readers and writers.readers_preference

readers_preference.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
int count = 0;
int share=0;
pthread_t maintid;
sem_t roomEmpty;
sem_t mutex;
void* reader(void *arg)
{
    pthread_join(maintid,NULL);
    int tid;
    tid=*(int*)arg;
    sem_wait(&mutex);
    count++;
    if (count==1)
        sem_wait(&roomEmpty);
    printf("count: %d.\n",count);
    sem_post(&mutex);
    printf("reader %d in critical position.\nreads %d\n",tid,share);
    sem_wait(&mutex);
    count--;
    if (count==0){
        printf("readers all left.\n");
        sem_post(&roomEmpty);
    }
    sem_post(&mutex);
    printf("reader %d left critical position.\n",tid);
    pthread_exit((void*)0);
}
void* writer(void *arg)
{
    pthread_join(maintid,NULL);
    int tid;
    tid=*(int*)arg;
    sem_wait(&roomEmpty);
    share++;
    printf("writer %d in.\nwrites %d\n",tid,share);
    sem_post(&roomEmpty);
    printf("writer %d left critical position.\n",tid);
    pthread_exit((void*)0);
}
int main(int argc,char *argv)
{
    int nr=0,nw=0;
    int i;
    int *idreaders,*idwriters;
    pthread_t *readers;
    pthread_t *writers;

    sem_init(&roomEmpty,0,1);
    sem_init(&mutex,0,1);
    maintid=pthread_self();
    printf("How many readers do you want?\n");
    scanf("%d",&nr);
    printf("How many writers do you want?\n");
    scanf("%d",&nw);
    readers=(pthread_t*)malloc(nr*sizeof(pthread_t));
    writers=(pthread_t*)malloc(nw*sizeof(pthread_t));
    idreaders=(int*)malloc(nr*sizeof(int));
    idwriters=(int*)malloc(nw*sizeof(int));
    for (i=0;i<nr;++i){
        idwriters[i]=i+1;
        pthread_create(readers+i,NULL,reader,idwriters+i);
    }

    for (i=0;i<nw;++i){
        idreaders[i]=i+1;
        pthread_create(writers+i,NULL,writer,idreaders+i);
    }
    for (i=0;i<nr;++i)
        pthread_detach(*(readers+i));
    for (i=0;i<nw;++i)
        pthread_detach(*(writers+i));
    /*sem_destroy(&mutex);
    sem_destroy(&roomEmpty);
    free(readers);
    free(writers);
    return 0;*/
    pthread_exit((void*)0);//now readers and writers all awake.
}