utimensat(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

UTIMENSAT(2)            Linux Programmer's Manual           UTIMENSAT(2)

NAME         top

       utimensat, futimens - change file timestamps with nanosecond
       precision

SYNOPSIS         top

       #include <fcntl.h>            /* Definition of AT_* constants */
       #include <sys/stat.h>

       int utimensat(int dirfd, const char *pathname,
                     const struct timespec times[2], int flags);
       int futimens(int fd, const struct timespec times[2]);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       utimensat():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

       futimens():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

DESCRIPTION         top

       utimensat() and futimens() update the timestamps of a file with
       nanosecond precision.  This contrasts with the historical
       utime(2) and utimes(2), which permit only second and microsecond
       precision, respectively, when setting file timestamps.

       With utimensat() the file is specified via the pathname given in
       pathname.  With futimens() the file whose timestamps are to be
       updated is specified via an open file descriptor, fd.

       For both calls, the new file timestamps are specified in the
       array times: times[0] specifies the new "last access time"
       (atime); times[1] specifies the new "last modification time"
       (mtime).  Each of the elements of times specifies a time as the
       number of seconds and nanoseconds since the Epoch, 1970-01-01
       00:00:00 +0000 (UTC).  This information is conveyed in a
       structure of the following form:

           struct timespec {
               time_t tv_sec;        /* seconds */
               long   tv_nsec;       /* nanoseconds */
           };

       Updated file timestamps are set to the greatest value supported
       by the filesystem that is not greater than the specified time.

       If the tv_nsec field of one of the timespec structures has the
       special value UTIME_NOW, then the corresponding file timestamp is
       set to the current time.  If the tv_nsec field of one of the
       timespec structures has the special value UTIME_OMIT, then the
       corresponding file timestamp is left unchanged.  In both of these
       cases, the value of the corresponding tv_sec field is ignored.

       If times is NULL, then both timestamps are set to the current
       time.

   Permissions requirements
       To set both file timestamps to the current time (i.e., times is
       NULL, or both tv_nsec fields specify UTIME_NOW), either:

       1. the caller must have write access to the file;

       2. the caller's effective user ID must match the owner of the
          file; or

       3. the caller must have appropriate privileges.

       To make any change other than setting both timestamps to the
       current time (i.e., times is not NULL, and neither tv_nsec field
       is UTIME_NOW and neither tv_nsec field is UTIME_OMIT), either
       condition 2 or 3 above must apply.

       If both tv_nsec fields are specified as UTIME_OMIT, then no file
       ownership or permission checks are performed, and the file
       timestamps are not modified, but other error conditions may still
       be detected.

   utimensat() specifics
       If pathname is relative, then by default it is interpreted
       relative to the directory referred to by the open file
       descriptor, dirfd (rather than relative to the current working
       directory of the calling process, as is done by utimes(2) for a
       relative pathname).  See openat(2) for an explanation of why this
       can be useful.

       If pathname is relative and dirfd is the special value AT_FDCWD,
       then pathname is interpreted relative to the current working
       directory of the calling process (like utimes(2)).

       If pathname is absolute, then dirfd is ignored.

       The flags field is a bit mask that may be 0, or include the
       following constant, defined in <fcntl.h>:

       AT_SYMLINK_NOFOLLOW
              If pathname specifies a symbolic link, then update the
              timestamps of the link, rather than the file to which it
              refers.

RETURN VALUE         top

       On success, utimensat() and futimens() return 0.  On error, -1 is
       returned and errno is set to indicate the error.

ERRORS         top

       EACCES times is NULL, or both tv_nsec values are UTIME_NOW, and
              the effective user ID of the caller does not match the
              owner of the file, the caller does not have write access
              to the file, and the caller is not privileged (Linux: does
              not have either the CAP_FOWNER or the CAP_DAC_OVERRIDE
              capability).

       EBADF  (futimens()) fd is not a valid file descriptor.

       EBADF  (utimensat()) pathname is a relative pathname, but dirfd
              is neither AT_FDCWD nor a valid file descriptor.

       EFAULT times pointed to an invalid address; or, dirfd was
              AT_FDCWD, and pathname is NULL or an invalid address.

       EINVAL Invalid value in flags.

       EINVAL Invalid value in one of the tv_nsec fields (value outside
              range 0 to 999,999,999, and not UTIME_NOW or UTIME_OMIT);
              or an invalid value in one of the tv_sec fields.

