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(libc.info)File Times

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File Times
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   Each file has three time stamps associated with it:  its access time,
its modification time, and its attribute modification time.  These
correspond to the `st_atime', `st_mtime', and `st_ctime' members of the
`stat' structure; see Note: 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
Note: 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 `unlink', `remove' or `rmdir'.  Renaming 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 `chmod')
updates its attribute change time field.

   You can also change some of the time stamps of a file explicitly
using the `utime' function--all except the attribute change time.  You
need to include the header file `utime.h' to use this facility.

 - Data Type: struct utimbuf
     The `utimbuf' structure is used with the `utime' function to
     specify new access and modification times for a file.  It contains
     the following members:

    `time_t actime'
          This is the access time for the file.

    `time_t modtime'
          This is the modification time for the file.

 - Function: int utime (const char *FILENAME, const struct utimbuf
          *TIMES)
     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 `actime' and `modtime' members
     (respectively) of the `utimbuf' structure 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).

     The `utime' function returns `0' if successful and `-1' on
     failure.  In addition to the usual file name errors (Note: File
     Name Errors), the following `errno' error conditions are defined
     for this function:

    `EACCES'
          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.

    `ENOENT'
          The file doesn't exist.

    `EPERM'
          If the TIMES argument is not a null pointer, you must either
          be the owner of the file or be a privileged user.

    `EROFS'
          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 `st_atime_usec',
`st_mtime_usec', and `st_ctime_usec'; 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 Note: High-Resolution
Calendar.

   The `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'.

 - Function: int utimes (const char *FILENAME, struct timeval TVP[2])
     This function sets the file access and modification times of the
     file FILENAME.  The new file access time is specified by `TVP[0]',
     and the new modification time by `TVP[1]'.  This function comes
     from BSD.

     The return values and error conditions are the same as for the
     `utime' function.