1UTIMENSAT(2) Linux Programmer's Manual UTIMENSAT(2)
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6 utimensat, futimens - change file timestamps with nanosecond precision
7
9 #include <sys/stat.h>
10 int utimensat(int dirfd, const char *pathname,
12 const struct timespec times[2], int flags);
13 int futimens(int fd, const struct timespec times[2]);
15 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17 utimensat():
19 Since glibc 2.10: _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
20 Before glibc 2.10: _ATFILE_SOURCE
21 futimens():
22 Since glibc 2.10: _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
23 Before glibc 2.10: _GNU_SOURCE
24
26 utimensat() and futimens() update the timestamps of a file with
27 nanosecond precision. This contrasts with the historical utime(2) and
28 utimes(2), which permit only second and microsecond precision, respec‐
29 tively, when setting file timestamps.
30 With utimensat() the file is specified via the pathname given in path‐
32 name. With futimens() the file whose timestamps are to be updated is
33 specified via an open file descriptor, fd.
34 For both calls, the new file timestamps are specified in the array
36 times: times[0] specifies the new "last access time" (atime); times[1]
37 specifies the new "last modification time" (mtime). Each of the ele‐
38 ments of times specifies a time as the the number of seconds and
39 nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). This
40 information is conveyed in a structure of the following form:
41 struct timespec {
43 time_t tv_sec; /* seconds */
44 long tv_nsec; /* nanoseconds */
45 };
46 Updated file timestamps are set to the greatest value supported by the
48 file system that is not greater than the specified time.
49 If the tv_nsec field of one of the timespec structures has the special
51 value UTIME_NOW, then the corresponding file timestamp is set to the
52 current time. If the tv_nsec field of one of the timespec structures
53 has the special value UTIME_OMIT, then the corresponding file timestamp
54 is left unchanged. In both of these cases, the value of the corre‐
55 sponding tv_sec field is ignored.
56 If times is NULL, then both timestamps are set to the current time.
58 Permissions requirements
60 To set both file timestamps to the current time (i.e., times is NULL,
61 or both tv_nsec fields specify UTIME_NOW), either:
62 1. the caller must have write access to the file;
64 2. the caller's effective user ID must match the owner of the file; or
66 3. the caller must have appropriate privileges.
68 To make any change other than setting both timestamps to the current
70 time (i.e., times is not NULL, and both tv_nsec fields are not
71 UTIME_NOW and both tv_nsec fields are not UTIME_OMIT), either condition
72 2 or 3 above must apply.
73 If both tv_nsec fields are specified as UTIME_OMIT, then no file owner‐
75 ship or permission checks are performed, and the file timestamps are
76 not modified, but other error conditions may still be detected.
77 utimensat() specifics
79 If pathname is relative, then by default it is interpreted relative to
80 the directory referred to by the open file descriptor, dirfd (rather
81 than relative to the current working directory of the calling process,
82 as is done by utimes(2) for a relative pathname). See openat(2) for an
83 explanation of why this can be useful.
84 If pathname is relative and dirfd is the special value AT_FDCWD, then
86 pathname is interpreted relative to the current working directory of
87 the calling process (like utimes(2)).
88 If pathname is absolute, then dirfd is ignored.
90 The flags field is a bit mask that may be 0, or include the following
92 constant, defined in <fcntl.h>:
93 AT_SYMLINK_NOFOLLOW
95 If pathname specifies a symbolic link, then update the time‐
96 stamps of the link, rather than the file to which it refers.
97
99 On success, utimensat() and futimens() return 0. On error, -1 is
100 returned and errno is set to indicate the error.
101
103 EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and:
104 * the effective user ID of the caller does not match the owner
105 of the file, the caller does not have write access to the
106 file, and the caller is not privileged (Linux: does not have
107 either the CAP_FOWNER or the CAP_DAC_OVERRIDE capability); or,
108 * the file is marked immutable (see chattr(1)).
109 EBADF (futimens()) fd is not a valid file descriptor.
111 EBADF (utimensat()) pathname is a relative pathname, but dirfd is nei‐
113 ther AT_FDCWD nor a valid file descriptor.
114 EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
116 pathname is NULL or an invalid address.
117 EINVAL Invalid value in flags.
119 EINVAL Invalid value in one of the tv_nsec fields (value outside range
121 0 to 999,999,999, and not UTIME_NOW or UTIME_OMIT); or an
122 invalid value in one of the tv_sec fields.
123 EINVAL pathname is NULL, dirfd is not AT_FDCWD, and flags contains
125 AT_SYMLINK_NOFOLLOW.
126 ELOOP (utimensat()) Too many symbolic links were encountered in
128 resolving pathname.
129 ENAMETOOLONG
131 (utimensat()) pathname is too long.
132 ENOENT (utimensat()) A component of pathname does not refer to an
134 existing directory or file, or pathname is an empty string.
135 ENOTDIR
137 (utimensat()) pathname is a relative pathname, but dirfd is nei‐
138 ther AT_FDCWD nor a file descriptor referring to a directory;
139 or, one of the prefix components of pathname is not a directory.
140 EPERM The caller attempted to change one or both timestamps to a value
142 other than the current time, or to change one of the timestamps
143 to the current time while leaving the other timestamp unchanged,
144 (i.e., times is not NULL, both tv_nsec fields are not UTIME_NOW,
145 and both tv_nsec fields are not UTIME_OMIT) and:
146 * the caller's effective user ID does not match the owner of
147 file, and the caller is not privileged (Linux: does not have
148 the CAP_FOWNER capability); or,
149 * the file is marked append-only or immutable (see chattr(1)).
150 EROFS The file is on a read-only file system.
152 ESRCH (utimensat()) Search permission is denied for one of the prefix
154 components of pathname.
155
157 utimensat() was added to Linux in kernel 2.6.22; glibc support was
158 added with version 2.6.
159 Support for futimens() first appeared in glibc 2.6.
161
163 futimens() and utimensat() are specified in POSIX.1-2008.
164
166 utimensat() obsoletes futimesat(2).
167 On Linux, timestamps cannot be changed for a file marked immutable, and
169 the only change permitted for files marked append-only is to set the
170 timestamps to the current time. (This is consistent with the histori‐
171 cal behavior of utime(2) and utimes(2) on Linux.)
172 On Linux, futimens() is a library function implemented on top of the
174 utimensat() system call. To support this, the Linux utimensat() system
175 call implements a nonstandard feature: if pathname is NULL, then the
176 call modifies the timestamps of the file referred to by the file
177 descriptor dirfd (which may refer to any type of file). Using this
178 feature, the call futimens(fd, times) is implemented as:
179 utimensat(fd, NULL, times, 0);
181
183 Several bugs afflict utimensat() and futimens() on kernels before
184 2.6.26. These bugs are either nonconformances with the POSIX.1 draft
185 specification or inconsistencies with historical Linux behavior.
186 * POSIX.1 specifies that if one of the tv_nsec fields has the value
188 UTIME_NOW or UTIME_OMIT, then the value of the corresponding tv_sec
189 field should be ignored. Instead, the value of the tv_sec field is
190 required to be 0 (or the error EINVAL results).
191 * Various bugs mean that for the purposes of permission checking, the
193 case where both tv_nsec fields are set to UTIME_NOW isn't always
194 treated the same as specifying times as NULL, and the case where one
195 tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
196 the same as specifying times as a pointer to an array of structures
197 containing arbitrary time values. As a result, in some cases: a)
198 file timestamps can be updated by a process that shouldn't have per‐
199 mission to perform updates; b) file timestamps can't be updated by a
200 process that should have permission to perform updates; and c) the
201 wrong errno value is returned in case of an error.
202 * POSIX.1 says that a process that has write access to the file can
204 make a call with times as NULL, or with times pointing to an array of
205 structures in which both tv_nsec fields are UTIME_NOW, in order to
206 update both timestamps to the current time. However, futimens()
207 instead checks whether the access mode of the file descriptor allows
208 writing.
209
211 chattr(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3),
212 path_resolution(7), symlink(7)
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215 This page is part of release 3.25 of the Linux man-pages project. A
216 description of the project, and information about reporting bugs, can
217 be found at http://www.kernel.org/doc/man-pages/.
218Linux 2009-12-13 UTIMENSAT(2)