>> Нужен таймер, программный, чтобы допустим каждые 500 миллисекунд, основная программа прерывалась
>> (как по прерыванию) исполнялся определенный код, и снова продолжалась основная программа
>> ...
> http://docs.libuv.org/en/v1.x/timer.html

Ну или еще вот так: man setitimer

#include <signal.h>                                                                                                                                                                                                                
#include <stdio.h>                                                                
#include <string.h>                                                                
#include <sys/time.h>                                                              
                                                                                  
void timer_handler (int signum)                                                    
{                                                                                  
    static int count = 0;                                                          
    printf ("timer expired %d times\n", ++count);                                  
}                                                                                  
                                                                                  
int main ()                                                                        
{                                                                                  
    struct sigaction sa;                                                          
    struct itimerval timer;                                                        
                                                                                  
    /* Install timer_handler as the signal handler for SIGVTALRM. */              
    memset (&sa, 0, sizeof (sa));                                                  
    sa.sa_handler = &timer_handler;                                                
    sigaction (SIGVTALRM, &sa, NULL);                                              
                                                                                  
    /* Configure the timer to expire after 250 msec... */                          
    timer.it_value.tv_sec = 0;                                                    
    timer.it_value.tv_usec = 250000;                                              
    /* ... and every 250 msec after that. */                                      
    timer.it_interval.tv_sec = 0;                                                  
    timer.it_interval.tv_usec = 250000;                                            
    /* Start a virtual timer. It counts down whenever this process is              
       executing. */                                                              
    setitimer (ITIMER_VIRTUAL, &timer, NULL);                                      
                                                                                  
    /* Do busy work. */                                                            
    while (1);                                                                    
}