How to protect a global variable shared by isr and regular function?

Question

Let\'s say I have function 1 and an isr routine, both share and update the same flag without any lock between them. the system is single th

Answer 1

This should help prevent missed interrupt. It is based on the wonderfully detailed ans from @artless_noise

Here the ISR posts on a semaphore(non blocking call). Adding the barrier to ensure that write completes. The thread will run as many times as the semaphore is posted.

sem = sem_open(argv[optind], flags, perms, 0); // Initialising semaphore to 0

function 1:

while(sem_getvalue(sem) > 0)
{
   flag = false;
   //Avoiding the barrier if this value isnt needed by ISR
   asm volatile ("" : : : "memory"); /* gcc barrier */
   //perform action
}

isr routine:
    do something
    flag=true;
    asm volatile ("" : : : "memory"); /* gcc barrier */
    sem_post(sem);