UTIMENSAT
Section: Linux Programmer's Manual (2)
Updated: 2020-06-09
Page Index
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:
-
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
-
_ATFILE_SOURCE
futimens():
-
- Since glibc 2.10:
-
_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 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.
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).
- 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, 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)).
- 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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
utimensat(),
futimens()
| Thread safety | MT-Safe
|
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.)
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.
C library/kernel ABI differences
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);
Note, however, that the glibc wrapper for
utimensat()
disallows passing NULL as the value for
pathname:
the wrapper function returns the error
EINVAL
in this case.
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.
SEE ALSO
chattr(1),
touch(1),
futimesat(2),
openat(2),
stat(2),
utimes(2),
futimes(3),
inode(7),
path_resolution(7),
symlink(7)
COLOPHON
This page is part of release 5.10 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.