3.2. Blinky with IPC demo

This example provides the same functionality as the one provided by the ‘blinky’ standalone application described in Blinky demo.

The main difference is that two applications are used: one ‘button’ app handling the button push events, and one ‘leds’ app handling the LEDs toggling. When the button is pushed, the ‘button’ app send a message to the ‘leds’ app by using the IPC mechanism.

To load the default Ada/SPARK version of the menuconfig, launch make with

make boards/32f407disco/configs/disco_blinky_ipc_ada_defconfig

3.2.1. The ‘button’ app Getting the leds application id

The button application must get the task id of the leds application to be able to communicate with it. During the initialization phase, it performs a sys_init(INIT_GETTASKID) syscall to get this id:

/* Get the LEDs task id to be able to communicate with it using IPCs */
ret = sys_init(INIT_GETTASKID, "leds", &id_leds); Sending a message by using IPCs

When the blue button is in the pushed state, the button application sends a message to the ‘leds’ application by using IPCs. EwoK IPC can be synchronous (blocking) of asynchronous (not blocking) as described in sys_ipc, Inter-Proccess Communication.

Here, the IPC_SEND_SYNC flag means that the syscall is blocking and that the task is blocked until the message is read by the receiver.

ret = sys_ipc(IPC_SEND_SYNC, id_leds, sizeof(button_pressed), (const char*) &button_pressed);

if (ret != SYS_E_DONE) {
    printf("sys_ipc(): error. Exiting.\n");
    return 1;
} Yielding

After sending the IPC to the leds task, the button task yields using the sys_yield() syscall. The task is put in a sleeping state and it will be awaken by a new interrupt. Yielding permits to release the CPU when no work have to be performed by the task.


3.2.2. The ‘leds’ app

That app is similar to the ‘button’ application. The main difference is that it wait for some messages from the button app. Receiving a message by using IPCs

The task checks for and read an incoming message using the sys_ipc(IPC_RECV_ASYNC) syscall:

while (1) {
    id = id_button;
    msg_size = sizeof(button_pressed);

    ret = sys_ipc(IPC_RECV_ASYNC, &id, &msg_size, (char*) &button_pressed);

IPC receiving can be synchronous or asynchronous. Here, the syscall is asynchronous, and thus not blocking.

Using a non blocking IPC permit to perform some other work when no message is available. Here, the app must blink the leds even when the button is not pushed.

Since the IPC reception is asynchronous, three cases can occur:

  • SYS_E_DONE: there is an awaiting message sent by the button application
  • SYS_E_BUSY: there is no awaiting message
  • SYS_E_DENIED or SYS_E_INVAL: these are syscall errors and should not occur in a nominal behavior. Possible causes are missing permissions or improper parameters (ie. invalid task id)
while (1) {
    id = id_button;
    msg_size = sizeof(button_pressed);

    ret = sys_ipc(IPC_RECV_ASYNC, &id, &msg_size, (char*) &button_pressed);

    switch (ret) {
        case SYS_E_DONE:
            printf("BUTTON sent message: %x\n", button_pressed);

            if (button_pressed == true) {
                /* Change leds state */
                green_state   = (green_state == ON) ? OFF : ON;
                orange_state  = (orange_state == ON) ? OFF : ON;
                red_state     = (red_state == ON) ? OFF : ON;
                blue_state    = (blue_state == ON) ? OFF : ON;

                /* Show leds */
                display_leds  = ON;

        case SYS_E_BUSY:
        case SYS_E_DENIED:
        case SYS_E_INVAL:
            printf("sys_ipc(): error. Exiting.\n");
            return 1;