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

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 change after next POSIX.1 revision */

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 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  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  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 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  times-
              tamps 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 effective 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 capabil-
                ity); 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
              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 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, 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 not specified in any  current  standard,
       but are included in the next revision of POSIX.1.

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.)

       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 a  structure  containing
         arbitrary  time  values.   As a result, in some cases: a) file times-
         tamps 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 a structure
         in  which  both  tv_nsec fields are UTIME_NOW, in order to update the
         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
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2008-07-15                      UTIMENSAT(2)

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