utimensat(2) System Calls Manual utimensat(2)
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, 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 as the number of seconds and nanosec-
onds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). This informa-
tion 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 corre-
sponding 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 owner-
ship 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 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 re-
turned and errno is set to indicate the error.
ERRORS
EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and the ef-
fective 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 in-
valid 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 re-
solving pathname.
ENAMETOOLONG
(utimensat()) pathname is too long.
ENOENT (utimensat()) A component of pathname does not refer to an ex-
isting directory or file, or pathname is an empty string.
ENOTDIR
(utimensat()) pathname is a relative pathname, but dirfd is nei-
ther 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.
VERSIONS
utimensat() was added in Linux 2.6.22; glibc support was added with
glibc 2.6.
Support for futimens() first appeared in glibc 2.6.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│utimensat(), futimens() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
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.)
If both tv_nsec fields are specified as UTIME_OMIT, then the Linux im-
plementation 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 de-
scriptor dirfd (which may refer to any type of file). Using this fea-
ture, 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.
BUGS
Several bugs afflict utimensat() and futimens() before Linux 2.6.26.
These bugs are either nonconformances with the POSIX.1 draft specifica-
tion 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 per-
mission 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), fu-
times(3), timespec(3), inode(7), path_resolution(7), symlink(7)
Linux man-pages 6.03 2023-02-12 utimensat(2)
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