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UTIMENSAT(2) Linux Programmer's Manual UTIMENSAT(2)
utimensat, futimens - change file timestamps with nanosecond preci‐
sion
#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
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.
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
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 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, 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.
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.
For an explanation of the terms used in this section, see
attributes(7).
┌────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────┼───────────────┼─────────┤
│utimensat(), futimens() │ Thread safety │ MT-Safe │
└────────────────────────┴───────────────┴─────────┘
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.)
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 file: the wrapper function returns the
error EINVAL in this case.
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), touch(1), futimesat(2), openat(2), stat(2), utimes(2),
futimes(3), inode(7), path_resolution(7), symlink(7)
This page is part of release 4.12 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/.
Linux 2017-05-03 UTIMENSAT(2)
Pages that refer to this page: fcntl(2), futimesat(2), open(2), syscalls(2), utime(2), futimes(3), inotify(7), signal-safety(7), symlink(7), xfs_io(8)