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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:
	       _XOPEN_SOURCE >=	700 || _POSIX_C_SOURCE >= 200809L
	   Before glibc	2.10:
	       _ATFILE_SOURCE
       futimens():
	   Since glibc 2.10:
		  _XOPEN_SOURCE	>= 700 || _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
       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	 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:
	      *	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,	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  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  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);

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() in-
	 stead 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 3.53 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				  2012-03-25			  UTIMENSAT(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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