The open() system call opens the file specified by pathname.  If the
specified file does not exist, it may optionally (if O_CREAT is
specified in flags) be created by open().

The return value of open() is a file descriptor, a small, nonnegative
integer that is used in subsequent system calls (read(2), write(2),
lseek(2), fcntl(2), etc.) to refer to the open file.  The file
descriptor returned by a successful call will be the lowest-numbered
file descriptor not currently open for the process.

By default, the new file descriptor is set to remain open across an
execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in
fcntl(2) is initially disabled); the O_CLOEXEC flag, described below,
can be used to change this default.  The file offset is set to the
beginning of the file (see lseek(2)).

A call to open() creates a new open file description, an entry in the
system-wide table of open files.  The open file description records
the file offset and the file status flags (see below).  A file
descriptor is a reference to an open file description; this reference
is unaffected if pathname is subsequently removed or modified to
refer to a different file.  For further details on open file
descriptions, see NOTES.

The argument flags must include one of the following access modes:
O_RDONLY, O_WRONLY, or O_RDWR.  These request opening the file read-
only, write-only, or read/write, respectively.

In addition, zero or more file creation flags and file status flags
can be bitwise-or'd in flags.  The file creation flags are O_CLOEXEC,
O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, and
O_TRUNC.  The file status flags are all of the remaining flags listed
below.  The distinction between these two groups of flags is that the
file creation flags affect the semantics of the open operation
itself, while the file status flags affect the semantics of
subsequent I/O operations.  The file status flags can be retrieved
and (in some cases) modified; see fcntl(2) for details.