1UTIMENSAT(2) Linux Programmer's Manual UTIMENSAT(2)
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6 utimensat, futimens - change file timestamps with nanosecond precision
7
9 #include <fcntl.h> /* Definition of AT_* constants */
10 #include <sys/stat.h>
11 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]);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 utimensat():
20 Since glibc 2.10:
21 _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
22 Before glibc 2.10:
23 _ATFILE_SOURCE
24 futimens():
25 Since glibc 2.10:
26 _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
27 Before glibc 2.10:
28 _GNU_SOURCE
29
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 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 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 struct timespec {
48 time_t tv_sec; /* seconds */
49 long tv_nsec; /* nanoseconds */
50 };
51 Updated file timestamps are set to the greatest value supported by the
53 file system that is not greater than the specified time.
54 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 If times is NULL, then both timestamps are set to the current time.
63 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 1. the caller must have write access to the file;
69 2. the caller's effective user ID must match the owner of the file; or
71 3. the caller must have appropriate privileges.
73 To make any change other than setting both timestamps to the current
75 time (i.e., times is not NULL, and both tv_nsec fields are not
76 UTIME_NOW and both tv_nsec fields are not UTIME_OMIT), either condition
77 2 or 3 above must apply.
78 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 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 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 If pathname is absolute, then dirfd is ignored.
95 The flags field is a bit mask that may be 0, or include the following
97 constant, defined in <fcntl.h>:
98 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
104 On success, utimensat() and futimens() return 0. On error, -1 is
105 returned and errno is set to indicate the error.
106
108 EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and:
109 * the effective user ID of the caller does not match the owner
110 of the file, the caller does not have write access to the
111 file, and the caller is not privileged (Linux: does not have
112 either the CAP_FOWNER or the CAP_DAC_OVERRIDE capability); or,
113 * the file is marked immutable (see chattr(1)).
114 EBADF (futimens()) fd is not a valid file descriptor.
116 EBADF (utimensat()) pathname is a relative pathname, but dirfd is nei‐
118 ther AT_FDCWD nor a valid file descriptor.
119 EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
121 pathname is NULL or an invalid address.
122 EINVAL Invalid value in flags.
124 EINVAL Invalid value in one of the tv_nsec fields (value outside range
126 0 to 999,999,999, and not UTIME_NOW or UTIME_OMIT); or an
127 invalid value in one of the tv_sec fields.
128 EINVAL pathname is NULL, dirfd is not AT_FDCWD, and flags contains
130 AT_SYMLINK_NOFOLLOW.
131 ELOOP (utimensat()) Too many symbolic links were encountered in
133 resolving pathname.
134 ENAMETOOLONG
136 (utimensat()) pathname is too long.
137 ENOENT (utimensat()) A component of pathname does not refer to an
139 existing directory or file, or pathname is an empty string.
140 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 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, both tv_nsec fields are not UTIME_NOW,
150 and both tv_nsec fields are not UTIME_OMIT) and:
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 * the file is marked append-only or immutable (see chattr(1)).
155 EROFS The file is on a read-only file system.
157 ESRCH (utimensat()) Search permission is denied for one of the prefix
159 components of pathname.
160
162 utimensat() was added to Linux in kernel 2.6.22; glibc support was
163 added with version 2.6.
164 Support for futimens() first appeared in glibc 2.6.
166
168 futimens() and utimensat() are specified in POSIX.1-2008.
169
171 utimensat() obsoletes futimesat(2).
172 On Linux, timestamps cannot be changed for a file marked immutable, and
174 the only change permitted for files marked append-only is to set the
175 timestamps to the current time. (This is consistent with the histori‐
176 cal behavior of utime(2) and utimes(2) on Linux.)
177 On Linux, futimens() is a library function implemented on top of the
179 utimensat() system call. To support this, the Linux utimensat() system
180 call implements a nonstandard feature: if pathname is NULL, then the
181 call modifies the timestamps of the file referred to by the file
182 descriptor dirfd (which may refer to any type of file). Using this
183 feature, the call futimens(fd, times) is implemented as:
184 utimensat(fd, NULL, times, 0);
186
188 Several bugs afflict utimensat() and futimens() on kernels before
189 2.6.26. These bugs are either nonconformances with the POSIX.1 draft
190 specification or inconsistencies with historical Linux behavior.
191 * POSIX.1 specifies that if one of the tv_nsec fields has the value
193 UTIME_NOW or UTIME_OMIT, then the value of the corresponding tv_sec
194 field should be ignored. Instead, the value of the tv_sec field is
195 required to be 0 (or the error EINVAL results).
196 * Various bugs mean that for the purposes of permission checking, the
198 case where both tv_nsec fields are set to UTIME_NOW isn't always
199 treated the same as specifying times as NULL, and the case where one
200 tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
201 the same as specifying times as a pointer to an array of structures
202 containing arbitrary time values. As a result, in some cases: a)
203 file timestamps can be updated by a process that shouldn't have per‐
204 mission to perform updates; b) file timestamps can't be updated by a
205 process that should have permission to perform updates; and c) the
206 wrong errno value is returned in case of an error.
207 * POSIX.1 says that a process that has write access to the file can
209 make a call with times as NULL, or with times pointing to an array of
210 structures in which both tv_nsec fields are UTIME_NOW, in order to
211 update both timestamps to the current time. However, futimens()
212 instead checks whether the access mode of the file descriptor allows
213 writing.
214
216 chattr(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3),
217 path_resolution(7), symlink(7)
218
220 This page is part of release 3.53 of the Linux man-pages project. A
221 description of the project, and information about reporting bugs, can
222 be found at http://www.kernel.org/doc/man-pages/.
223Linux 2012-03-25 UTIMENSAT(2)