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Linux 2.6 - man page for utimensat (linux section 2)

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():
	   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, respectively, when setting file timestamps.

       With  utimensat()  the  file  is specified via the pathname given in pathname.  With futi-
       mens() the file whose timestamps are to be updated is specified via an open file  descrip-
       tor, fd.

       For  both calls, the new file timestamps are specified in the array times: times[0] speci-
       fies the new "last access time" (atime); times[1] specifies  the  new  "last  modification
       time"  (mtime).	 Each  of the elements of times specifies a time as the number of seconds
       and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000  (UTC).   This  information  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 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  work-
       ing  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  inter-
       preted 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 path-
	      name 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 path-
	      name.

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 nonstandard fea-
       ture: 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  nonconformances	with the POSIX.1 draft specification or inconsistencies with his-
       torical 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  con-
	 taining  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  time-
	 stamps 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,  futi-
	 mens() 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 3.27 of the Linux man-pages project.  A  description  of  the
       project,   and	information  about  reporting  bugs,  can  be  found  at  http://www.ker-
       nel.org/doc/man-pages/.

Linux					    2009-12-13				     UTIMENSAT(2)


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