1UTIMENSAT(2)               Linux Programmer's Manual              UTIMENSAT(2)
2
3
4

NAME

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

SYNOPSIS

9       #include <fcntl.h>            /* Definition of AT_* constants */
10       #include <sys/stat.h>
11
12       int utimensat(int dirfd, const char *pathname,
13                     const struct timespec times[2], int flags);
14       int futimens(int fd, const struct timespec times[2]);
15
16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17
18       utimensat():
19           Since glibc 2.10:
20               _POSIX_C_SOURCE >= 200809L
21           Before glibc 2.10:
22               _ATFILE_SOURCE
23
24       futimens():
25           Since glibc 2.10:
26               _POSIX_C_SOURCE >= 200809L
27           Before glibc 2.10:
28               _GNU_SOURCE
29

DESCRIPTION

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

RETURN VALUE

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

ERRORS

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

VERSIONS

163       utimensat() was added to Linux in  kernel  2.6.22;  glibc  support  was
164       added with version 2.6.
165
166       Support for futimens() first appeared in glibc 2.6.
167

ATTRIBUTES

169       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
170       tributes(7).
171
172       ┌────────────────────────────────────────────┬───────────────┬─────────┐
173Interface                                   Attribute     Value   
174       ├────────────────────────────────────────────┼───────────────┼─────────┤
175utimensat(), futimens()                     │ Thread safety │ MT-Safe │
176       └────────────────────────────────────────────┴───────────────┴─────────┘
177

CONFORMING TO

179       futimens() and utimensat() are specified in POSIX.1-2008.
180

NOTES

182       utimensat() obsoletes futimesat(2).
183
184       On Linux, timestamps cannot be changed for a file marked immutable, and
185       the  only  change  permitted for files marked append-only is to set the
186       timestamps to the current time.  (This is consistent with the  histori‐
187       cal behavior of utime(2) and utimes(2) on Linux.)
188
189       If  both tv_nsec fields are specified as UTIME_OMIT, then the Linux im‐
190       plementation of utimensat() succeeds even if the file  referred  to  by
191       dirfd and pathname does not exist.
192
193   C library/kernel ABI differences
194       On  Linux,  futimens()  is a library function implemented on top of the
195       utimensat() system call.  To support this, the Linux utimensat() system
196       call  implements  a  nonstandard feature: if pathname is NULL, then the
197       call modifies the timestamps of the file referred to by  the  file  de‐
198       scriptor  dirfd (which may refer to any type of file).  Using this fea‐
199       ture, the call futimens(fd, times) is implemented as:
200
201           utimensat(fd, NULL, times, 0);
202
203       Note, however, that the glibc wrapper for utimensat() disallows passing
204       NULL  as the value for pathname: the wrapper function returns the error
205       EINVAL in this case.
206

BUGS

208       Several bugs afflict  utimensat()  and  futimens()  on  kernels  before
209       2.6.26.   These  bugs are either nonconformances with the POSIX.1 draft
210       specification or inconsistencies with historical Linux behavior.
211
212       *  POSIX.1 specifies that if one of the tv_nsec fields  has  the  value
213          UTIME_NOW  or UTIME_OMIT, then the value of the corresponding tv_sec
214          field should be ignored.  Instead, the value of the tv_sec field  is
215          required to be 0 (or the error EINVAL results).
216
217       *  Various  bugs mean that for the purposes of permission checking, the
218          case where both tv_nsec fields are set  to  UTIME_NOW  isn't  always
219          treated the same as specifying times as NULL, and the case where one
220          tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
221          the  same as specifying times as a pointer to an array of structures
222          containing arbitrary time values.  As a result, in  some  cases:  a)
223          file timestamps can be updated by a process that shouldn't have per‐
224          mission to perform updates; b) file timestamps can't be updated by a
225          process  that  should have permission to perform updates; and c) the
226          wrong errno value is returned in case of an error.
227
228       *  POSIX.1 says that a process that has write access to  the  file  can
229          make  a  call with times as NULL, or with times pointing to an array
230          of structures in which both tv_nsec fields are UTIME_NOW,  in  order
231          to  update both timestamps to the current time.  However, futimens()
232          instead checks whether the access mode of the file descriptor allows
233          writing.
234

SEE ALSO

236       chattr(1),  touch(1),  futimesat(2), openat(2), stat(2), utimes(2), fu‐
237       times(3), inode(7), path_resolution(7), symlink(7)
238

COLOPHON

240       This page is part of release 5.12 of the Linux  man-pages  project.   A
241       description  of  the project, information about reporting bugs, and the
242       latest    version    of    this    page,    can     be     found     at
243       https://www.kernel.org/doc/man-pages/.
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247Linux                             2021-03-22                      UTIMENSAT(2)
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