| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
UTIMENSAT(2) Linux Programmer's Manual UTIMENSAT(2)
utimensat, futimens - change file timestamps with nanosecond precision
#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
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 file
system 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.
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.
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.
On success, utimensat() and futimens() return 0. On error, -1 is returned and
errno is set to indicate the error.
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 file system.
ESRCH (utimensat()) Search permission is denied for one of the prefix
components of pathname.
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.
futimens() and utimensat() are specified in 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.)
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);
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.
chattr(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3),
path_resolution(7), symlink(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-12-13 UTIMENSAT(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface