Larry, Moe, Curly Mutual Exclusion

Problem Statement:

"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."

I've typed up a program from some pseudocode that I was given, but I'm getting the error:

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);
    }
}

Solution

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;

That's not how you initialise semaphores. They're complex things, not just counters that can be assigned to. That's the reason there's the function sem_init, which you're using, but not correctly. Read up on it in a reference of your choice.

IIRC sem_t is a typedef similar to

typedef struct whatever * sem_t;

Thus each of the above lines initialises a pointer from an integer.

Furthermore, your functions should return what you declare them to return. This is especially true for the functions used for the threads: They (need to) declare that they return a void pointer, so (if you don't have anything meaningful to return) just use

return NULL;

at the end of reach of these.

Moreover:

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

You want these calls to different thread_ts, not all to the same (btid).