specifies the new "last modification time" (mtime).  Each of  the  ele-
       ments  of  times specifies a time as the number of seconds and nanosec-
       onds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).   This  informa-
       tion is conveyed in a structure of the following form:

           struct timespec {
               time_t tv_sec;        /* seconds */
               long   tv_nsec;       /* nanoseconds */
           };

       Updated  file timestamps are set to the greatest value supported by the
       filesystem that is not greater than the specified time.

       If the tv_nsec field of one of the timespec structures has the  special
       value  UTIME_NOW,  then  the corresponding file timestamp is set to the
       current time.  If the tv_nsec field of one of the  timespec  structures
       has the special value UTIME_OMIT, then the corresponding file timestamp
       is left unchanged.  In both of these cases, the  value  of  the  corre-
       sponding tv_sec field is ignored.

       If times is NULL, then both timestamps are set to the current time.

   Permissions requirements
       To  set  both file timestamps to the current time (i.e., times is NULL,
       or both tv_nsec fields specify UTIME_NOW), either:

       1. the caller must have write access to the file;

       2. the caller's effective user ID must match the owner of the file; or

       3. the caller must have appropriate privileges.

       To make any change other than setting both timestamps  to  the  current
       time  (i.e.,  times is not NULL, and neither tv_nsec field is UTIME_NOW
       and neither tv_nsec field is UTIME_OMIT), either condition 2 or 3 above
       must apply.

       If both tv_nsec fields are specified as UTIME_OMIT, then no file owner-
       ship or permission checks are performed, and the  file  timestamps  are
       not modified, but other error conditions may still be detected.

   utimensat() specifics
       If  pathname is relative, then by default it is interpreted relative to
       the directory referred to by the open file  descriptor,  dirfd  (rather
       than  relative to the current working directory of the calling process,
       as is done by utimes(2) for a relative pathname).  See openat(2) for an
       explanation of why this can be useful.

       If  pathname  is relative and dirfd is the special value AT_FDCWD, then
       pathname is interpreted relative to the current  working  directory  of
       the calling process (like utimes(2)).

       If pathname is absolute, then dirfd is ignored.

       The  flags  field is a bit mask that may be 0, or include the following
       constant, defined in <fcntl.h>:

       AT_SYMLINK_NOFOLLOW
              If pathname specifies a symbolic link,  then  update  the  time-
              stamps of the link, rather than the file to which it refers.

RETURN VALUE
       On  success,  utimensat() and futimens() return 0.  On error, -1 is re-
       turned and errno is set to indicate the error.

ERRORS
       EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and either:

              *  the effective user ID of the caller does not match the  owner
                 of  the  file,  the  caller does not have write access to the
                 file, and the caller is not privileged (Linux: does not  have
                 either  the  CAP_FOWNER  or the CAP_DAC_OVERRIDE capability);
                 or,

              *  the file is marked immutable (see chattr(1)).

       EBADF  (futimens()) fd is not a valid file descriptor.

       EBADF  (utimensat()) pathname is a relative pathname, but dirfd is nei-
              ther AT_FDCWD nor a valid file descriptor.

       EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
              pathname is NULL or an invalid address.

       EINVAL Invalid value in flags.

       EINVAL Invalid value in one of the tv_nsec fields (value outside  range
              0  to  999,999,999,  and not UTIME_NOW or UTIME_OMIT); or an in-
              valid value in one of the tv_sec fields.

       EINVAL pathname is NULL, dirfd is  not  AT_FDCWD,  and  flags  contains
              AT_SYMLINK_NOFOLLOW.

       ELOOP  (utimensat())  Too  many  symbolic links were encountered in re-
              solving pathname.

       ENAMETOOLONG
              (utimensat()) pathname is too long.

       ENOENT (utimensat()) A component of pathname does not refer to  an  ex-
              isting directory or file, or pathname is an empty string.

       ENOTDIR
              (utimensat()) pathname is a relative pathname, but dirfd is nei-
              ther AT_FDCWD nor a file descriptor referring  to  a  directory;
              or, one of the prefix components of pathname is not a directory.

