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UTIMENSAT(2)		   Linux Programmers Manual		 UTIMENSAT(2)



NAME
       utimensat, futimens - change file timestamps with nanosecond precision

SYNOPSIS
       #include 

       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 callers 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 :

       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 callers 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 isnt 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 isnt 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 shouldnt have  permission  to
	 perform  updates;  b)	file  timestamps cant 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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