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(): _ATFILE_SOURCE
19 futimens(): _GNU_SOURCE /* Will likely change after POSIX.1-2008
20 changes are incorporated into glibc */
21
23 utimensat() and futimens() update the timestamps of a file with
24 nanosecond precision. This contrasts with the historical utime(2) and
25 utimes(2), which permit only second and microsecond precision, respec‐
26 tively, when setting file timestamps.
27 With utimensat() the file is specified via the pathname given in path‐
29 name. With futimens() the file whose timestamps are to be updated is
30 specified via an open file descriptor, fd.
31 For both calls, the new file timestamps are specified in the array
33 times: times[0] specifies the new "last access time" (atime); times[1]
34 specifies the new "last modification time" (mtime). Each of the ele‐
35 ments of times specifies a time in seconds and nanoseconds since the
36 Epoch (00:00:00, 1 Jan 1970, UTC), in a structure of the following
37 form:
38 struct timespec {
40 time_t tv_sec; /* seconds */
41 long tv_nsec; /* nanoseconds */
42 };
43 Updated file timestamps are set to the greatest value supported by the
45 file system that is not greater than the specified time.
46 If the tv_nsec field of one of the timespec structures has the special
48 value UTIME_NOW, then the corresponding file timestamp is set to the
49 current time. If the tv_nsec field of one of the timespec structures
50 has the special value UTIME_OMIT, then the corresponding file timestamp
51 is left unchanged. In both of these cases, the value of the corre‐
52 sponding tv_sec field is ignored.
53 If times is NULL, then both timestamps are set to the current time.
55 Permissions requirements
57 To set both file timestamps to the current time (i.e., times is NULL,
58 or both tv_nsec fields specify UTIME_NOW), either:
59 1. the caller must have write access to the file;
61 2. the caller's effective user ID must match the owner of the file; or
63 3. the caller must have appropriate privileges.
65 To make any change other than setting both timestamps to the current
67 time (i.e., times is not NULL, and both tv_nsec fields are not
68 UTIME_NOW and both tv_nsec fields are not UTIME_OMIT), either condition
69 2 or 3 above must apply.
70 If both tv_nsec fields are specified as UTIME_OMIT, then no file owner‐
72 ship or permission checks are performed, and the file timestamps are
73 not modified, but other error conditions may still be detected.
74 utimensat() specifics
76 If pathname is relative, then by default it is interpreted relative to
77 the directory referred to by the open file descriptor, dirfd (rather
78 than relative to the current working directory of the calling process,
79 as is done by utimes(2) for a relative pathname). See openat(2) for an
80 explanation of why this can be useful.
81 If pathname is relative and dirfd is the special value AT_FDCWD, then
83 pathname is interpreted relative to the current working directory of
84 the calling process (like utimes(2)).
85 If pathname is absolute, then dirfd is ignored.
87 The flags field is a bit mask that may be 0, or include the following
89 constant, defined in <fcntl.h>:
90 AT_SYMLINK_NOFOLLOW
92 If pathname specifies a symbolic link, then update the time‐
93 stamps of the link, rather than the file to which it refers.
94
96 On success, utimensat() and futimens() return 0. On error, -1 is
97 returned and errno is set to indicate the error.
98
100 EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and:
101 * the effective user ID of the caller does not match the owner
102 of the file, the caller does not have write access to the
103 file, and the caller is not privileged (Linux: does not have
104 either the CAP_FOWNER or the CAP_DAC_OVERRIDE capability); or,
105 * the file is marked immutable (see chattr(1)).
106 EBADF (futimens()) fd is not a valid file descriptor.
108 EBADF (utimensat()) pathname is a relative pathname, but dirfd is nei‐
110 ther AT_FDCWD nor a valid file descriptor.
111 EFAULT times pointed to an invalid address; or, dirfd was AT_FDCWD, and
113 pathname is NULL or an invalid address.
114 EINVAL Invalid value in flags.
116 EINVAL Invalid value in one of the tv_nsec fields (value outside range
118 0 to 999,999,999, and not UTIME_NOW or UTIME_OMIT); or an
119 invalid value in one of the tv_sec fields.
