.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk
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.TH UTIMENSAT 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
utimensat, futimens \- change file timestamps with nanosecond precision
.SH SYNOPSIS
.nf
.B #include <fcntl.h>           /* Definition of AT_* constants */
.B #include <sys/stat.h>
.PP
.BI "int utimensat(int " dirfd ", const char *" pathname ,
.BI "              const struct timespec " times "[2], int " flags );
.PP
.BI "int futimens(int " fd ", const struct timespec " times [2]);
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.ad l
.PD 0
.BR utimensat ():
.RS 4
.TP 4
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.PP
.BR futimens ():
.RS 4
.TP
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_GNU_SOURCE
.RE
.PD
.ad
.SH DESCRIPTION
.BR utimensat ()
and
.BR futimens ()
update the timestamps of a file with nanosecond precision.
This contrasts with the historical
.BR utime (2)
and
.BR utimes (2),
which permit only second and microsecond precision, respectively,
when setting file timestamps.
.PP
With
.BR utimensat ()
the file is specified via the pathname given in
.IR pathname .
With
.BR futimens ()
the file whose timestamps are to be updated is specified via
an open file descriptor,
.IR fd .
.PP
For both calls, the new file timestamps are specified in the array
.IR times :
.IR times [0]
specifies the new "last access time" (\fIatime\fP);
.IR times [1]
specifies the new "last modification time" (\fImtime\fP).
Each of the elements of
.I 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:
.PP
.in +4n
.EX
struct timespec {
    time_t tv_sec;        /* seconds */
    long   tv_nsec;       /* nanoseconds */
};
.EE
.in
.PP
Updated file timestamps are set to the greatest value
supported by the filesystem that is not greater than the specified time.
.PP
If the
.I tv_nsec
field of one of the
.I timespec
structures has the special value
.BR UTIME_NOW ,
then the corresponding file timestamp is set to the current time.
If the
.I tv_nsec
field of one of the
.I timespec
structures has the special value
.BR UTIME_OMIT ,
then the corresponding file timestamp is left unchanged.
In both of these cases, the value of the corresponding
.I tv_sec
.\" 2.6.22 was broken: it is not ignored
field is ignored.
.PP
If
.I times
is NULL, then both timestamps are set to the current time.
.\"
.SS Permissions requirements
To set both file timestamps to the current time (i.e.,
.I times
is NULL, or both
.I tv_nsec
fields specify
.BR UTIME_NOW ),
either:
.IP 1. 3
the caller must have write access to the file;
.\" 2.6.22 was broken here -- for futimens() the check is
.\" based on whether or not the file descriptor is writable,
.\" not on whether the caller's effective UID has write
.\" permission for the file referred to by the descriptor.
.IP 2.
the caller's effective user ID must match the owner of the file; or
.IP 3.
the caller must have appropriate privileges.
.PP
To make any change other than setting both timestamps to the
current time (i.e.,
.I times
is not NULL, and neither
.I tv_nsec
field is
.B UTIME_NOW
.\" 2.6.22 was broken here:
.\" both must be something other than *either* UTIME_OMIT *or* UTIME_NOW.
and neither
.I tv_nsec
field is
.BR UTIME_OMIT ),
either condition 2 or 3 above must apply.
.PP
If both
.I tv_nsec
fields are specified as
.BR 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.
.\"
.\"
.SS utimensat() specifics
If
.I pathname
is relative, then by default it is interpreted relative to the
directory referred to by the open file descriptor,
.IR dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR utimes (2)
for a relative pathname).
See
.BR openat (2)
for an explanation of why this can be useful.
.PP
If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR utimes (2)).
.PP
If
.I pathname
is absolute, then
.I dirfd
is ignored.
.PP
The
.I flags
field is a bit mask that may be 0, or include the following constant,
defined in
.IR <fcntl.h> :
.TP
.B AT_SYMLINK_NOFOLLOW
If
.I pathname
specifies a symbolic link, then update the timestamps of the link,
rather than the file to which it refers.
.SH RETURN VALUE
On success,
.BR utimensat ()
and
.BR futimens ()
return 0.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
.I times
is NULL,
or both
.I tv_nsec
values are
.BR UTIME_NOW ,
and either:
.RS
.IP * 3
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
.B CAP_FOWNER
or the
.B CAP_DAC_OVERRIDE
capability); or,
.\" But Linux 2.6.22 was broken here.
.\" Traditionally, utime()/utimes() gives the error EACCES for the case
.\" where the timestamp pointer argument is NULL (i.e., set both timestamps
.\" to the current time), and the file is owned by a user other than the
.\" effective UID of the caller, and the file is not writable by the
.\" effective UID of the program.  utimensat() also gives this error in the
.\" same case.  However, in the same circumstances, when utimensat() is
.\" given a 'times' array in which both tv_nsec fields are UTIME_NOW, which
.\" provides equivalent functionality to specifying 'times' as NULL, the
.\" call succeeds.  It should fail with the error EACCES in this case.
