Programming Assignment  4 solution

This assignment requires you to write a multi-threaded C program for the same problem as in the Assignment 3: the bounded-buffer producer/consumer approach for file copying, but using mutex locks and condition variables (in POSIX thread library) instead of semaphores. Make sure that your submitted program compiles and runs in Athena. 1. Your program should be run with two file names provided at the command line: infile and outfile. Otherwise, your program prompts the user with the message: “Correct Usage: pc infile outfile” and then terminates. 2. In your program, the main thread checks for correct number of input, verifies and opens the infile, and creates the (empty) outfile. The main thread then spawns a producer thread and a consumer thread, and waits for both the producer and the consumer to finish before it terminates. The producer and the consumer share a buffer of 9 slots with each slot having a size of 18 bytes. The producer reads a string of the buffer slot size from the infile and places it into the next available buffer slot. The consumer takes the next available string from a buffer slot and writes it into the outfile. The outfile is again a verbatim copy of the infile. 3. The buffer can only be accessed in a mutually exclusive fashion, which is enforced through the use of a mutex lock buf_lock and pthread_mutex_lock and pthread_mutex_unlock operations. 4. When the buffer is full, the producer must wait until a buffer slot becomes available before it can place a string into it. When the buffer is empty, the consumer must wait for an item (a string) to be available. These synchronization conditions between the producer and the consumer are facilitated through two condition variables empty_slot and avail_item, and the pthread_cond_wait and pthread_cond_signal operations. 5. A condition variable is always used in conjunction with a mutex lock and a condition, and should be declared global. A thread uses a condition variable to (a) either notify other cooperating threads (with access to the same condition variable) that a condition has been met (pthread_cond_signal); (b) or block and wait for some condition to be met (pthread_cond_wait). When a thread blocks on a condition variable, it automatically releases the associated mutex lock, allowing other threads to gain the mutex lock. In the context of the producer/consumer approach, the producer can be synchronized with the consumer in the following manner when the buffer is full: When the producer has the lock to the buffer first, and then realizes that the buffer is full, it uses a wait operation to block itself on the condition variable empty_slot and releases buf_lock. Later when the consumer gains buf_lock and empties a slot, it uses a signal operation on the condition variable empty_slot to notify the producer that there is an empty slot so that the producer can continue. After being awakened, the producer reacquires buf_lock and continues from the line immediately after where it was blocked. Alternatively, the consumer can be synchronized with the producer in a similar manner when the buffer is empty (it should wait on the condition variable avail_item instead). A wait operation will unconditionally block the calling thread, but a signal operation on a condition variable on which there is not any waiting thread is not remembered. Producer pthread_mutex_lock(&buf_lock); while (buffer is full) { pthread_cond_wait(&empty_slot, &buf_lock); } {fill a buffer slot, update variables, ……} pthread_cond_signal(&avail_item); pthread_mutex_unlock(&buf_lock); Consumer pthread_mutex_lock(&buf_lock); while (buffer is empty) { pthread_cond_wait(&avail_item, &buf_lock); } {empty a buffer slot, update variables, ……} pthread_cond_signal(&empty_slot); pthread_mutex_unlock(&buf_lock); 6. A condition variable can be declared and statically initialized as follows: pthread_cond_t empty_slot = PTHREAD_COND_INITIALIZER; Condition variable can be destroyed using pthread_cond_destroy(&empty_slot). The wait operation takes both a condition variable and a mutex lock, while the signal operation just takes a condition variable as its argument. pthread_cond_wait(&empty_slot, &buf_lock); pthread_cond_signal(&empty_slot); 7. A mutex lock can be declared and initialized as follows: pthread_mutex_t buf_lock = PTHREAD_MUTEX_INITIALIZER The following calls can be used with a mutex lock: pthread_mutex_lock(&buf_lock); pthread_mutex_unlock(&buf_lock); 8. The data type for the buffer are given as follows: #define SLOTSIZE 18 #define SLOTCNT 9 char buffer[SLOTCNT][SLOTSIZE]; Like in Assignment 3, you need to have some additional variables to handle things like (a) the next available slot for the producer; (b) the next available item for the consumer; (c) number of items available in the buffer; (d) number of bytes in a slot; and (e) a flag to indicate when producing/consuming process ends. 9. Test your program with your own data. Then make sure that your program works for the following as the content of the infile: Instead of using semaphores to synchronize concurrent activities, This assignment offers an opportunity for you to use the mutex locks and condition variables to coordinate the producer and the consumer activities in the context of the file copying application. In contrast to the semaphore-based solution where the order of two semaphore wait operations is of pivotal importance to avoid potential deadlocks, the mutex-lock-condition-variable approach allows a thread to suspend on a particular condition and release the mutex lock at the same time via the pthread_cond_wait operation, therefore avoiding potential deadlocks. This exercise helps reinforce the point that there are different synchronization approaches to a given problem.
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