Using semaphores with shared memory while running multiple processes - c

Question

I am learning some simple producer/consumer examples involving semaphores, but having a hard time explaining to myself why I am getting a certain result (because I am a noobie to C).
When I run two concurrent processes of the following producer, both processes get hung up and never finish:
#include <stdio.h>
#include <sys/types.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/shm.h>
#define BUFF_SIZE 20
typedef struct {
char buffer[BUFF_SIZE];
int nextIn;
int nextOut;
} shared_data;
shared_data *shm, *s;
char sem_name1[] = "mutex";
char sem_name2[] = "empty_slots";
char sem_name3[] = "full_slots";
sem_t *empty_slots;
sem_t *full_slots;
sem_t *mutex;
void Put(char item)
{
sem_wait(empty_slots);
sem_wait(mutex);
s->buffer[s->nextIn] = item;
s->nextIn = (s->nextIn + 1) % BUFF_SIZE;
sem_post(mutex);
printf("Producing %c ... with pid = %d\n", item, getpid());
sem_post(full_slots);
}
void Producer()
{
int i;
for(i = 0; i < 10; i++)
{
sleep(rand()%3);
Put((char)('A'+ i % 26));
}
}
void main()
{
//Create and initialize the semaphores
mutex=sem_open(sem_name1, O_CREAT,0644, 1);
full_slots=sem_open(sem_name3, O_CREAT,0644, 0);
empty_slots=sem_open(sem_name2, O_CREAT,0644, 10);
//Create a key for the segment
key_t key;
key = 1234;
//create the segment
int shmid;
if ((shmid = shmget(key, sizeof(shared_data), IPC_CREAT |0666)) <0)
{
perror("Shmget");
exit(1);
}
//attach to the segment
if ((shm = (shared_data *) shmat(shmid, NULL, 0))==(shared_data *) -1)
{
perror("Shmat");
exit(1);
}
s=shm;
s->nextIn = 0;
Producer();
//detach from the segment
shmdt((void *) shm);
}
The output from the first and second processes, respectively:
Can someone help me understand why this happens (i.e. why do these processes never finish)? Have I created a deadlock? Why or why not? I am completely stumped.
Thanks!