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  Constant:  O_APPEND, O_CREAT, O_EXCL, O_RDONLY, O_RDWR, O_TRUNC,
             O_WRONLY
 
  Include:   <fcntl.h>
 
  Context:   open, sopen, _dos_open
 
  Synopsis:  The integer expression formed from one or more of these
             constants determines the type of reading or writing
             operations permitted. It is formed by combining one or
             more constants with a translation-mode constant (see
             O_BINARY or O_TEXT).
 
     The file constants are described below:
 
     Constant     Meaning
 
     O_APPEND     Repositions the file pointer to the end of the file
                  before every write operation.
 
     O_CREAT      Creates and opens a new file for writing; this has no
                  effect if the file specified by the path name exists.
 
     O_EXCL       Returns an error value if the file specified by the
                  path name exists. Only applies when used with
                  O_CREAT.
 
     O_RDONLY     Opens file for reading only; if this flag is given,
                  neither O_RDWR nor O_WRONLY can be given.
 
     O_RDWR       Opens file for both reading and writing; if this flag
                  is given, neither O_RDONLY nor O_WRONLY can be given.
 
     O_TRUNC      Opens and truncates an existing file to zero length;
                  the file must have write permission. The contents of
                  the file are destroyed.
 
     O_WRONLY     Opens file for writing only; if this flag is given,
                  neither O_RDONLY nor O_RDWR can be given.