Fix bug where size of file name buffer was less than maximum file name size

Add support for some basic system calls and args handling correctly.

See merge request lab2425_autumn/pintos_22!29

.gitignore

@@ -31,3 +31,6 @@
 *.nav
 *.toc
 
+#ignore files from CLion/VSCode IDEs
+.idea
+.vscode

.gitlab-ci.yml

@@ -0,0 +1,44 @@
+stages:
+  - test
+
+.pintos_tests:
+  stage: test
+  image: gitlab.doc.ic.ac.uk:4567/lab2425_autumn/pintos_22/pintos-testing:latest
+  artifacts:
+    when: always
+    paths:
+      - src/$DIR/build/tests/$DIR/
+  before_script:
+    - cd src/utils
+    - make
+    - export PATH=$PWD:$PATH
+    - cd ../..
+  script:
+    - cd src/$DIR
+    - make check | tee build.log
+    - grep -q "FAIL tests/$DIR" build.log && exit 1 || exit 0
+
+test_devices:
+  extends: .pintos_tests
+  variables:
+    DIR: devices
+
+test_filesys:
+  extends: .pintos_tests
+  variables:
+    DIR: filesys
+
+test_threads:
+  extends: .pintos_tests
+  variables:
+    DIR: threads
+
+test_userprog:
+  extends: .pintos_tests
+  variables:
+    DIR: userprog
+
+test_vm:
+  extends: .pintos_tests
+  variables:
+    DIR: vm

Dockerfile.devel

@@ -0,0 +1,3 @@
+FROM debian:12-slim
+
+RUN apt update && apt install gcc perl make qemu-system-i386 gdb -y

src/devices/timer.c

@@ -119,9 +119,6 @@ timer_sleep (int64_t ticks)
                        NULL);
   intr_set_level (old_level);
   sema_down (&st.semaphore);
-  old_level = intr_disable ();
-  list_remove (&st.elem);
-  intr_set_level (old_level);
 }
 
 /* Sleeps for approximately MS milliseconds.  Interrupts must be
@@ -204,7 +201,10 @@ timer_interrupt (struct intr_frame *args UNUSED)
     {
       struct asleep_thread *st = list_entry (e, struct asleep_thread, elem);
       if (ticks >= st->end_at)
-        sema_up (&st->semaphore);
+        {
+          list_remove (&st->elem);
+          sema_up (&st->semaphore);
+        }
       else
         break;
     }

src/filesys/file.h

@@ -4,6 +4,9 @@
 #include "filesys/off_t.h"
 #include <stdbool.h>
 
+/* The maximum length of a file name in PintOS. */
+#define FNAME_MAX_LEN 14
+
 struct inode;
 
 /* Opening and closing files. */

src/lib/user/syscall.c

@@ -166,7 +166,7 @@ mkdir (const char *dir)
 }
 
 bool
-readdir (int fd, char name[READDIR_MAX_LEN + 1]) 
+readdir (int fd, char name[FNAME_MAX_LEN + 1]) 
 {
   return syscall2 (SYS_READDIR, fd, name);
 }

src/lib/user/syscall.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 #include <debug.h>
+#include "../../filesys/file.h"
 
 /* Process identifier. */
 typedef int pid_t;
@@ -12,9 +13,6 @@ typedef int pid_t;
 typedef int mapid_t;
 #define MAP_FAILED ((mapid_t) -1)
 
-/* Maximum characters in a filename written by readdir(). */
-#define READDIR_MAX_LEN 14
-
 /* Typical return values from main() and arguments to exit(). */
 #define EXIT_SUCCESS 0          /* Successful execution. */
 #define EXIT_FAILURE 1          /* Unsuccessful execution. */
@@ -41,7 +39,7 @@ void munmap (mapid_t);
 /* Task 4 only. */
 bool chdir (const char *dir);
 bool mkdir (const char *dir);
-bool readdir (int fd, char name[READDIR_MAX_LEN + 1]);
+bool readdir (int fd, char name[FNAME_MAX_LEN + 1]);
 bool isdir (int fd);
 int inumber (int fd);
 