.\"
.\" POSIX.1-2008 has the following:
.\" .TP
.\" .B EACCES
.\" .RB ( utimensat ())
.\" .I fd
.\" was not opened with
.\" .B O_SEARCH
.\" and the permissions of the directory to which
.\" .I fd
.\" refers do not allow searches.
.IP *
the file is marked immutable (see
.BR chattr (1)).
.\" EXT2_IMMUTABLE_FL and similar flags for other filesystems.
.RE
.TP
.B EBADF
.RB ( futimens ())
.I fd
is not a valid file descriptor.
.TP
.B EBADF
.RB ( utimensat ())
.I pathname
is a relative pathname, but
.I dirfd
is neither
.BR AT_FDCWD
nor a valid file descriptor.
.TP
.B EFAULT
.I times
pointed to an invalid address; or,
.I dirfd
was
.BR AT_FDCWD ,
and
.I pathname
is NULL or an invalid address.
.TP
.B EINVAL
Invalid value in
.IR flags .
.TP
.B EINVAL
Invalid value in one of the
.I tv_nsec
fields (value outside range 0 to 999,999,999, and not
.B UTIME_NOW
or
.BR UTIME_OMIT );
or an invalid value in one of the
.I tv_sec
fields.
.TP
.B EINVAL
.\" SUSv4 does not specify this error.
.I pathname
is NULL,
.I dirfd
is not
.BR AT_FDCWD ,
and
.I flags
contains
.BR AT_SYMLINK_NOFOLLOW .
.TP
.B ELOOP
.RB ( utimensat ())
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
.B ENAMETOOLONG
.RB ( utimensat ())
.I pathname
is too long.
.TP
.B ENOENT
.RB ( utimensat ())
A component of
.I pathname
does not refer to an existing directory or file,
or
.I pathname
is an empty string.
.TP
.B ENOTDIR
.RB ( utimensat ())
.I pathname
is a relative pathname, but
.I dirfd
is neither
.B AT_FDCWD
nor a file descriptor referring to a directory;
or, one of the prefix components of
.I pathname
is not a directory.
.TP
.B 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.,
.I times
is not NULL, neither
.I tv_nsec
field is
.BR UTIME_NOW ,
and neither
.I tv_nsec
field is
.BR UTIME_OMIT )
and either:
.RS
.IP * 3
the caller's effective user ID does not match the owner of file,
and the caller is not privileged
(Linux: does not have the
.BR CAP_FOWNER
capability); or,
.IP *
.\" Linux 2.6.22 was broken here:
.\" it was not consistent with the old utimes() implementation,
.\" since the case when both tv_nsec fields are UTIME_NOW, was not
.\" treated like the (times == NULL) case.
the file is marked append-only or immutable (see
.BR chattr (1)).
.\" EXT2_IMMUTABLE_FL EXT_APPPEND_FL and similar flags for
.\" other filesystems.
.\"
.\" Why the inconsistency (which is described under NOTES) between
.\" EACCES and EPERM, where only EPERM tests for append-only.
.\" (This was also so for the older utimes() implementation.)
.RE
.TP
.B EROFS
The file is on a read-only filesystem.
.TP
.B ESRCH
.RB ( utimensat ())
Search permission is denied for one of the prefix components of
.IR pathname .
.SH VERSIONS
.BR utimensat ()
was added to Linux in kernel 2.6.22;
glibc support was added with version 2.6.
.PP
Support for
.BR futimens ()
first appeared in glibc 2.6.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw23 lb lb
l l l.
Interface	Attribute	Value
T{
.BR utimensat (),
.BR futimens ()
T}	Thread safety	MT-Safe
.TE
.sp 1
.SH CONFORMING TO
.BR futimens ()
and
.BR utimensat ()
are specified in POSIX.1-2008.
.SH NOTES
.BR utimensat ()
obsoletes
.BR futimesat (2).
.PP
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
.BR utime (2)
and
.BR utimes (2)
on Linux.)
.PP
If both
.I tv_nsec
fields are specified as
.BR UTIME_OMIT ,
then the Linux implementation of
.BR utimensat ()
succeeds even if the file referred to by
.IR dirfd
and
.I pathname
does not exist.
.SS C library/kernel ABI differences
On Linux,
.BR futimens ()
is a library function implemented on top of the
.BR utimensat ()
system call.
