File status flags are used to specify attributes of the opening of a
file. Unlike the file descriptor flags discussed in Descriptor Flags, the file status flags are shared by duplicated file descriptors
resulting from a single opening of the file. The file status flags are
specified with the flags argument to open;
see Opening and Closing Files.
File status flags fall into three categories, which are described in the
Access Modes, specify what type of access is allowed to the
file: reading, writing, or both. They are set by open and are
returned by fcntl, but cannot be changed.
Open-time Flags, control details of what open will do.
These flags are not preserved after the open call.
Operating Modes, affect how operations such as read and
write are done. They are set by open, and can be fetched or
changed with fcntl.
The symbols in this section are defined in the header file