Manpages - utimensat.2

utimensat, futimens - change file timestamps with nanosecond precision

  #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

*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.

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

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 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.

*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.

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

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

*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.

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.

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

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