Why do I need two different connections to wpa_supplicant (wpa_cli - ctrl_conn and mon_conn)

Question

I am writing my own C library to manage wlan in linux. I base on wpa_cli interface, but I can not understand, why do they use two wpa_ctrl structures:

static struct wpa_ctrl *ctrl_conn;
static struct wpa_ctrl *mon_conn;

It works also when I open and attach only with ctrl_conn ?

Answer 1

wpa_cli works two ways: interactive and non-interactive

When you have a prompt you are using wpa_cli interactively and vice versa.

Here is the interactive mode:

$ wpa_cli -i wlan0
wpa_cli v2.1
Copyright (c) 2004-2014, Jouni Malinen <[email protected]> and contributors

This software may be distributed under the terms of the BSD license.
See README for more details.

Interactive mode

> status
wpa_state=INACTIVE
address=98:fc:11:d1:89:68
uuid=0cb62eb3-776e-55d2-a4f9-983cdd3e48d2

And is the non-interactive mode:

$ wpa_cli -i wlan0 status
wpa_state=INACTIVE
address=98:fc:11:d1:89:68
uuid=0cb62eb3-776e-55d2-a4f9-983cdd3e48d2

It seems that when you are using the interactive mode, wpa_cli use both ctrl_conn and mon_conn. ctrl_conn is used to send commands only, and mon_conn is used to get events (i.e it is the one to be attached via wpa_ctrl_attach()).

And when you are using the non-interactive mode, wpa_cli use only ctrl_conn because there is no events returned.

If you plan to use the wpa_supplicant events (and I hope you will do), I think it is better to use two different connections as explained in the wpa_ctrl_request() comments concerning the msg_cb argument:

/**
 * wpa_ctrl_request - Send a command to wpa_supplicant/hostapd
 * @ctrl: Control interface data from wpa_ctrl_open()
 * @cmd: Command; usually, ASCII text, e.g., "PING"
 * @cmd_len: Length of the cmd in bytes
 * @reply: Buffer for the response
 * @reply_len: Reply buffer length
 * @msg_cb: Callback function for unsolicited messages or %NULL if not used
 * Returns: 0 on success, -1 on error (send or receive failed), -2 on timeout
 *
 * This function is used to send commands to wpa_supplicant/hostapd. Received
 * response will be written to reply and reply_len is set to the actual length
 * of the reply. This function will block for up to two seconds while waiting
 * for the reply. If unsolicited messages are received, the blocking time may
 * be longer.
 *
 * msg_cb can be used to register a callback function that will be called for
 * unsolicited messages received while waiting for the command response. These
 * messages may be received if wpa_ctrl_request() is called at the same time as
 * wpa_supplicant/hostapd is sending such a message. This can happen only if
 * the program has used wpa_ctrl_attach() to register itself as a monitor for
 * event messages. Alternatively to msg_cb, programs can register two control
 * interface connections and use one of them for commands and the other one for
 * receiving event messages, in other words, call wpa_ctrl_attach() only for
 * the control interface connection that will be used for event messages.
 */
int wpa_ctrl_request(struct wpa_ctrl *ctrl, const char *cmd, size_t cmd_len,
             char *reply, size_t *reply_len,
             void (*msg_cb)(char *msg, size_t len));