120 EINVAL pathname is NULL, dirfd is not AT_FDCWD, and flags contains
122 AT_SYMLINK_NOFOLLOW.
123 ELOOP (utimensat()) Too many symbolic links were encountered in
125 resolving pathname.
126 ENAMETOOLONG
128 (utimensat()) pathname is too long.
129 ENOENT (utimensat()) A component of pathname does not refer to an
131 existing directory or file, or pathname is an empty string.
132 ENOTDIR
134 (utimensat()) pathname is a relative pathname, but dirfd is nei‐
135 ther AT_FDCWD nor a file descriptor referring to a directory;
136 or, one of the prefix components of pathname is not a directory.
137 EPERM The caller attempted to change one or both timestamps to a value
139 other than the current time, or to change one of the timestamps
140 to the current time while leaving the other timestamp unchanged,
141 (i.e., times is not NULL, both tv_nsec fields are not UTIME_NOW,
142 and both tv_nsec fields are not UTIME_OMIT) and:
143 * the caller's effective user ID does not match the owner of
144 file, and the caller is not privileged (Linux: does not have
145 the CAP_FOWNER capability); or,
146 * the file is marked append-only or immutable (see chattr(1)).
147 EROFS The file is on a read-only file system.
149 ESRCH (utimensat()) Search permission is denied for one of the prefix
151 components of pathname.
152
154 utimensat() was added to Linux in kernel 2.6.22; glibc support was
155 added with version 2.6.
156 Support for futimens() first appeared in glibc 2.6.
158
160 futimens() and utimensat() are specified in POSIX.1-2008.
161
163 utimensat() obsoletes futimesat(2).
164 On Linux, timestamps cannot be changed for a file marked immutable, and
166 the only change permitted for files marked append-only is to set the
167 timestamps to the current time. (This is consistent with the histori‐
168 cal behavior of utime(2) and utimes(2) on Linux.)
169 On Linux, futimens() is a library function implemented on top of the
171 utimensat() system call. To support this, the Linux utimensat() system
172 call implements a non-standard feature: if pathname is NULL, then the
173 call modifies the timestamps of the file referred to by the file
174 descriptor dirfd (which may refer to any type of file). Using this
175 feature, the call futimens(fd, times) is implemented as:
176 utimensat(fd, NULL, times, 0);
178
180 Several bugs afflict utimensat() and futimens() on kernels before
181 2.6.26. These bugs are either non-conformances with the POSIX.1 draft
182 specification or inconsistencies with historical Linux behavior.
183 * POSIX.1 specifies that if one of the tv_nsec fields has the value
185 UTIME_NOW or UTIME_OMIT, then the value of the corresponding tv_sec
186 field should be ignored. Instead, the value of the tv_sec field is
187 required to be 0 (or the error EINVAL results).
188 * Various bugs mean that for the purposes of permission checking, the
190 case where both tv_nsec fields are set to UTIME_NOW isn't always
191 treated the same as specifying times as NULL, and the case where one
192 tv_nsec value is UTIME_NOW and the other is UTIME_OMIT isn't treated
193 the same as specifying times as a pointer to an array of structures
194 containing arbitrary time values. As a result, in some cases: a)
195 file timestamps can be updated by a process that shouldn't have per‐
196 mission to perform updates; b) file timestamps can't be updated by a
197 process that should have permission to perform updates; and c) the
198 wrong errno value is returned in case of an error.
199 * POSIX.1 says that a process that has write access to the file can
201 make a call with times as NULL, or with times pointing to an array of
202 structures in which both tv_nsec fields are UTIME_NOW, in order to
203 update both timestamps to the current time. However, futimens()
204 instead checks whether the access mode of the file descriptor allows
205 writing.
206
208 chattr(1), futimesat(2), openat(2), stat(2), utimes(2), futimes(3),
209 path_resolution(7), symlink(7)
210
212 This page is part of release 3.22 of the Linux man-pages project. A
213 description of the project, and information about reporting bugs, can
214 be found at http://www.kernel.org/doc/man-pages/.
215Linux 2008-09-29 UTIMENSAT(2)