1utimensat(2)                  System Calls Manual                 utimensat(2)
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3
4

NAME

6       utimensat, futimens - change file timestamps with nanosecond precision
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <fcntl.h>            /* Definition of AT_* constants */
13       #include <sys/stat.h>
14
15       int utimensat(int dirfd, const char *pathname,
16                     const struct timespec times[_Nullable 2], int flags);
17       int futimens(int fd, const struct timespec times[_Nullable 2]);
18
19   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20
21       utimensat():
22           Since glibc 2.10:
23               _POSIX_C_SOURCE >= 200809L
24           Before glibc 2.10:
25               _ATFILE_SOURCE
26
27       futimens():
28           Since glibc 2.10:
29               _POSIX_C_SOURCE >= 200809L
30           Before glibc 2.10:
31               _GNU_SOURCE
32

DESCRIPTION

34       utimensat()  and  futimens()  update  the  timestamps  of  a  file with
35       nanosecond precision.  This contrasts with the historical utime(2)  and
36       utimes(2),  which permit only second and microsecond precision, respec‐
37       tively, when setting file timestamps.
38
39       With utimensat() the file is specified via the pathname given in  path‐
40       name.   With  futimens() the file whose timestamps are to be updated is
41       specified via an open file descriptor, fd.
42
43       For both calls, the new file timestamps  are  specified  in  the  array
44       times:  times[0] specifies the new "last access time" (atime); times[1]
45       specifies the new "last modification time" (mtime).  Each of  the  ele‐
46       ments  of  times specifies a time as the number of seconds and nanosec‐
47       onds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).   This  informa‐
48       tion is conveyed in a timespec(3) structure.
49
50       Updated  file timestamps are set to the greatest value supported by the
51       filesystem that is not greater than the specified time.
52
53       If the tv_nsec field of one of the timespec structures has the  special
54       value  UTIME_NOW,  then  the corresponding file timestamp is set to the
55       current time.  If the tv_nsec field of one of the  timespec  structures
56       has the special value UTIME_OMIT, then the corresponding file timestamp
57       is left unchanged.  In both of these cases, the  value  of  the  corre‐
58       sponding tv_sec field is ignored.
59
60       If times is NULL, then both timestamps are set to the current time.
61
62       The status change time (ctime) will be set to the current time, even if
63       the other time stamps don't actually change.
64
65   Permissions requirements
66       To set both file timestamps to the current time (i.e., times  is  NULL,
67       or both tv_nsec fields specify UTIME_NOW), either:
68
69       •  the caller must have write access to the file;
70
71       •  the caller's effective user ID must match the owner of the file; or
72
73       •  the caller must have appropriate privileges.
74
75       To  make  any  change other than setting both timestamps to the current
76       time (i.e., times is not NULL, and neither tv_nsec field  is  UTIME_NOW
77       and neither tv_nsec field is UTIME_OMIT), either condition 2 or 3 above
78       must apply.
79
80       If both tv_nsec fields are specified as UTIME_OMIT, then no file owner‐
81       ship  or  permission  checks are performed, and the file timestamps are
82       not modified, but other error conditions may still be detected.
83
84   utimensat() specifics
85       If pathname is relative, then by default it is interpreted relative  to
86       the  directory  referred  to by the open file descriptor, dirfd (rather
87       than relative to the current working directory of the calling  process,
88       as is done by utimes(2) for a relative pathname).  See openat(2) for an
89       explanation of why this can be useful.
90
91       If pathname is relative and dirfd is the special value  AT_FDCWD,  then
92       pathname  is  interpreted  relative to the current working directory of
93       the calling process (like utimes(2)).
94
95       If pathname is absolute, then dirfd is ignored.
96
97       The flags field is a bit mask that may be 0, or include  the  following
98       constant, defined in <fcntl.h>:
99
100       AT_SYMLINK_NOFOLLOW
101              If  pathname  specifies  a  symbolic link, then update the time‐
102              stamps of the link, rather than the file to which it refers.
103

RETURN VALUE

105       On success, utimensat() and futimens() return 0.  On error, -1  is  re‐
106       turned and errno is set to indicate the error.
107