To support this, the Linux
.BR utimensat ()
system call implements a nonstandard feature: if
.I pathname
is NULL, then the call modifies the timestamps of
the file referred to by the file descriptor
.I dirfd
(which may refer to any type of file).
Using this feature, the call
.I "futimens(fd,\ times)"
is implemented as:
.PP
.in +4n
.EX
utimensat(fd, NULL, times, 0);
.EE
.in
.PP
Note, however, that the glibc wrapper for
.BR utimensat ()
disallows passing NULL as the value for
.IR pathname :
the wrapper function returns the error
.IR EINVAL
in this case.
.SH BUGS
Several bugs afflict
.BR utimensat ()
and
.BR futimens ()
on kernels before 2.6.26.
These bugs are either nonconformances with the POSIX.1 draft specification
or inconsistencies with historical Linux behavior.
.IP * 3
POSIX.1 specifies that if one of the
.I tv_nsec
fields has the value
.B UTIME_NOW
or
.BR UTIME_OMIT ,
then the value of the corresponding
.I tv_sec
field should be ignored.
Instead, the value of the
.I tv_sec
field is required to be 0 (or the error
.B EINVAL
results).
.IP *
Various bugs mean that for the purposes of permission checking,
the case where both
.I tv_nsec
fields are set to
.BR UTIME_NOW
isn't always treated the same as specifying
.I times
as NULL,
and the case where one
.I tv_nsec
value is
.B UTIME_NOW
and the other is
.B UTIME_OMIT
isn't treated the same as specifying
.I 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
.I errno
value is returned in case of an error.
.\" Below, the long description of the errors from the previous bullet
.\" point (abridged because it's too much detail for a man page).
.\" .IP *
.\" If one of the
.\" .I tv_nsec
.\" fields is
.\" .BR UTIME_OMIT
.\" and the other is
.\" .BR UTIME_NOW ,
.\" then the error
.\" .B EPERM
.\" should occur if the process's effective user ID does not match
.\" the file owner and the process is not privileged.
.\" Instead, the call successfully changes one of the timestamps.
.\" .IP *
.\" If file is not writable by the effective user ID of the process and
.\" the process's effective user ID does not match the file owner and
.\" the process is not privileged,
.\" and
.\" .I times
.\" is NULL, then the error
.\" .B EACCES
.\" results.
.\" This error should also occur if
.\" .I times
.\" points to an array of structures in which both
.\" .I tv_nsec
.\" fields are
.\" .BR UTIME_NOW .
.\" Instead the call succeeds.
.\" .IP *
.\" If a file is marked as append-only (see
.\" .BR chattr (1)),
.\" then Linux traditionally
.\" (i.e.,
.\" .BR utime (2),
.\" .BR utimes (2)),
.\" permits a NULL
.\" .I times
.\" argument to be used in order to update both timestamps to the current time.
.\" For consistency,
.\" .BR utimensat ()
.\" and
.\" .BR futimens ()
.\" should also produce the same result when given a
.\" .I times
.\" argument that points to an array of structures in which both
.\" .I tv_nsec
.\" fields are
.\" .BR UTIME_NOW .
.\" Instead, the call fails with the error
.\" .BR EPERM .
.\" .IP *
.\" If a file is marked as immutable (see
.\" .BR chattr (1)),
.\" then Linux traditionally
.\" (i.e.,
.\" .BR utime (2),
.\" .BR utimes (2)),
.\" gives an
.\" .B EACCES
.\" error if
.\" .I times
.\" is NULL.
.\" For consistency,
.\" .BR utimensat ()
.\" and
.\" .BR futimens ()
.\" should also produce the same result when given a
.\" .I times
.\" that points to an array of structures in which both
.\" .I tv_nsec
.\" fields are
.\" .BR UTIME_NOW .
.\" Instead, the call fails with the error
.\" .BR EPERM .
.IP *
POSIX.1 says that a process that has \fIwrite access to the file\fP
can make a call with
.I times
as NULL, or with
.I times
pointing to an array of structures in which both
.I tv_nsec
fields are
.BR UTIME_NOW ,
in order to update both timestamps to the current time.
However,
.BR futimens ()
instead checks whether the
.IR "access mode of the file descriptor allows writing" .
.\" This means that a process with a file descriptor that allows
.\" writing could change the timestamps of a file for which it
.\" does not have write permission;
.\" conversely, a process with a read-only file descriptor won't
.\" be able to update the timestamps of a file,
.\" even if it has write permission on the file.
.SH SEE ALSO
.BR chattr (1),
.BR touch (1),
.BR futimesat (2),
.BR openat (2),
.BR stat (2),
.BR utimes (2),
.BR futimes (3),
.BR inode (7),
.BR path_resolution (7),
.BR symlink (7)