Time measuring in Synchronized threaded C language - c

Question

The time measuring in thread_work function is not working.
Code is a little bit nasty but I just want you to look at the thread_work function
and teach me why the print_time function keeps generating 0 value.
(I write the whole code just in case, I'm sorry for your eyes, really)
#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include <stdlib.h>
#include <semaphore.h>
#include <unistd.h>
#define num_thread 20
char str[11];
void *thread_work(void *tid);
void generate_str(int n);
void str_sort(int n);
void check_sort(void);
void print_time(struct timespec *myclock);
void print_time_start(struct timespec *myclock);
void print_time_end(struct timespec *myclock);
sem_t my_sem;
int main(void)
{
pthread_t tid[num_thread];
int ret;
int t;
struct timespec t1[2];
srand(time(NULL));
ret = sem_init(&my_sem, 0, 1);
clock_gettime(CLOCK_REALTIME, &t1[0]);
print_time_start(t1);
for(t=0; t<num_thread; t++)
{
ret = pthread_create(&tid[t], NULL, thread_work, (void *)t);
usleep(1);
}
for(t=0; t<num_thread; t++)
ret = pthread_join(tid[t], NULL);
clock_gettime(CLOCK_REALTIME, &t1[1]);
print_time_end(t1);
sem_destroy(&my_sem);
return 0;
}
void *thread_work(void *t)
{
int n = (int )t;
struct timespec t2[2];
printf("########## Thread #%2d starting ########## \n",n);
sleep(1);
sem_wait(&my_sem); //Entry Section
clock_gettime(CLOCK_REALTIME, &t2[0]); //Critical Section Start
generate_str(n);
str_sort(n);
check_sort();
clock_gettime(CLOCK_REALTIME, &t2[1]);
print_time(t2); //Critical Section End
sem_post(&my_sem); //Exit Section
}
void str_sort(int n)
{
int temp;
int i, j;
for(i=0; i<9; i++)
for(j=0; j<9-i; j++)
{
if(str[j]>str[j+1])
{
temp=str[j];
str[j]=str[j+1];
str[j+1]=temp;
}
}
printf("[%2d] ",n);
for(i=0; i<10; i++)
printf("%2c", str[i]);
}
void generate_str(int n)
{
int i;
int num;
srand(n); //differentiate the string of each threads
for(i=0; i<10; i++)
{
num = (97+rand()%26);
str[i]=num;
}
str[10]='\0';
}
void check_sort(void)
{
int i;
int count=0;
for(i=0; i<9; i++)
{
if(str[i]>str[i+1])
count++;
}
if(count != 0)
printf(" [X]FALSE ");
else
printf(" [O]TRUE ");
}
void print_time(struct timespec *myclock)
{
long delay, temp, temp_n, sec;
sec = myclock[0].tv_sec % 60;
printf(" %ld.%ld -> ", sec, myclock[0].tv_nsec);
sec = myclock[1].tv_sec % 60;
printf("%ld.%ld", sec, myclock[1].tv_nsec);
if(myclock[1].tv_nsec >= myclock[0].tv_nsec)
{
temp = myclock[1].tv_sec - myclock[0].tv_sec;
temp_n = myclock[1].tv_nsec - myclock[0].tv_nsec;
delay = 1000000000 * temp + temp_n;
}
else
{
temp = myclock[1].tv_sec - myclock[0].tv_sec - 1;
temp_n = 1000000000 + myclock[1].tv_nsec - myclock[0].tv_nsec;
delay = 1000000000 * temp + temp_n;
}
printf(", Interval : %ld ns\n", delay);
}
void print_time_start(struct timespec *myclock)
{
long sec;
sec = myclock[0].tv_sec % 60;
printf("########## Thread: Start Time -> %ld.%ld\n", sec, myclock[0].tv_nsec);
}
void print_time_end(struct timespec *myclock)
{
long delay, temp, temp_n, sec;
sec = myclock[1].tv_sec % 60;
printf("########## Thread: End Time -> %ld.%ld ", sec, myclock[1].tv_nsec);
if (myclock[1].tv_nsec >= myclock[0].tv_nsec)
{
temp = myclock[1].tv_sec - myclock[0].tv_sec;
temp_n = myclock[1].tv_nsec - myclock[0].tv_nsec;
delay = 1000000000 * temp + temp_n; //The unit of delay is nano second
}
else
{
temp = myclock[1].tv_sec - myclock[0].tv_sec - 1;
temp_n = 1000000000 + myclock[1].tv_nsec - myclock[0].tv_nsec;
delay = 1000000000 * temp + temp_n; //The unit of delay is nano second
}
delay = delay / 1000; //The unit of delay is now micro second
printf("(Thread Execution Time -> %ld micro second)\n", delay);
}

Answer 1

clock_gettime(CLOCK_REALTIME, &t2[0]); //Critical Section Start
generate_str(n);
str_sort(n);
check_sort();
clock_gettime(CLOCK_REALTIME, &t2[1]);
Could be that the three methods execute so fast that the system clock doesn't progress. You could try and get a higher solution by changing CLOCK_REALTIME to CLOCK_THREAD_CPUTIME_ID or CLOCK_PROCESS_CPUTIME_ID.