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

NAME

       utimensat, futimens - change file timestamps with nanosecond precision

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

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

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

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 timespec(3) structure.

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.

The status change time (ctime) will be set to the current time, even if the other time
stamps don't actually change.

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:

• the caller must have write access to the file;

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

• 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

       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).

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

       EBADF  (utimensat())  pathname  is relative 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, 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

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

       Support for futimens() first appeared in glibc 2.6.

ATTRIBUTES

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

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

STANDARDS

       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.)

       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

       Several bugs afflict utimensat() and futimens()  before  Linux  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.