c - C 中使用 pthread 和信号量的读写器问题

Question

我正在尝试通过 C 代码创建读者-作者场景，但我坚持尝试在中间中断读者代码，以便读者不只是进入和退出，它会留下来，更多的读者可以添加。

我现在的程序不会进入多个读者，只要一个进入，它就会离开，然后作家就会来。

我尝试过使用 sleep() 但包含 unistd 库确实会破坏一半的错误代码

#include<semaphore.h>
#include<stdio.h>
#include<stdlib.h>
sem_t x,y;
pthread_t tid;
pthread_t writerthreads[100],readerthreads[100];
int readercount;

void *reader(void* param)
{
    sem_wait(&x);
    readercount++;
    if(readercount==1)
    sem_wait(&y);
    sem_post(&x);
    printf("\n%d reader is inside",readercount);
    sem_wait(&x);
    readercount--;
    if(readercount==0)
    {
        sem_post(&y);
    }
    sem_post(&x);
    printf("\n%d Reader is leaving",readercount+1);
}

void *writer(void* param)
{
    printf("\nWriter is trying to enter");
    sem_wait(&y);
    printf("\nWriter has entered");
    sem_post(&y);
    printf("\nWriter is leaving");
}

int main()
{
    int n2,i;
    printf("Enter the number of readers:");
    scanf("%d",&n2);
    int n1[n2];
    sem_init(&x,0,1);
    sem_init(&y,0,1);
    for(i=0;i<n2;i++)
    {
        pthread_create(&writerthreads[i],NULL,reader,NULL);
        pthread_create(&readerthreads[i],NULL,writer,NULL);
    }
    for(i=0;i<n2;i++)
    {
        pthread_join(writerthreads[i],NULL);
        pthread_join(readerthreads[i],NULL);
    }

}

输出的形式为

reader is inside
reader is leaving
reader is inside 
reader is leaving
writer is trying to enter
writer has entered
writer is leaving

依此类推，我希望程序中一次有多个读者。

Answer 1

你的代码工作正常。只需给阅读器添加一点延迟，就可以让其他阅读器有时间进入。否则，阅读器完成得太快，并且很少会看到重叠的阅读器。您还忘记将 readercount 初始化为零。

#include<semaphore.h>
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<pthread.h>
sem_t x,y;
pthread_t tid;
pthread_t writerthreads[100],readerthreads[100];
int readercount = 0;

void *reader(void* param)
{
    sem_wait(&x);
    readercount++;
    if(readercount==1)
        sem_wait(&y);
    sem_post(&x);
    printf("%d reader is inside\n",readercount);
    usleep(3);
    sem_wait(&x);
    readercount--;
    if(readercount==0)
    {
        sem_post(&y);
    }
    sem_post(&x);
    printf("%d Reader is leaving\n",readercount+1);
    return NULL;
}

void *writer(void* param)
{
    printf("Writer is trying to enter\n");
    sem_wait(&y);
    printf("Writer has entered\n");
    sem_post(&y);
    printf("Writer is leaving\n");
    return NULL;
}

int main()
{
    int n2,i;
    printf("Enter the number of readers:");
    scanf("%d",&n2);
    printf("\n");
    int n1[n2];
    sem_init(&x,0,1);
    sem_init(&y,0,1);
    for(i=0;i<n2;i++)
    {
        pthread_create(&writerthreads[i],NULL,reader,NULL);
        pthread_create(&readerthreads[i],NULL,writer,NULL);
    }
    for(i=0;i<n2;i++)
    {
        pthread_join(writerthreads[i],NULL);
        pthread_join(readerthreads[i],NULL);
    }

}

1 reader is inside
Writer is trying to enter
2 reader is inside
2 Reader is leaving
2 reader is inside
Writer is trying to enter
2 Reader is leaving
1 Reader is leaving