Ubuntu Manpage: utimensat, futimens - change file timestamps with nanosecond precision

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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, respectively, when setting 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 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 filesystem 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
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 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
              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 filesystem.

       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  nonstandard
       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 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  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 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.

COLOPHON

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