Each file has three time stamps associated with it: its access time,
its modification time, and its attribute modification time. These
correspond to the
members of the
stat structure; see File Attributes.
All of these times are represented in calendar time format, as
time_t objects. This data type is defined in time.h.
For more information about representation and manipulation of time
values, see Calendar Time.
Reading from a file updates its access time attribute, and writing
updates its modification time. When a file is created, all three
time stamps for that file are set to the current time. In addition, the
attribute change time and modification time fields of the directory that
contains the new entry are updated.
Adding a new name for a file with the
link function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name. These same fields are affected if a file name
is deleted with
a file with
rename affects only the attribute change time and
modification time fields of the two parent directories involved, and not
the times for the file being renamed.
Changing the attributes of a file (for example, with
updates its attribute change time field.
You can also change some of the time stamps of a file explicitly using
utime function—all except the attribute change time. You
need to include the header file utime.h to use this facility.
utimbufstructure is used with the
utimefunction to specify new access and modification times for a file. It contains the following members:
- This is the access time for the file.
- This is the modification time for the file.
This function is used to modify the file times associated with the file named filename.
If times is a null pointer, then the access and modification times of the file are set to the current time. Otherwise, they are set to the values from the
modtimemembers (respectively) of the
utimbufstructure pointed to by times.
The attribute modification time for the file is set to the current time in either case (since changing the time stamps is itself a modification of the file attributes).
0if successful and
-1on failure. In addition to the usual file name errors (see File Name Errors), the following
errnoerror conditions are defined for this function:
- There is a permission problem in the case where a null pointer was passed as the times argument. In order to update the time stamp on the file, you must either be the owner of the file, have write permission for the file, or be a privileged user.
- The file doesn't exist.
- If the times argument is not a null pointer, you must either be the owner of the file or be a privileged user.
- The file lives on a read-only file system.
Each of the three time stamps has a corresponding microsecond part,
which extends its resolution. These fields are called
each has a value between 0 and 999,999, which indicates the time in
microseconds. They correspond to the
tv_usec field of a
timeval structure; see High-Resolution Calendar.
utimes function is like
utime, but also lets you specify
the fractional part of the file times. The prototype for this function is
in the header file sys/time.h.
This function sets the file access and modification times of the file filename. The new file access time is specified by tvp
, and the new modification time by tvp
. This function comes from BSD.
The return values and error conditions are the same as for the