/* * alarm_mutex.c * * This is an enhancement to the alarm_thread.c program, which * created an "alarm thread" for each alarm command. This new * version uses a single alarm thread, which reads the next * entry in a list. The main thread places new requests onto the * list, in order of absolute expiration time. The list is * protected by a mutex, and the alarm thread sleeps for at * least 1 second, each iteration, to ensure that the main * thread can lock the mutex to add new work to the list. */ #include #include #include "errors.h"

/* * The "alarm" structure now contains the time_t (time since the * Epoch, in seconds) for each alarm, so that they can be * sorted. Storing the requested number of seconds would not be * enough, since the "alarm thread" cannot tell how long it has * been on the list. */ typedef struct alarm_tag { struct alarm_tag *link; int seconds; time_t time; /* seconds from EPOCH */ char message[64]; } alarm_t;

pthread_mutex_t alarm_mutex = PTHREAD_MUTEX_INITIALIZER; alarm_t *alarm_list = NULL;

/* * The alarm thread's start routine. */ void *alarm_thread (void *arg) { alarm_t *alarm; int sleep_time; time_t now; int status;

/* * Loop forever, processing commands. The alarm thread will * be disintegrated when the process exits. */ while (1) { status = pthread_mutex_lock (&alarm_mutex); if (status != 0) err_abort (status, "Lock mutex"); alarm = alarm_list;

/* * If the alarm list is empty, wait for one second. This * allows the main thread to run, and read another * command. If the list is not empty, remove the first * item. Compute the number of seconds to wait -- if the * result is less than 0 (the time has passed), then set * the sleep_time to 0. */ if (alarm == NULL) sleep_time = 1; else { alarm_list = alarm->link; now = time (NULL); if (alarm->time time - now; #ifdef DEBUG printf ("[waiting: %d(%d)\"%s\"] ", alarm->time, sleep_time, alarm->message); #endif }

/* * Unlock the mutex before waiting, so that the main * thread can lock it to insert a new alarm request. If * the sleep_time is 0, then call sched_yield, giving * the main thread a chance to run if it has been * readied by user input, without delaying the message * if there's no input. */ status = pthread_mutex_unlock (&alarm_mutex); if (status != 0) err_abort (status, "Unlock mutex"); if (sleep_time > 0) sleep (sleep_time); else sched_yield ();

/* * If a timer expired, print the message and free the * structure. */ if (alarm != NULL) { printf ("(%d) %s ", alarm->seconds, alarm->message); free (alarm); } } }

int main (int argc, char *argv[]) { int status; char line[128]; alarm_t *alarm, **last, *next; pthread_t thread;

status = pthread_create ( &thread, NULL, alarm_thread, NULL); if (status != 0) err_abort (status, "Create alarm thread"); while (1) { printf ("alarm> "); if (fgets (line, sizeof (line), stdin) == NULL) exit (0); if (strlen (line)

/* * Parse input line into seconds (%d) and a message * (%64[^ ]), consisting of up to 64 characters * separated from the seconds by whitespace. */ if (sscanf (line, "%d %64[^ ]", &alarm->seconds, alarm->message) time = time (NULL) + alarm->seconds;

/* * Insert the new alarm into the list of alarms, * sorted by expiration time. */ last = &alarm_list; next = *last; while (next != NULL) { if (next->time >= alarm->time) { alarm->link = next; *last = alarm; break; } last = &next->link; next = next->link; } /* * If we reached the end of the list, insert the new * alarm there. ("next" is NULL, and "last" points * to the link field of the last item, or to the * list header). */ if (next == NULL) { *last = alarm; alarm->link = NULL; } #ifdef DEBUG printf ("[list: "); for (next = alarm_list; next != NULL; next = next->link) printf ("%d(%d)[\"%s\"] ", next->time, next->time - time (NULL), next->message); printf ("] "); #endif status = pthread_mutex_unlock (&alarm_mutex); if (status != 0) err_abort (status, "Unlock mutex"); } } }

