Execution is transferred to second process while waiting for an event in contiki

I have created two processes in Contiki. In the first process, I want to keep calling a function after a certain amount of time for this I have set an event timer. But while waiting for the event for the expiration of the timer the control of execution is transferred to the second process.

Code:

PROCESS(hello_world_process, "Hello world process");
PROCESS(hello_world_process2, "Hello world process2");
AUTOSTART_PROCESSES(&hello_world_process,&hello_world_process2);
#define TIMEOUT              (CLOCK_SECOND / 4)
static struct etimer timer;

void setFlag(void)
{
    int i;
    i = random_rand();
    if (i>0 && i<32767)
    {
        printf("Setting flag\n");
        flag =1;
    } 

}
PROCESS_THREAD(hello_world_process, ev, data)
{
    PROCESS_BEGIN();
    printf("1st process started\n");
    do 
    {
        etimer_set(&timer, TIMEOUT);
        PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&timer));
        setFlag();
    } while(!flag);
    printf("1st process completed\n");
    PROCESS_END();
}
PROCESS_THREAD(hello_world_process2, ev, data)
{
    printf("2nd process started\n");
    PROCESS_BEGIN();
    printf("2nd process completed\n");
    PROCESS_END();
}

Output:

Contiki-list-1532-g2ca33d4 started with IPV6, RPL
Rime started with address 1.2.3.4.5.6.7.8
MAC nullmac RDC nullrdc NETWORK sicslowpan
Tentative link-local IPv6 address fe80:0000:0000:0000:0302:0304:0506:0708
1st process started
2nd process started
2nd process completed
Setting flag
1st process completed

I want that second process should only be executed once the first process is executed completely i.e. while waiting for the event, the control should not get transferred to the second process.

Try this:

AUTOSTART_PROCESSES(&hello_world_process);

// ...

PROCESS_THREAD(hello_world_process, ev, data)
{
    PROCESS_BEGIN();
    printf("1st process started\n");
    do 
    {
        etimer_set(&timer, TIMEOUT);
        PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&timer));
        setFlag();
    } while(!flag);
    printf("1st process completed\n");
    process_start(&hello_world_process2, NULL); // start the second process
    PROCESS_END();
} 

See https://github.com/contiki-os/contiki/wiki/Processes.

Also, don't put any executable code before PROCESS_BEGIN()! You will not get a warning, but the result may not be what you expect, as that code will be executed every time when the process is resumed.