Threading program issues [closed] - c

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The problem with the following program is that the main thread ends before the other threads get a chance to display their results. Another issue is that the threads display isn't in order.
The output needs to be:
This is thread 0.
This is thread 1.
This is thread 2. etc.
code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
void *text(void *arg);
long code[] = { 4, 6, 3, 1, 5, 0, 2 }; // Order in which to start threads
int num = 0;
int main()
{
int i;
pthread_t tid[7];
// Initialize random number generator
time_t seconds;
time(&seconds);
srand((unsigned int) seconds);
// Create our threads
for (i = 0; i < 7; i++)
pthread_create(&tid[i], NULL, text, (void*)code[i]);
// Exit main
return 0;
}
void *text(void *arg)
{
long n = (long)arg;
int rand_sec = rand() % (3 - 1 + 1) + 1; // Random num seconds to sleep
while (num != n) {} // Busy wait used to wait for our turn
num++; // Let next thread go
sleep(rand_sec); // Sleep for random amount of time
printf("This is thread %d.\n", n);
// Exit thread
pthread_exit(0);
}
Can anyone help with using a mutex to fix the synchronization problem?

Answer 1

You can use pthread_join to make your main routine wait until your other threads have finished, e.g.
int main()
{
int i;
pthread_t tid[7];
// Initialize random number generator
time_t seconds;
time(&seconds);
srand((unsigned int) seconds);
// Create our threads
for (i = 0; i < 7; i++)
pthread_create(&tid[i], NULL, text, (void*)code[i]);
for(i = 0; i < 7; i++)
pthread_join(tid[i], NULL);
// Exit main
return 0;
}
Also, your code which waits for num to be the thread ID might have some issues when you start turning optimisations on. Your num variable should be declared volatile if it is shared between threads, like so:
volatile int num;
That said, busy waiting on num like so is very, very inefficient. You should consider using condition variables to notify threads when num changes, so they can check if it is their turn once, then sleep if not. Have a look at all of the pthread_cond_* functions for more information.