Threads_posix, C (gcc)

Threads_posix

//Threads in C

/* Includes */
#include <unistd.h>     /* Symbolic Constants */
//#include <sys/types.h>  /* Primitive System Data Types */ 
//#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <stdlib.h>     /* General Utilities */
#include <pthread.h>    /* POSIX Threads */
#include <string.h>     /* String handling */

/* prototype for thread routine */
void *read_func(void *ptr);
void *write_func(void *ptr);

#define NUM_READERS 5
#define NUM_WRITERS 5

int g_shared, g_readers, g_waiting_readers;
pthread_mutex_t g_lock; 
pthread_cond_t p_read = PTHREAD_COND_INITIALIZER;
pthread_cond_t p_write = PTHREAD_COND_INITIALIZER;

int main()
{
    pthread_t r_thread[NUM_READERS], w_thread[NUM_WRITERS];  /* thread variables */
    int r_id[NUM_READERS], w_id[NUM_WRITERS];
    int i;
    
    if (pthread_mutex_init(&g_lock, NULL) != 0) {
        printf("\n mutex init has failed\n"); 
        return 1; 
    }
    
    for (i = 0; i < NUM_READERS; i++) {
        r_id[i] = i;
        pthread_create (&r_thread[i], NULL, (void *) &read_func, &r_id[i]);
    }

for (i = 0; i < NUM_WRITERS; i++) {
        w_id[i] = i;
        pthread_create (&w_thread[i], NULL, (void *) &write_func, &w_id[i]);
    }
    
    for (i = 0; i < NUM_READERS; i++)
        pthread_join(r_thread[i], NULL);
    for (i = 0; i < NUM_READERS; i++)
        pthread_join(w_thread[i], NULL);

return 0;
} /* main() */

void *read_func(void *data)
{
    int *r_id = (int *)data;
    int i;
 
    for (i = 0; i < NUM_READERS; i++) {
        usleep(1000 * (random() % NUM_READERS + NUM_WRITERS)); /*wait random*/
        
        pthread_mutex_lock(&g_lock);
        g_waiting_readers++;
        while (g_readers == -1)
            pthread_cond_wait(&p_read, &g_lock);
        g_waiting_readers--;
        g_readers++;
        pthread_mutex_unlock(&g_lock);
        
        printf("Reader_id:%d Shared_value %d num_readers:%d\n", *r_id, g_shared, g_readers);
        
        pthread_mutex_lock(&g_lock);
        g_readers--;
        if (g_readers == 0)
            pthread_cond_signal(&p_write);
        pthread_mutex_unlock(&g_lock);
    }
    
    return NULL;
}

void *write_func(void *data)
{
    int *w_id = (int *)data;
    int i;
 
    for (i = 0; i < NUM_WRITERS; i++) {
        usleep(100 * (random() % NUM_READERS + NUM_WRITERS)); /*wait random*/
        
        pthread_mutex_lock(&g_lock);
        while (g_readers != 0)
            pthread_cond_wait(&p_write, &g_lock);
        g_readers = -1;
        pthread_mutex_unlock(&g_lock);

printf("Writer_id:%d Shared_value %d num_readers:%d\n", *w_id, ++g_shared, g_readers + 1);
        
        pthread_mutex_lock(&g_lock);
        g_readers = 0;
        if (g_waiting_readers > 0) {
            pthread_cond_signal(&p_read);
        } else {
            pthread_cond_signal(&p_write);
        }
        pthread_mutex_unlock(&g_lock);
    }
    
    return NULL;
}


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