1UTIMENSAT(2)               Linux Programmer's Manual              UTIMENSAT(2)
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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
15       int futimens(int fd, const struct timespec times[2]);
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18
19       utimensat():
20           Since glibc 2.10:
21               _POSIX_C_SOURCE >= 200809L
22           Before glibc 2.10:
23               _ATFILE_SOURCE
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
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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
105       returned and errno is set to indicate the error.
106

ERRORS

108       EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and either:
109
110              *  the effective user ID of the caller does not match the  owner
111                 of  the  file,  the  caller does not have write access to the
112                 file, and the caller is not privileged (Linux: does not  have
113                 either  the  CAP_FOWNER  or the CAP_DAC_OVERRIDE capability);
114                 or,
115
116              *  the file is marked immutable (see chattr(1)).
117
118       EBADF  (futimens()) fd is not a valid file descriptor.
119
120       EBADF  (utimensat()) pathname is a relative pathname, but dirfd is nei‐
121              ther AT_FDCWD nor a valid file descriptor.
122
123       EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
124              pathname is NULL or an invalid address.
125
126       EINVAL Invalid value in flags.
127
128       EINVAL Invalid value in one of the tv_nsec fields (value outside  range
129              0  to  999,999,999,  and  not  UTIME_NOW  or  UTIME_OMIT); or an
130              invalid value in one of the tv_sec fields.
131
132       EINVAL pathname is NULL, dirfd is  not  AT_FDCWD,  and  flags  contains
133              AT_SYMLINK_NOFOLLOW.
134
135       ELOOP  (utimensat())  Too  many  symbolic  links  were  encountered  in
136              resolving pathname.
137
138       ENAMETOOLONG
139              (utimensat()) pathname is too long.
140
141       ENOENT (utimensat()) A component of  pathname  does  not  refer  to  an
142              existing directory or file, or pathname is an empty string.
143
144       ENOTDIR
145              (utimensat()) pathname is a relative pathname, but dirfd is nei‐
146              ther AT_FDCWD nor a file descriptor referring  to  a  directory;
147              or, one of the prefix components of pathname is not a directory.
148
149       EPERM  The caller attempted to change one or both timestamps to a value
150              other than the current time, or to change one of the  timestamps
151              to the current time while leaving the other timestamp unchanged,
152              (i.e., times is not NULL, neither tv_nsec  field  is  UTIME_NOW,
153              and neither tv_nsec field is UTIME_OMIT) and either:
154
155              *  the  caller's  effective  user ID does not match the owner of
156                 file, and the caller is not privileged (Linux: does not  have
157                 the CAP_FOWNER capability); or,
158
159              *  the file is marked append-only or immutable (see chattr(1)).
160
161       EROFS  The file is on a read-only filesystem.
162
163       ESRCH  (utimensat())  Search permission is denied for one of the prefix
164              components of pathname.
165

VERSIONS

167       utimensat() was added to Linux in  kernel  2.6.22;  glibc  support  was
168       added with version 2.6.
169
170       Support for futimens() first appeared in glibc 2.6.
171

ATTRIBUTES

173       For   an   explanation   of   the  terms  used  in  this  section,  see
174       attributes(7).
175
176       ┌────────────────────────┬───────────────┬─────────┐
177Interface               Attribute     Value   
178       ├────────────────────────┼───────────────┼─────────┤
179utimensat(), futimens() │ Thread safety │ MT-Safe │
180       └────────────────────────┴───────────────┴─────────┘
181

CONFORMING TO

183       futimens() and utimensat() are specified in POSIX.1-2008.
184

NOTES

186       utimensat() obsoletes futimesat(2).
187
188       On Linux, timestamps cannot be changed for a file marked immutable, and
189       the  only  change  permitted for files marked append-only is to set the
190       timestamps to the current time.  (This is consistent with the  histori‐
191       cal behavior of utime(2) and utimes(2) on Linux.)
192
193       If  both  tv_nsec  fields  are  specified as UTIME_OMIT, then the Linux
194       implementation of utimensat() succeeds even if the file referred to  by
195       dirfd and pathname does not exist.
196
197   C library/kernel ABI differences
198       On  Linux,  futimens()  is a library function implemented on top of the
199       utimensat() system call.  To support this, the Linux utimensat() system
200       call  implements  a  nonstandard feature: if pathname is NULL, then the
201       call modifies the timestamps of  the  file  referred  to  by  the  file
202       descriptor  dirfd  (which  may  refer to any type of file).  Using this
203       feature, the call futimens(fd, times) is implemented as:
204
205           utimensat(fd, NULL, times, 0);
206
207       Note, however, that the glibc wrapper for utimensat() disallows passing
208       NULL  as the value for pathname: the wrapper function returns the error
209       EINVAL in this case.
210

BUGS

212       Several bugs afflict  utimensat()  and  futimens()  on  kernels  before
213       2.6.26.   These  bugs are either nonconformances with the POSIX.1 draft
214       specification or inconsistencies with historical Linux behavior.
215
216       *  POSIX.1 specifies that if one of the tv_nsec fields  has  the  value
217          UTIME_NOW  or UTIME_OMIT, then the value of the corresponding tv_sec
218          field should be ignored.  Instead, the value of the tv_sec field  is
219          required to be 0 (or the error EINVAL results).
220
221       *  Various  bugs mean that for the purposes of permission checking, the
222          case where both tv_nsec fields are set  to  UTIME_NOW  isn't  always
223          treated the same as specifying times as NULL, and the case where one
224          tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
225          the  same as specifying times as a pointer to an array of structures
226          containing arbitrary time values.  As a result, in  some  cases:  a)
227          file timestamps can be updated by a process that shouldn't have per‐
228          mission to perform updates; b) file timestamps can't be updated by a
229          process  that  should have permission to perform updates; and c) the
230          wrong errno value is returned in case of an error.
231
232       *  POSIX.1 says that a process that has write access to  the  file  can
233          make  a  call with times as NULL, or with times pointing to an array
234          of structures in which both tv_nsec fields are UTIME_NOW,  in  order
235          to  update both timestamps to the current time.  However, futimens()
236          instead checks whether the access mode of the file descriptor allows
237          writing.
238

SEE ALSO

240       chattr(1),   touch(1),  futimesat(2),  openat(2),  stat(2),  utimes(2),
241       futimes(3), inode(7), path_resolution(7), symlink(7)
242

COLOPHON

244       This page is part of release 5.02 of the Linux  man-pages  project.   A
245       description  of  the project, information about reporting bugs, and the
246       latest    version    of    this    page,    can     be     found     at
247       https://www.kernel.org/doc/man-pages/.
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251Linux                             2017-09-15                      UTIMENSAT(2)
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