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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON |
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utimensat(2) System Calls Manual utimensat(2)
utimensat, futimens - change file timestamps with nanosecond
precision
Standard C library (libc, -lc)
#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
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.
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).
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.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ utimensat(), futimens() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
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.
POSIX.1-2008.
utimensat()
Linux 2.6.22, glibc 2.6. POSIX.1-2008.
futimens()
glibc 2.6. 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.
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.
chattr(1), touch(1), futimesat(2), openat(2), stat(2), utimes(2),
futimes(3), timespec(3), inode(7), path_resolution(7), symlink(7)
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-05-02 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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