hostapd/doc/porting.doxygen

/**
\page porting Porting to different target boards and operating systems

wpa_supplicant was designed to be easily portable to different
hardware (board, CPU) and software (OS, drivers) targets. It is
already used with number of operating systems and numerous wireless
card models and drivers. The main wpa_supplicant repository includes
support for Linux, FreeBSD, and Windows. In addition, the code has been
ported to number of other operating systems like VxWorks, PalmOS,
Windows CE, and Windows Mobile. On the hardware
side, wpa_supplicant is used on various systems: desktops, laptops,
PDAs, and embedded devices with CPUs including x86, PowerPC,
arm/xscale, and MIPS. Both big and little endian configurations are
supported.


\section ansi_c_extra Extra functions on top of ANSI C

wpa_supplicant is mostly using ANSI C functions that are available on
most targets. However, couple of additional functions that are common
on modern UNIX systems are used. Number of these are listed with
prototypes in \ref common.h (the \verbatim #ifdef CONFIG_ANSI_C_EXTRA \endverbatim
block). These functions may need to be implemented or at least defined
as macros to native functions in the target OS or C library.

Many of the common ANSI C functions are used through a wrapper
definitions in \ref os.h to allow these to be replaced easily with a
platform specific version in case standard C libraries are not
available. In addition, \ref os.h defines couple of common platform
specific functions that are implemented in \ref os_unix.c for UNIX like
targets and in \ref os_win32.c for Win32 API. If the target platform does
not support either of these examples, a new os_*.c file may need to be
added.

Unless OS_NO_C_LIB_DEFINES is defined, the standard ANSI C and POSIX
functions are used by defining the os_*() wrappers to use them
directly in order to avoid extra cost in size and speed. If the target
platform needs different versions of the functions, \ref os.h can be
modified to define the suitable macros or alternatively,
OS_NO_C_LIB_DEFINES may be defined for the build and the wrapper
functions can then be implemented in a new os_*.c wrapper file.

\ref common.h defines number of helper macros for handling integers of
different size and byte order. Suitable version of these definitions
may need to be added for the target platform.


\section configuration_backend Configuration backend

wpa_supplicant implements a configuration interface that allows the
backend to be easily replaced in order to read configuration data from
a suitable source depending on the target platform. \ref config.c
implements the generic code that can be shared with all configuration
backends. Each backend is implemented in its own config_*.c file.

The included \ref config_file.c backend uses a text file for configuration
and \ref config_winreg.c uses Windows registry. These files can be used as
an example for a new configuration backend if the target platform uses
different mechanism for configuration parameters. In addition,
\ref config_none.c can be used as an empty starting point for building a
new configuration backend.


\section driver_iface_porting Driver interface

Unless the target OS and driver is already supported, most porting
projects have to implement a driver wrapper. This may be done by
adding a new driver interface module or modifying an existing module
(driver_*.c) if the new target is similar to one of them. \ref
driver_wrapper "Driver wrapper implementation" describes the details
of the driver interface and discusses the tasks involved in porting
this part of wpa_supplicant.


\section l2_packet_porting l2_packet (link layer access)

wpa_supplicant needs to have access to sending and receiving layer 2
(link layer) packets with two Ethertypes: EAP-over-LAN (EAPOL) 0x888e
and RSN pre-authentication 0x88c7. \ref l2_packet.h defines the interfaces
used for this in the core wpa_supplicant implementation.

If the target operating system supports a generic mechanism for link
layer access, that is likely the best mechanism for providing the
needed functionality for wpa_supplicant. Linux packet socket is an
example of such a generic mechanism. If this is not available, a
separate interface may need to be implemented to the network stack or
driver. This is usually an intermediate or protocol driver that is
operating between the device driver and the OS network stack. If such
a mechanism is not feasible, the interface can also be implemented
directly in the device driver.

The main wpa_supplicant repository includes l2_packet implementations
for Linux using packet sockets (\ref l2_packet_linux.c), more portable
version using libpcap/libdnet libraries (\ref l2_packet_pcap.c; this
supports WinPcap, too), and FreeBSD specific version of libpcap
interface (\ref l2_packet_freebsd.c).

