Ubuntu Manpage: utimensat, futimens - change file timestamps with nanosecond precision

Provided by: manpages-dev_3.54-1ubuntu1_all

NAME

       utimensat, futimens - change file timestamps with nanosecond precision

SYNOPSIS

       #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:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE
       futimens():
           Since glibc 2.10:
                  _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
                  _GNU_SOURCE

DESCRIPTION

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
both tv_nsec fields are not UTIME_NOW and both tv_nsec fields are not 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

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

ERRORS

       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); or,
              * the file is marked immutable (see chattr(1)).

       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, both tv_nsec fields are not UTIME_NOW, and both tv_nsec fields  are  not
              UTIME_OMIT) and:
              * 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

       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.

CONFORMING TO

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

NOTES

       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.)

       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);

BUGS

       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.

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.