utimensat, futimens - change file timestamps with nanosecond precision
#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.
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.
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.
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.
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)