Ajouté verrou contre la famine

TP11and12/readers_writers_exercice.c

@@ -10,19 +10,16 @@
 
 #include "semaph.h"
 
+#define FILE_NB_READERS_NAME "readers.dat"
+
 static void usage(void);
 
 sem_t *lock_for_shared_variables;
 sem_t *lock_for_readers_writers;
+sem_t *lock_against_starvation;
 
-/* System file descriptor containing le number 
- * of readers accessing the shared ressource */
 FILE *number_of_readers_file;
 
-/* Macro: name of the file containing the number of readers accessint the shared resource */
-#define FILE_NB_READERS_NAME "readers.dat"
-
-/* Prototypes of functions */
 void readers_function(int);
 void writers_function(int);
 char is_first();
@@ -42,6 +39,7 @@ main(int argc, char *argv[])
 {
 	int i, max_writers, max_readers, child_state;
 	int initial_sem_counter = 1; // initial value for semaphores counter
+
 	if (argc != 3)
 		usage();
 
@@ -58,6 +56,9 @@ main(int argc, char *argv[])
 	if ((lock_for_readers_writers = sem_open(LOCK_FOR_READERS_WRITERS_NAME, O_CREAT, 0644, initial_sem_counter)) == SEM_FAILED)
 		err(EXIT_FAILURE, "sem_open");
 
+	if ((lock_against_starvation = sem_open(LOCK_AGAINST_STARVATION_NAME, O_CREAT, 0644, initial_sem_counter)) == SEM_FAILED)
+		err(EXIT_FAILURE, "sem_open");
+
 	/*
 	 * Open and initialise the file containing the number of readers
 	 * accessing the shared resource.
@@ -118,6 +119,7 @@ writers_function(int writer_id)
 	/* Loop of three writings */
 	for (i = 0; i < 3; ++i) {
 		/*****  Begining of the critical section for the readers/writers mechanism *****/
+		sem_wait(lock_against_starvation);
 		sem_wait(lock_for_readers_writers);
 
 		/* get the current date */
@@ -142,6 +144,7 @@ writers_function(int writer_id)
 		printf(" ******************** Writer %d about to exit the critical section -- Date :  %02d:%02d (iteration : %d)\n", writer_id, minutes, seconds,  i);
 
 		/*****  End of the critical section for the readers/writers mechanism *****/
+		sem_post(lock_against_starvation);
 		sem_post(lock_for_readers_writers);
 		sleep(2 + i);
 	}
@@ -158,6 +161,8 @@ readers_function(int reader_id)
 	time_t the_clock;
 	struct tm *the_date;
 
+	sem_wait(lock_against_starvation);
+	sem_post(lock_against_starvation);
 	sem_wait(lock_for_shared_variables);
 
 	if (is_first()) {
@@ -190,6 +195,7 @@ readers_function(int reader_id)
 	printf("========================= Reader %d about to exit the critical section -- Date :  %02d:%02d \n", reader_id, minutes, seconds);
 
 	sem_wait(lock_for_shared_variables);
+
 	if (is_last()) {
 		printf("========================= Reader %d is last\n", reader_id);
 		/*****  End of the critical section for the readers/writers mechanism *****/