       EPERM  The caller attempted to change one or both timestamps to a value
              other than the current time, or to change one of the  timestamps
              to the current time while leaving the other timestamp unchanged,
              (i.e., times is not NULL, neither tv_nsec  field  is  UTIME_NOW,
              and neither tv_nsec field is UTIME_OMIT) and either:

              *  the  caller's  effective  user ID does not match the owner of
                 file, and the caller is not privileged (Linux: does not  have
                 the CAP_FOWNER capability); or,

              *  the file is marked append-only or immutable (see chattr(1)).

       EROFS  The file is on a read-only filesystem.

       ESRCH  (utimensat())  Search permission is denied for one of the prefix
              components of pathname.

VERSIONS
       utimensat() was added to Linux in  kernel  2.6.22;  glibc  support  was
       added with version 2.6.

       Support for futimens() first appeared in glibc 2.6.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +------------------------+---------------+---------+
       |Interface               | Attribute     | Value   |
       +------------------------+---------------+---------+
       |utimensat(), futimens() | Thread safety | MT-Safe |
       +------------------------+---------------+---------+

CONFORMING TO
       futimens() and utimensat() are specified in POSIX.1-2008.

NOTES
       utimensat() obsoletes futimesat(2).

       On Linux, timestamps cannot be changed for a file marked immutable, and
       the  only  change  permitted for files marked append-only is to set the
       timestamps to the current time.  (This is consistent with the  histori-
       cal behavior of utime(2) and utimes(2) on Linux.)

       If  both tv_nsec fields are specified as UTIME_OMIT, then the Linux im-
       plementation of utimensat() succeeds even if the file  referred  to  by
       dirfd and pathname does not exist.

   C library/kernel ABI differences
       On  Linux,  futimens()  is a library function implemented on top of the
       utimensat() system call.  To support this, the Linux utimensat() system
       call  implements  a  nonstandard feature: if pathname is NULL, then the
       call modifies the timestamps of the file referred to by  the  file  de-
       scriptor  dirfd (which may refer to any type of file).  Using this fea-
       ture, the call futimens(fd, times) is implemented as:

           utimensat(fd, NULL, times, 0);

       Note, however, that the glibc wrapper for utimensat() disallows passing
       NULL  as the value for pathname: the wrapper function returns the error
       EINVAL in this case.

BUGS
       Several bugs afflict  utimensat()  and  futimens()  on  kernels  before
       2.6.26.   These  bugs are either nonconformances with the POSIX.1 draft
       specification or inconsistencies with historical Linux behavior.

       *  POSIX.1 specifies that if one of the tv_nsec fields  has  the  value
          UTIME_NOW  or UTIME_OMIT, then the value of the corresponding tv_sec
          field should be ignored.  Instead, the value of the tv_sec field  is
          required to be 0 (or the error EINVAL results).

       *  Various  bugs mean that for the purposes of permission checking, the
          case where both tv_nsec fields are set  to  UTIME_NOW  isn't  always
          treated the same as specifying times as NULL, and the case where one
          tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
          the  same as specifying times as a pointer to an array of structures
          containing arbitrary time values.  As a result, in  some  cases:  a)
          file timestamps can be updated by a process that shouldn't have per-
          mission to perform updates; b) file timestamps can't be updated by a
          process  that  should have permission to perform updates; and c) the
          wrong errno value is returned in case of an error.

       *  POSIX.1 says that a process that has write access to  the  file  can
          make  a  call with times as NULL, or with times pointing to an array
          of structures in which both tv_nsec fields are UTIME_NOW,  in  order
          to  update both timestamps to the current time.  However, futimens()
          instead checks whether the access mode of the file descriptor allows
          writing.

SEE ALSO
       chattr(1),  touch(1),  futimesat(2), openat(2), stat(2), utimes(2), fu-
       times(3), inode(7), path_resolution(7), symlink(7)

COLOPHON
       This page is part of release 4.16 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/.

Linux                             2017-09-15                      UTIMENSAT(2)

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