utimensat, futimens - change file timestamps with nanosecond precision
Standard C library (libc, -lc)
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/stat.h>
int utimensat(int dirfd, const char *pathname,
const struct timespec times[_Nullable 2], int flags);
int futimens(int fd, const struct timespec times[_Nullable 2]);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
timespec(3) structure.
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.
The status change time (ctime) will be set to the current time, even if the other time stamps don't actually change.
To set both file timestamps to the current time (i.e., times
is NULL, or both tv_nsec fields specify
UTIME_NOW), either:
the caller must have write access to the file;
the caller's effective user ID must match the owner of the file; or
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 neither tv_nsec
field is UTIME_NOW and neither tv_nsec field
is 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.
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 argument is a bit mask created by ORing together
zero or more of the following values defined in
<fcntl.h>:
If pathname is an empty string, operate on the file referred
to by dirfd (which may have been obtained using the
open(2) O_PATH flag). In this case,
dirfd can refer to any type of file, not just a directory. If
dirfd is AT_FDCWD, the call operates on the
current working directory. This flag is Linux-specific; define
_GNU_SOURCE to obtain its definition.
If pathname specifies a symbolic link, then update the
timestamps of the link, rather than the file to which it refers.
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).
(futimens()) fd is not a valid file
descriptor.
(utimensat()) pathname is relative but
dirfd is neither AT_FDCWD nor a valid file
descriptor.
times pointed to an invalid address; or, dirfd was
AT_FDCWD, and pathname is NULL or an invalid
address.
Invalid value in flags.
Invalid value in one of the tv_nsec fields (value outside
range [0, 999,999,999], and not UTIME_NOW or
UTIME_OMIT); or an invalid value in one of the
tv_sec fields.
pathname is NULL, dirfd is not
AT_FDCWD, and flags contains
AT_SYMLINK_NOFOLLOW.
(utimensat()) Too many symbolic links were
encountered in resolving pathname.
(utimensat()) pathname is too long.
(utimensat()) A component of pathname does
not refer to an existing directory or file, or pathname is an
empty string.
(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.
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, neither tv_nsec field is
UTIME_NOW, and neither tv_nsec field is
UTIME_OMIT) and either:
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)).
The file is on a read-only filesystem.
(utimensat()) Search permission is denied for one of
the prefix components of pathname.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| Thread safety | MT-Safe |
POSIX.1-2008.
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.)
If both tv_nsec fields are specified as
UTIME_OMIT, then the Linux implementation of
utimensat() succeeds even if the file referred to by
dirfd and pathname does not exist.
Several bugs afflict utimensat() and futimens() before Linux 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.