       EINVAL pathname is NULL, dirfd is not AT_FDCWD, and flags
              contains AT_SYMLINK_NOFOLLOW.

       ELOOP  (utimensat()) Too many symbolic links were encountered in
              resolving pathname.

       ENAMETOOLONG
              (utimensat()) pathname is too long.

       ENOENT (utimensat()) A component of pathname does not refer to an
              existing directory or file, or pathname is an empty
              string.

       ENOTDIR
              (utimensat()) pathname is a relative pathname, but dirfd
              is neither AT_FDCWD nor a file descriptor referring to a
              directory; or, one of the prefix components of pathname is
              not a directory.

       EPERM  The caller attempted to change one or both timestamps to a
              value other than the current time, or to change one of the
              timestamps to the current time while leaving the other
              timestamp unchanged, (i.e., times is not NULL, neither
              tv_nsec field is UTIME_NOW, and neither tv_nsec field is
              UTIME_OMIT) and either:

              *  the caller's effective user ID does not match the owner
                 of file, and the caller is not privileged (Linux: does
                 not have the CAP_FOWNER capability); or,

              *  the file is marked append-only or immutable (see
                 chattr(1)).

       EROFS  The file is on a read-only filesystem.

       ESRCH  (utimensat()) Search permission is denied for one of the
              prefix components of pathname.

VERSIONS         top

       utimensat() was added to Linux in kernel 2.6.22; glibc support
       was added with version 2.6.

       Support for futimens() first appeared in glibc 2.6.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │utimensat(), futimens()               │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO         top

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

NOTES         top

       utimensat() obsoletes futimesat(2).

       On Linux, timestamps cannot be changed for a file marked
       immutable, and the only change permitted for files marked append-
       only is to set the timestamps to the current time.  (This is
       consistent with the historical behavior of utime(2) and utimes(2)
       on Linux.)

       If both tv_nsec fields are specified as UTIME_OMIT, then the
       Linux implementation of utimensat() succeeds even if the file
       referred to by dirfd and pathname does not exist.

   C library/kernel ABI differences
       On Linux, futimens() is a library function implemented on top of
       the utimensat() system call.  To support this, the Linux
       utimensat() system call implements a nonstandard feature: if
       pathname is NULL, then the call modifies the timestamps of the
       file referred to by the file descriptor dirfd (which may refer to
       any type of file).  Using this feature, the call
       futimens(fd, times) is implemented as:

           utimensat(fd, NULL, times, 0);

       Note, however, that the glibc wrapper for utimensat() disallows
       passing NULL as the value for pathname: the wrapper function
       returns the error EINVAL in this case.

BUGS         top

       Several bugs afflict utimensat() and futimens() on kernels before
       2.6.26.  These bugs are either nonconformances with the POSIX.1
       draft specification or inconsistencies with historical Linux
       behavior.

       *  POSIX.1 specifies that if one of the tv_nsec fields has the
          value UTIME_NOW or UTIME_OMIT, then the value of the
          corresponding tv_sec field should be ignored.  Instead, the
          value of the tv_sec field is required to be 0 (or the error
          EINVAL results).

       *  Various bugs mean that for the purposes of permission
          checking, the case where both tv_nsec fields are set to
          UTIME_NOW isn't always treated the same as specifying times as
          NULL, and the case where one tv_nsec value is UTIME_NOW and
          the other is UTIME_OMIT isn't treated the same as specifying
          times as a pointer to an array of structures containing
          arbitrary time values.  As a result, in some cases: a) file
          timestamps can be updated by a process that shouldn't have
          permission to perform updates; b) file timestamps can't be
          updated by a process that should have permission to perform
          updates; and c) the wrong errno value is returned in case of
          an error.

       *  POSIX.1 says that a process that has write access to the file
          can make a call with times as NULL, or with times pointing to
          an array of structures in which both tv_nsec fields are
          UTIME_NOW, in order to update both timestamps to the current
          time.  However, futimens() instead checks whether the access
          mode of the file descriptor allows writing.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                   UTIMENSAT(2)

Pages that refer to this page: fcntl(2)futimesat(2)open(2)syscalls(2)utime(2)futimes(3)inotify(7)signal-safety(7)symlink(7)xfs_io(8)