SYNOPSIS
#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(): _ATFILE_SOURCE
futimens(): _GNU_SOURCE /* Will likely change after POSIX.1-2008
changes are incorporated into glibc */
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, respec-
tively, when setting file timestamps.
With utimensat() the file is specified via the pathname given in path-
name. 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 ele-
ments of times specifies a time in seconds and nanoseconds since the
Epoch (00:00:00, 1 Jan 1970, UTC), 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 corre-
sponding 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;
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 time-
stamps 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); 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 nei-
ther 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.
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.
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.
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 histori-
cal 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 non-standard 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);
BUGS
Several bugs afflict utimensat() and futimens() on kernels before
2.6.26. These bugs are either non-conformances 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
writing.
SEE ALSO
chattr(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3),
path_resolution(7), symlink(7)
COLOPHON
This page is part of release 3.23 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/.
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