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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 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  set-
       ting 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 spe-
       cial 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  descrip-
              tor.

       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  modi-
       fies 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-confor-
       mances 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 pro-
         cess 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
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-09-29                      UTIMENSAT(2)