A3. You are required to make the following changes to the program "alarm mutex.c" to produce a program named "New_Alarm Mutex.c". A3.1 Iwo tvpes of "Alarm requests" in "New Alarm Mutex.c" Two types of "Alarm requests", "Start_Alarm", and "Cancel _Alarm" requests, are recognized and handled by "New_Alarm Mutex.c", as follows (a) "Start Alarm" requests with the following format: Alam> Start_Alarm(Alarm_ID): Time Message . "Start_Alarm" is a keyword - "Alarm_ID" is a positive integer. Time" has the same meaning and syntax as in the program "alarm mutex.c". "Message" has the same meaning and syntax as in the program "alam mutexc For example: Alarm> Start_Alarm(2345) 5 Will meet you at Grandma's house at 6 pm (b) "Cancel Alarm" requests with the following format: Alarm> Cancel_Alarm(Alarm ID) Cancel Alarm:" is a keyword. "Alarm_ID" is a positive integer. For example: Alarm> Cancel_Alarm(2345) If the user types in something other than one of the above two types of valid alarm requests, then an error message will be displayed, and the invalid request will be discarded A3.2. The main thread in "New Alarm Mutex.c" The main thread in "New Alarm Mutex.c" will first determine whether the format of each alarm request is consistent with the formats specified above; otherwise the alarm request will be rejected with an error message. If a Message exceeds 128 characters, it will be truncated to 128 characters A.3.2.1. For each Start Alarm request received, the main thread will insert the corresponding alarm with the specified AlarmID into the alarm list, in which all the alarms are placed in the order of their Alarm IDs. Then the main thread will print "Alarm( ) Inserted by Main Thread Into Alarm List at : " Note that above, and in each of the messages printed by the various threads below, is the thread identifier>, insert time>, , , , tenninate time>, etc., are the actual times at which the corresponding request was processed by the main thread, those times are all expressed as the number of seconds from the Unix Epoch Jan 1 1970 00:00; correspond to "Time" and "Message" as specified in A3.1 (a) above A.3.2.2. After receiving a Start_Alarm request and inserting the alarm with the specified Alarm ID into the alarm list, the main thread will do the following: A.3.2.2.1. If: a) no display alarm thread currently exists in the system; o b) all existing (previously created) display alarm threads are currentl assigned a total of three (3) alarms that have not been canceled Then: The main thread will create a new display alarm thread and assign the most recently received alarm to that newly created display alarm thread which will be responsible for periodically printing the most recently received alarm message; then the main thread will print: "Created New Display Alarm Thread For Alarm( ) at : " Otherwise A.3.2.2.2. If some existing display alarm thread is currently assigned less than three (3) alarms, then the main thread will assign the most recently received alarm to that existing display alarm thread, and that existing display alarm thread will be responsible for periodically printing the most recently received alarm message (in addition to periodically printing any alarm messages that were previously assigned to it and that have not been canceled); then the main thread will print "Alarm( at : ". A.3.2.3. For each Cancel_Alarm request received, the main thread will remove the alarm with the specified Alarm Id from the alarm list, and the display alarm to which the alarm was previously assigned to will stop printing the corresponding alarm message; then the main thread will print Alarm() Canceled at : ". Furthermore, the main thread will do the following If any existing displav alarm thread does not have any alarm assigned to it (that is all alarms previously assigned to it have been canceled), then the main thread will terminate that display alarm thread;, then the main thread will print "Display Alarm Thread Terminated at ". A3.3. Display alarm threads in "New Alarm Mutex.c" After each display alarm thread in "New_Alarm Mutex.c" has been created by the main thread, that alarm thread will periodically check the alarms in the alarm list for alarms that have been assigned to it. If a display alarm thread finds an alarm in the alarm list that has been assigned to it, the alarm thread will periodically print, every Time seconds, where Time is the first parameter in Start Alarm requests as described in A3.1 (a) above "Alarn (salarn id>) Printed by Alarm Thread : " Note that each alarm can only be assigned to exactly one display alarm thread,, however, as described above, up to three (3) alarms can be assigned to each display alarm thread B. Platform on Which The Programs Are to be Implemented The programs should to be implemented using the ANSI C programming language and using the Linux system at York. You should use POSIX system calls or POSIX functions whenever possible