In this project, we will design a programming solution to the bounded-buffer problem using the producer and consumer processes shown in Figure 5.9 and Figure 5.10.
Figure 5.9 The structure of the producer process.
do {
...
/* produce an item in next_produced */
...
wait(empty);
wait(mutex);
...
/* add next_produced to the buffer */
...
signal(mutex);
signal(full);
} while(true);
Figure 5.10 The structure of the consumer process.
do {
wait(full);
wait(mutex);
...
/* remove an item from buffer to next_consumed */
...
signal(mutex);
signal(empty);
...
/* consume the item in next_consumed */
...
} while(true);
This solution uses three semaphores: mutex, empty, and full.
int n;
semaphore mutex = 1;
semaphore empty = n;
semaphore full = 0
empty and full, which count the number of empty and full slots in the buffer, and mutex, which is a binary (or mutual exclusive) semaphore that protects the actual insertion or removal of items in the buffer. For this project, standard counting semaphores will be used for empty and full, and, rather than a binary semaphore, a mutex lock will be used to represent mutex. The producer and consumer - running as separate threads will move items to and from a buffer that is synchronized with these empty, full, and mutex structures. You are required to use the pthread package to solve the problem in this assignment.
Internally, the buffer will consist of a fixed-size array of type buffer_item (which will be defined using a typedef). The array of buffer_item objects will be manipulated as a circular queue. The definition of buffer_item, along with the size of the buffer, can be stored in a header file such as the following:
/* buffer.h */
typedef int buffer_item;
#define BUFFER_SIZE 5
The buffer will be manipulated with two functions, insert_item() and remove_item(), which are called by the producer and consumer threads, respectively. A skeleton outlining these functions appears as:
#include “buffer.h”
/* the buffer */
buffer_item buffer[BUFFER_SIZE];
int insert_item(buffer_item item) {
/* insert an item into buffer */
printf (“producer produced %d\n”, item);
/* return 0 if successful, otherwise
return -1 indicating an error condition */
}
int remove_item(buffer_item *item) {
/* remove an object from buffer and placing it in item*/
printf(“consumer consumed %d\n”, rand);
/* return 0 if successful, otherwise
return -1 indicating an error condition */
}
The insert_item() and remove_item() functions will synchronize the producer and consumer using the algorithms outlined in Figure 5.9 and 5.10. The buffer will also require an initialization function that initializes the mutual exclusive object mutex along with the empty and full semaphores.
The main() function will initialize the buffer and create the separate producer and consumer threads. Once it has created the producer and consumer threads, the main() function will sleep for a period of time and, upon awakening, will terminate the application. The main() function will be passed three parameters on the command line:
A skeleton for this function appears as:
#include “buffer.h”
int main(int argc, char*argv[]) {
/* 1. Get command line arguments argv[1], argv[2], argv[3] */
/* 2. Initialize buffer, mutex, semaphores, and other global vars */
/* 3. Create producer thread(s) */
/* 4. Create consumer thread(s) */
/* 5. Sleep */
/* 6. Release resources, e.g. destroy mutex and semaphores */
/* 7. Exit */
}
The producer thread will alternate between sleeping for a random period of time and inserting a random integer into the buffer. Random numbers will be produced using the rand_r(unsigned int *seed) function, which produces random integers between 0 and RAND_MAX safely in multithreaded processes. The consumer will also sleep for a random period of time and, upon awakening, will attempt to remove an item from the buffer. An outline of the producer and consumer threads appears as:
#include < stdlib.h> /* required for rand_r(...) */
#include “buffer.h”
void *producer(void *param) {
buffer_item rand;
while (1) {
/* sleep for a random period of time */
sleep(...);
/* generate a random number */
rand = rand_r(...);
if (insert_item(rand) < 0)
printf(...); // report error condition
}
}
void *consumer(void *param) {
buffer_item rand;
while (1) {
/* sleep for a random period of time */
sleep (...);
if (remove_item(&rand) < 0)
printf(...); // report error condition
}
}
The following code sample demonstrates the pthread APIs for creating a new thread:
#include < pthread.h>
void *thread_entry(void *param) { /* the entry point of a new thread */
...
}
int main(…) { pthread_t
tid; pthread_attr_t
attr;
/* get the default attribute */
pthread_attr_init(&attr);
/* create a new thread */
pthread_create(&tid, &attr, thread_entry, NULL);
...
}
The pthread package provides pthread_attr_init() function to set the default attributes for the new thread. The function pthread_create() creates a new thread, which starts the execution from the entry point specified by the third argument.
The following code sample illustrates how mutex locks available in the pthread API can be used to protect a critical section:
#include < pthread.h>
pthread_mutex_t mutex;
/* create the mutex lock */
pthread_mutex_init(&mutex, NULL);
/* acquire the mutex lock */
pthread_mutex_lock(&mutex);
/*** critical section ***/
/* release the mutex lock */
pthread_mutex_unlock(&mutex);
The pthread package uses the pthread_mutex_t data type for mutex locks. A mutex is created with the pthread_mutex_init(&mutex, NULL) function, with the first parameter being a pointer to the mutex. By passing NULL as a second parameter, we initialize the mutex to its default attributes. The mutex is acquired and released with the pthread_mutex_lock() and pthread_mutex_unlock() functions. If the mutex lock is unavailable when pthread_mutex_lock() is invoked, the calling thread is blocked until the owner invokes pthread_mutex_unlock(). All mutex functions return a value of 0 with correct operation; if an error occurs, these functions return a nonzero value.
The pthread package provides two types of semaphores - named and unnamed. For this project, we use unnamed semaphores. The code below illustrates how a semaphore is created:
#include < semaphore.h>
sem_t sem;
/* create the semaphore and initialize it to 5 */
sem_init(&sem, 0, BUFFER_SIZE);
The sem_init() creates a semaphore and initialize it. This function is passed three parameters:
In this example, by passing the flag 0, we are indicating that this semaphore can only be shared by threads belonging to the same process that created the semaphore. A nonzero value would allow other processes to access the semaphore as well. In this example, we initialize the semaphore to the value 5.
For the semaphore operations wait (or down, P) and signal (or up, V) discussed in class, the pthread package names them sem_wait() and sem_post(), respectively. The code example below creates a binary semaphore mutex with an initial value of 1 and illustrates its use in protecting a critical section: (Note: The code below is only for illustration purposes. Do not use this binary semaphore for protecting critical section. Instead, you are required to use the mutex locks provided by the pthread package for protecting critical section.)
#include < semaphore.h>
sem_t sem_mutex;
/* create the semaphore */
sem_init(&sem_mutex, 0, 1);
/* acquire the semaphore */
sem_wait(&sem_mutex);
/*** critical section ***/
/* release the semaphore */
sem_post(&mutex);
You need to link the pthread package two special libraries to provide multithreaded and semaphore support using the command "gcc -lpthread < files>"
You can start use one producer thread and one consumer thread for testing, and gradually use more producer and consumer threads. For each test case, you need to make sure that the random numbers generated by producer threads should exactly match the random numbers consumed by consumer threads (both their orders and their values).