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NAME
utimensat, futimens - change file timestamps with nanosecond precision
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/stat.h>
int utimensat(int dirfd, const char *pathname,
const struct timespec times[_Nullable 2], int flags);
int futimens(int fd, const struct timespec times[_Nullable 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
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 timespec(3) structure.
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.
The status change time (ctime) will be set to the current time, even if the other time
stamps don't actually change.
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'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 argument is a bit mask created by ORing together zero or more of the following
values defined in <fcntl.h>:
AT_EMPTY_PATH (since Linux 5.8)
If pathname is an empty string, operate on the file referred to by dirfd (which may
have been obtained using the open(2) O_PATH flag). In this case, dirfd can refer
to any type of file, not just a directory. If dirfd is AT_FDCWD, the call operates
on the current working directory. This flag is Linux-specific; define _GNU_SOURCE
to obtain its definition.
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 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).
EBADF (futimens()) fd is not a valid file descriptor.
EBADF (utimensat()) pathname is relative 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, 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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│utimensat(), futimens() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
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.
STANDARDS
POSIX.1-2008.
VERSIONS
utimensat()
Linux 2.6.22, glibc 2.6. POSIX.1-2008.
futimens()
glibc 2.6. 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 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.
BUGS
Several bugs afflict utimensat() and futimens() before Linux 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.
SEE ALSO
chattr(1), touch(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3), timespec(3),
inode(7), path_resolution(7), symlink(7)