If the target operating system is supported by libpcap (receiving) and
libdnet (sending), \ref l2_packet_pcap.c can likely be used with minimal or
no changes. If this is not a case or a proprietary interface for link
layer is required, a new l2_packet module may need to be
added. Alternatively, for hostapd,
struct \ref wpa_driver_ops::hapd_send_eapol() handler can
be used to override the l2_packet library if the link layer access is
integrated with the driver interface implementation.


\section eloop_porting Event loop

wpa_supplicant uses a single process/thread model and an event loop
to provide callbacks on events (registered timeout, received packet,
signal). eloop.h defines the event loop interface. \ref eloop.c is an
implementation of such an event loop using select() and sockets. This
is suitable for most UNIX/POSIX systems. When porting to other
operating systems, it may be necessary to replace that implementation
with OS specific mechanisms that provide similar functionality.


\section ctrl_iface_porting Control interface

wpa_supplicant uses a \ref ctrl_iface_page "control interface"
to allow external processed
to get status information and to control the operations. Currently,
this is implemented with socket based communication; both UNIX domain
sockets and UDP sockets are supported. If the target OS does not
support sockets, this interface will likely need to be modified to use
another mechanism like message queues. The control interface is
optional component, so it is also possible to run wpa_supplicant
without porting this part.

The wpa_supplicant side of the control interface is implemented in
\ref wpa_supplicant/ctrl_iface.c. Matching client side is implemented as a control
interface library in \ref wpa_ctrl.c.


\section entry_point Program entry point

wpa_supplicant defines a set of functions that can be used to
initialize main supplicant processing. Each operating system has a
mechanism for starting new processing or threads. This is usually a
function with a specific set of arguments and calling convention. This
function is responsible on initializing wpa_supplicant.

\ref wpa_supplicant/main.c includes an entry point for UNIX-like
operating system, i.e., main() function that uses command line arguments
for setting parameters for wpa_supplicant. When porting to other
operating systems, similar OS-specific entry point implementation is
needed. It can be implemented in a new file that is then linked with
wpa_supplicant instead of main.o. \ref wpa_supplicant/main.c is also a
good example on how the initialization process should be done.

The supplicant initialization functions are defined in
\ref wpa_supplicant_i.h. In most cases, the entry point function should
start by fetching configuration parameters. After this, a global
wpa_supplicant context is initialized with a call to
\ref wpa_supplicant_init(). After this, existing network interfaces can be
added with \ref wpa_supplicant_add_iface(). \ref wpa_supplicant_run() is then
used to start the main event loop. Once this returns at program
termination time, \ref wpa_supplicant_deinit() is used to release global
context data.

\ref wpa_supplicant_add_iface() and \ref wpa_supplicant_remove_iface() can be
used dynamically to add and remove interfaces based on when
wpa_supplicant processing is needed for them. This can be done, e.g.,
when hotplug network adapters are being inserted and ejected. It is
also possible to do this when a network interface is being
enabled/disabled if it is desirable that wpa_supplicant processing
for the interface is fully enabled/disabled at the same time.


\section simple_build Simple build example

One way to start a porting project is to begin with a very simple
build of wpa_supplicant with WPA-PSK support and once that is
building correctly, start adding features.

Following command can be used to build very simple version of
wpa_supplicant:

\verbatim
cc -o wpa_supplicant config.c eloop.c common.c md5.c rc4.c sha1.c \
	config_none.c l2_packet_none.c tls_none.c wpa.c preauth.c \
	aes_wrap.c wpa_supplicant.c events.c main_none.c drivers.c
\endverbatim

The end result is not really very useful since it uses empty functions
for configuration parsing and layer 2 packet access and does not
include a driver interface. However, this is a good starting point
since the build is complete in the sense that all functions are
present and this is easy to configure to a build system by just
including the listed C files.

Once this version can be build successfully, the end result can be
made functional by adding a proper program entry point (main*.c),
driver interface (driver_*.c and matching CONFIG_DRIVER_* define for
registration in \ref drivers.c), configuration parser/writer (config_*.c),
and layer 2 packet access implementation (l2_packet_*.c). After these
components have been added, the end result should be a working
WPA/WPA2-PSK enabled supplicant.

After the basic functionality has been verified to work, more features
can be added by linking in more files and defining C pre-processor
defines. Currently, the best source of information for what options
are available and which files needs to be included is in the Makefile
used for building the supplicant with make. Similar configuration will
be needed for build systems that either use different type of make
tool or a GUI-based project configuration.

*/