Multi-thread synchronization of the readers write the problem

Problem Definition:

    An existing shared memory, multiple processes and multiple read-write process. Read more processes can be read at the same time, but when there is a write process is writing, any other process to read or write process can not be executed.

The problem has three variants. The first is called "Reader Priority" (readers-preference). In this case, as long as there is progress in reading and writing process would have to wait.

To achieve the following:

#include <stdio.h> 
#include <stdlib.h> 
#include <pthread.h> #define M. 6         // Number of Readers #define N 2         // Number of Writers int ReadCount = 0 ; // Current Number of Readers 
the mutex = PTHREAD_MUTEX_INITIALIZER pthread_mutex_t;     // for modifying readCount mutex variable 
pthread_mutex_t RW = PTHREAD_MUTEX_INITIALIZER;       // for reading, writing and writing are mutually exclusive write void * write ( void * Arg) 
{ 
    the pthread_mutex_lock ( & RW) ; 
    printf (





"writer %d is writing\n", arg);
    sleep(1);
    printf("writer %d is leaving\n", arg);
    pthread_mutex_unlock(&rw);
}

void* read(void *arg)
{
    pthread_mutex_lock(&mutex);
    if (readCount == 0)
        pthread_mutex_lock(&rw);
    ++readCount;
    pthread_mutex_unlock(&mutex);

    printf("reader %d is reading\n", arg);
    sleep(1);
    printf("reader %d is leaving\n", arg);

    pthread_mutex_lock(&mutex);
    --readCount;
    if (readCount == 0)
        pthread_mutex_unlock(&rw);
    pthread_mutex_unlock(&mutex);
}

int main()
{
    pthread_t readers[M], writers[N];
    int i;
    for (i = 0; i < M; ++i)
        pthread_create(&readers[i], NULL, read, (void*)i);
    for (i = 0; i < N; ++i)
        pthread_create(&writers[i], NULL, write, (void*)i);
    sleep(10);
    return 0;
}

In this case, likely to cause the phenomenon of writer starvation.

Reproduced in: https: //www.cnblogs.com/gattaca/p/4734590.html