ERRORS

109       EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and the ef‐
110              fective user ID of the caller does not match the  owner  of  the
111              file, the caller does not have write access to the file, and the
112              caller is not  privileged  (Linux:  does  not  have  either  the
113              CAP_FOWNER or the CAP_DAC_OVERRIDE capability).
114
115       EBADF  (futimens()) fd is not a valid file descriptor.
116
117       EBADF  (utimensat()) pathname is relative but dirfd is neither AT_FDCWD
118              nor a valid file descriptor.
119
120       EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
121              pathname is NULL or an invalid address.
122
123       EINVAL Invalid value in flags.
124
125       EINVAL Invalid  value in one of the tv_nsec fields (value outside range
126              [0, 999,999,999], and not UTIME_NOW or UTIME_OMIT);  or  an  in‐
127              valid value in one of the tv_sec fields.
128
129       EINVAL pathname  is  NULL,  dirfd  is  not AT_FDCWD, and flags contains
130              AT_SYMLINK_NOFOLLOW.
131
132       ELOOP  (utimensat()) Too many symbolic links were  encountered  in  re‐
133              solving pathname.
134
135       ENAMETOOLONG
136              (utimensat()) pathname is too long.
137
138       ENOENT (utimensat())  A  component of pathname does not refer to an ex‐
139              isting directory or file, or pathname is an empty string.
140
141       ENOTDIR
142              (utimensat()) pathname is a relative pathname, but dirfd is nei‐
143              ther  AT_FDCWD  nor  a file descriptor referring to a directory;
144              or, one of the prefix components of pathname is not a directory.
145
146       EPERM  The caller attempted to change one or both timestamps to a value
147              other  than the current time, or to change one of the timestamps
148              to the current time while leaving the other timestamp unchanged,
149              (i.e.,  times  is  not NULL, neither tv_nsec field is UTIME_NOW,
150              and neither tv_nsec field is UTIME_OMIT) and either:
151
152              •  the caller's effective user ID does not match  the  owner  of
153                 file,  and the caller is not privileged (Linux: does not have
154                 the CAP_FOWNER capability); or,
155
156              •  the file is marked append-only or immutable (see chattr(1)).
157
158       EROFS  The file is on a read-only filesystem.
159
160       ESRCH  (utimensat()) Search permission is denied for one of the  prefix
161              components of pathname.
162

ATTRIBUTES

164       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
165       tributes(7).
166
167       ┌────────────────────────────────────────────┬───────────────┬─────────┐
168Interface                                   Attribute     Value   
169       ├────────────────────────────────────────────┼───────────────┼─────────┤
170utimensat(), futimens()                     │ Thread safety │ MT-Safe │
171       └────────────────────────────────────────────┴───────────────┴─────────┘
172

VERSIONS

174   C library/kernel ABI differences
175       On Linux, futimens() is a library function implemented on  top  of  the
176       utimensat() system call.  To support this, the Linux utimensat() system
177       call implements a nonstandard feature: if pathname is  NULL,  then  the
178       call  modifies  the  timestamps of the file referred to by the file de‐
179       scriptor dirfd (which may refer to any type of file).  Using this  fea‐
180       ture, the call futimens(fd, times) is implemented as:
181
182           utimensat(fd, NULL, times, 0);
183
184       Note, however, that the glibc wrapper for utimensat() disallows passing
185       NULL as the value for pathname: the wrapper function returns the  error
186       EINVAL in this case.
187

STANDARDS

189       POSIX.1-2008.
190

VERSIONS

192       utimensat()
193              Linux 2.6.22, glibc 2.6.  POSIX.1-2008.
194
195       futimens()
196              glibc 2.6.  POSIX.1-2008.
197

NOTES

199       utimensat() obsoletes futimesat(2).
200
201       On Linux, timestamps cannot be changed for a file marked immutable, and
202       the only change permitted for files marked append-only is  to  set  the
203       timestamps  to the current time.  (This is consistent with the histori‐
204       cal behavior of utime(2) and utimes(2) on Linux.)
205
206       If both tv_nsec fields are specified as UTIME_OMIT, then the Linux  im‐
207       plementation  of  utimensat()  succeeds even if the file referred to by
208       dirfd and pathname does not exist.
209

BUGS

211       Several bugs afflict utimensat() and futimens()  before  Linux  2.6.26.
212       These bugs are either nonconformances with the POSIX.1 draft specifica‐
213       tion or inconsistencies with historical Linux behavior.
214
215       •  POSIX.1 specifies that if one of the tv_nsec fields  has  the  value
216          UTIME_NOW  or UTIME_OMIT, then the value of the corresponding tv_sec
217          field should be ignored.  Instead, the value of the tv_sec field  is
218          required to be 0 (or the error EINVAL results).
219
220       •  Various  bugs mean that for the purposes of permission checking, the
221          case where both tv_nsec fields are set  to  UTIME_NOW  isn't  always
222          treated the same as specifying times as NULL, and the case where one
223          tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
224          the  same as specifying times as a pointer to an array of structures
225          containing arbitrary time values.  As a result, in  some  cases:  a)
226          file timestamps can be updated by a process that shouldn't have per‐
227          mission to perform updates; b) file timestamps can't be updated by a
228          process  that  should have permission to perform updates; and c) the
229          wrong errno value is returned in case of an error.
230
231       •  POSIX.1 says that a process that has write access to  the  file  can
232          make  a  call with times as NULL, or with times pointing to an array
233          of structures in which both tv_nsec fields are UTIME_NOW,  in  order
234          to  update both timestamps to the current time.  However, futimens()
235          instead checks whether the access mode of the file descriptor allows
236          writing.
237

SEE ALSO

239       chattr(1),  touch(1),  futimesat(2), openat(2), stat(2), utimes(2), fu‐
240       times(3), timespec(3), inode(7), path_resolution(7), symlink(7)
241
242
243
244Linux man-pages 6.04              2023-03-30                      utimensat(2)
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