Section (2) utimensat
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]) ; |
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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:
-
the caller must have write access to the file;
-
the caller_zsingle_quotesz_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.
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 bothtv_nsec
values areUTIME_NOW
, and either:-
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 theCAP_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, butdirfd
is neitherAT_FDCWD
nor a valid file descriptor. - EFAULT
-
times
pointed to an invalid address; or,dirfd
wasAT_FDCWD
, andpathname
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 notUTIME_NOW
orUTIME_OMIT
); or an invalid value in one of thetv_sec
fields. - EINVAL
-
pathname
is NULL,dirfd
is notAT_FDCWD
, andflags
containsAT_SYMLINK_NOFOLLOW
. - ELOOP
-
(
utimensat
()) Too many symbolic links were encountered in resolvingpathname
. - ENAMETOOLONG
-
(
utimensat
())pathname
is too long. - ENOENT
-
(
utimensat
()) A component ofpathname
does not refer to an existing directory or file, orpathname
is an empty string. - ENOTDIR
-
(
utimensat
())pathname
is a relative pathname, butdirfd
is neitherAT_FDCWD
nor a file descriptor referring to a directory; or, one of the prefix components ofpathname
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, neithertv_nsec
field isUTIME_NOW
, and neithertv_nsec
field isUTIME_OMIT
) and either:-
the caller_zsingle_quotesz_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 ofpathname
.
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 |
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 valueUTIME_NOW
orUTIME_OMIT
, then the value of the correspondingtv_sec
field should be ignored. Instead, the value of thetv_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 toUTIME_NOW
isn_zsingle_quotesz_t always treated the same as specifyingtimes
as NULL, and the case where onetv_nsec
value isUTIME_NOW
and the other isUTIME_OMIT
isn_zsingle_quotesz_t treated the same as specifyingtimes
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_zsingle_quotesz_t have permission to perform updates; b) file timestamps can_zsingle_quotesz_t be updated by a process that should have permission to perform updates; and c) the wrongerrno
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 withtimes
pointing to an array of structures in which bothtv_nsec
fields areUTIME_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 4.16 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/.
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