Larry, Moe, Curly 互斥

Question

问题陈述：

"Larry, Moe, and Curly are planting seeds. Larry digs the holes. Moe then places a seed in each hole. Curly then fills the hole up. There are several synchronization constraints:

Moe cannot plant a seed unless at least one empty hole exists, but Moe does not care how far Larry gets ahead of Moe.

Curly cannot fill a hole unless at least one hole exists in which Moe has planted a seed, but the hole has not yet been filled. Curly does not care how far Moe gets ahead of Curly.

Curly does care that Larry does not get more than MAX holes ahead of Curly. Thus, if there are MAX unfilled holes, Larry has to wait.

There is only one shovel with which both Larry and Curly need to dig and fill the holes, respectively.

Design, implement and test a solution for this IPC problem, which represent Larry, Curly, and Moe. Use semaphores as the synchronization mechanism."

我已经根据收到的一些伪代码输入了一个程序，但出现错误：

project2part3.c:13:13: warning: initialization makes pointer from integer without a cast [-Wint-conversion] 
 #define MAX 5
             ^
project2part3.c:22:18: note: in expansion of macro 'MAX'
 sem_t unfilled = MAX;
                   ^

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>

#define MAX 5

void *larry();
void *moe();
void *curly();

pthread_mutex_t shovel = PTHREAD_MUTEX_INITIALIZER;
sem_t empty;
sem_t seeded;
sem_t unfilled;

int main(){
    pthread_t ltid;
    pthread_t mtid;
    pthread_t ctid;
    //initializing the semaphores
    sem_init(&empty, 0, 0);
    sem_init(&seeded, 0, 0);
    sem_init(&unfilled, 0, 0);

    pthread_create(&ltid, NULL, larry, NULL);  //create the larry thread
    pthread_create(&mtid, NULL, moe, NULL);  //create the moe thread
    pthread_create(&ctid, NULL, curly, NULL);  //create the curly thread

    pthread_join(ltid,NULL);
    pthread_join(mtid,NULL);
    pthread_join(ctid,NULL);
}

void *larry(){
    while(1){
        sem_wait(unfilled);
        sem_wait(shovel);
        //Dig the hole
        printf("Digging");
        sem_post(shovel);
        sem_post(empty);
    }
}

void *moe(){
    while(1){
        sem_wait(empty);
        //Seed the hole
        printf("Seeding");
        sem_post(seeded);
    }
}

void *curly(){
    while(1){
        sem_wait(seeded);
        sem_wait(shovel);
        //Fill the hole
        printf("Filling");
        sem_post(shovel);
        sem_post(unfilled);
    }
}

Answer 1

sem_t shovel = 1; // Note: Is a lock in the updated question code
sem_t empty = 0;
sem_t seeded = 0;
sem_t unfilled = MAX;

那是不是初始化信号量的方式。它们是复杂的东西，而不仅仅是可以分配给的计数器。这就是您正在使用但不正确的函数 sem_init 的原因。 Read up on it in a reference of your choice.

IIRC sem_t 是类似于

的类型定义

typedef struct whatever * sem_t;

因此，上面的每一行都从一个整数初始化了一个指针。

此外，您的函数应该 return 您声明它们的内容 return。对于用于线程的函数尤其如此：它们（需要）声明它们 return 一个空指针，所以（如果你对 return 没有任何意义）只需使用

return NULL;

在这些到达的尽头。

此外：

pthread_create(&btid, NULL, larry, NULL);  //create the larry thread
pthread_create(&btid, NULL, moe, NULL);  //create the moe thread
pthread_create(&btid, NULL, curly, NULL);  //create the curly thread

您希望对不同的 thread_t 进行这些调用，而不是对同一个 (btid)。

Larry, Moe, Curly 互斥

Larry, Moe, Curly Mutual Exclusion

c

semaphore

pthreads

mutual-exclusion