wpa_supplicant is a WPA Supplicant for Linux, BSD, Mac OS X, and Windows with support for WPA and WPA2 (IEEE 802.11i / RSN). It is suitable for both desktop/laptop computers and embedded systems. Supplicant is the IEEE 802.1X/WPA component that is used in the client stations. It implements key negotiation with a WPA Authenticator and it controls the roaming and IEEE 802.11 authentication/association of the wlan driver.
wpa_supplicant is designed to be a “daemon” program that runs in the background and acts as the backend component controlling the wireless connection. wpa_supplicant supports separate frontend programs and a text-based frontend (wpa_cli) and a GUI (wpa_gui) are included with wpa_supplicant.
wpa_supplicant uses a flexible build configuration that can be used to select which features are included. This allows minimal code size (from ca. 50 kB binary for WPA/WPA2-Personal and 130 kB binary for WPA/WPA2-Enterprise without debugging code to 450 kB with most features and full debugging support; these example sizes are from a build for x86 target).
wpa_supplicant config example
http://w1.fi/ home of wpa_supplicant and hostapd