#define _XOPEN_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#include <inttypes.h>
#include <sys/time.h>

uint32_t COUNTER;
int64_t t0, t1; 

int64_t timeInMS ()
{
    struct timeval t;
 
    gettimeofday(&t, NULL);
    return (
        (int64_t)t.tv_sec * 1000 +
        (int64_t)t.tv_usec / 1000
    );
}

ucontext_t start_context, tasks[2];

void threads(
    int id
)
{
    for (;;) {
        if ((COUNTER & 0xfff) == 0) {
            t1 = timeInMS();
            if (t1 - t0 >= 1000) {
                // finished
                setcontext(&start_context);
            }
        }
        COUNTER++; 
        swapcontext(&tasks[id], &tasks[1-id]); 
    }
}

#define panic printf("panic: %u\n", __LINE__); exit(1);
int main(void)
{
    char task1_stack[SIGSTKSZ];
    char task2_stack[SIGSTKSZ];

    volatile int flag = 0;

    COUNTER = 0;
    getcontext(&tasks[0]);
    getcontext(&tasks[1]);
    if (flag) 
    {
        panic;
    }
    flag = 1;

    tasks[0].uc_link = 0;
    tasks[0].uc_stack.ss_sp = task1_stack;
    tasks[0].uc_stack.ss_size = sizeof(task1_stack);
    makecontext(&tasks[0], (void (*)(void))threads, 1, 0);

    tasks[1].uc_link = 0;
    tasks[1].uc_stack.ss_sp = task2_stack;
    tasks[1].uc_stack.ss_size = sizeof(task2_stack);
    makecontext(&tasks[1], (void (*)(void))threads, 1, 1);

    t0 = timeInMS();
    if (-1 == swapcontext(&start_context, &tasks[1]))
    {
        panic;
    }
    t1 = timeInMS() - t0;

    // Correct the number of context switches accordingly
    COUNTER = (uint32_t)(((uint64_t)COUNTER * 1000) / t1);
    printf("Number of ucontext switches in about one second was %u\n", COUNTER);
    exit(0);
}