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NAME

       utimensat, futimens - change file timestamps with nanosecond precision

SYNOPSIS

       #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(): _ATFILE_SOURCE
       futimens():  _GNU_SOURCE   /*  Will  likely  change  after POSIX.1-2008
       changes are incorporated into glibc */

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,
       respectively, when setting file timestamps.

       With  utimensat()  the  file  is  specified  via  the pathname given in
       pathname.  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
       elements of times specifies a time in seconds and nanoseconds since the
       Epoch  (00:00:00,  1  Jan  1970,  UTC), 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
       file system 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
       corresponding 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  both  tv_nsec  fields  are  not
       UTIME_NOW and both tv_nsec fields are not UTIME_OMIT), either condition
       2 or 3 above must apply.

       If both tv_nsec fields  are  specified  as  UTIME_OMIT,  then  no  file
       ownership  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
              timestamps of the link, rather than the file to which it refers.

RETURN VALUE

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

ERRORS

       EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and:
              * 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
              neither 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
              invalid 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
              resolving pathname.

       ENAMETOOLONG
              (utimensat()) pathname is too long.

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

       ENOTDIR
              (utimensat())  pathname  is  a  relative  pathname, but dirfd is
              neither 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, both tv_nsec fields are not UTIME_NOW,
              and both tv_nsec fields are not UTIME_OMIT) and:
              * 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 file system.

       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.

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
       historical behavior of utime(2) and utimes(2) on Linux.)

       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 non-standard feature: if pathname is NULL, then the
       call modifies the timestamps of  the  file  referred  to  by  the  file
       descriptor  dirfd  (which  may  refer to any type of file).  Using this
       feature, the call futimens(fd, times) is implemented as:

           utimensat(fd, NULL, times, 0);

BUGS

       Several bugs afflict  utimensat()  and  futimens()  on  kernels  before
       2.6.26.   These bugs are either non-conformances 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
         permission 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),  futimesat(2),  openat(2),  stat(2),  utimes(2), futimes(3),
       path_resolution(7), symlink(7)

COLOPHON

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