src/threads/fixed-point.h

@@ -0,0 +1,100 @@
+#include <stdint.h>
+#ifndef FIXED_POINT_H
+#define FIXED_POINT_H
+
+typedef struct
+  {
+    int32_t raw;
+  } fp32_t;
+
+/* Fixed Point Arithmetic bit count constants */
+#define NUM_FRAC_BITS 14
+#define NUM_INT_BITS (31 - NUM_FRAC_BITS)
+#define CONVERSION_FACTOR (1 << NUM_FRAC_BITS) /* f = 2^q, (2^14) */
+
+/* Fixed Point Arithmetic conversion operations */
+/* Converts an integer n to a fixed point number */
+inline fp32_t
+fp_from_int (int32_t n)
+{
+  return (fp32_t){ n * CONVERSION_FACTOR };
+}
+
+/* Handles conversion of fixed point to integer,
+   with truncation */
+inline int32_t
+fp_floor (fp32_t x)
+{
+  return x.raw / CONVERSION_FACTOR;
+}
+
+/* Handles conversion of fixed point to integer,
+   with rounding */
+inline int32_t
+fp_round (fp32_t x)
+{
+  if (x.raw >= 0)
+    return (x.raw + CONVERSION_FACTOR / 2) / CONVERSION_FACTOR;
+  else
+    return (x.raw - CONVERSION_FACTOR / 2) / CONVERSION_FACTOR;
+}
+
+/* Add two fixed points */
+inline fp32_t
+fp_add (fp32_t x, fp32_t y)
+{
+  return (fp32_t){ x.raw + y.raw };
+}
+
+/* Subtract two fixed points */
+inline fp32_t
+fp_sub (fp32_t x, fp32_t y)
+{
+  return (fp32_t){ x.raw - y.raw };
+}
+
+
+
+/* Multiple two fixed points */
+inline fp32_t
+fp_mul (fp32_t x, fp32_t y)
+{
+  return (fp32_t){ ((int64_t)x.raw) * y.raw / CONVERSION_FACTOR };
+}
+
+/* Divide two fixed points */
+inline fp32_t
+fp_div (fp32_t x, fp32_t y)
+{
+  return (fp32_t){ ((int64_t)x.raw) * CONVERSION_FACTOR / y.raw };
+}
+
+/* Multiply fixed point and integer */
+inline fp32_t
+fp_mul_int (fp32_t x, int32_t n)
+{
+  return (fp32_t){ x.raw * n };
+}
+
+/* Divide fixed point by integer */
+inline fp32_t
+fp_div_int (fp32_t x, int32_t n)
+{
+  return (fp32_t){ x.raw / n };
+}
+
+/* Add fixed point to integer */
+inline fp32_t
+fp_add_int (fp32_t x, int32_t n)
+{
+  return (fp32_t){ x.raw + n * CONVERSION_FACTOR };
+}
+
+/* Subtract integer from fixed point */
+inline fp32_t
+fp_sub_int (fp32_t x, int32_t n)
+{
+  return (fp32_t){ x.raw - n * CONVERSION_FACTOR };
+}
+
+#endif //FIXED_POINT_H

src/threads/synch.c

@@ -32,6 +32,10 @@
 #include "threads/interrupt.h"
 #include "threads/thread.h"
 
+static bool
+priority_less (const struct list_elem *a_, const struct list_elem *b_,
+               void *aux UNUSED);
+
 /* Initializes semaphore SEMA to VALUE.  A semaphore is a
    nonnegative integer along with two atomic operators for
    manipulating it:
@@ -68,8 +72,7 @@ sema_down (struct semaphore *sema)
   old_level = intr_disable ();
   while (sema->value == 0) 
     {
-      list_insert_ordered(&sema->waiters, &thread_current ()->elem,
-                          priority_more, NULL);
+      list_push_back (&sema->waiters, &thread_current ()->elem);
       thread_block ();
     }
   sema->value--;
@@ -110,26 +113,33 @@ void
 sema_up (struct semaphore *sema)
 {
   enum intr_level old_level;
+  bool thread_unblocked = false; /* Flag to track if any thread was woken up. */
 
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
-
-  /* Wake up (unblock) the highest priority thread from the waiters list */
-  struct thread *t = NULL;
   if (!list_empty (&sema->waiters))
-    {
-      t = list_entry (list_pop_front (&sema->waiters), struct thread, elem);
-      thread_unblock (t);
-    }
-
+  {
+    /* Enforces wake-up of the highest priority thread waiting for the
+       semaphore. */
+    struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
+    list_remove (e);
+    thread_unblock (list_entry (e, struct thread, elem));
+    thread_unblocked = true;
+  }
   sema->value++;
   intr_set_level (old_level);
 
-  /* If the unblocked thread has higher priority than the current running thread
-     then yield the CPU to the unblocked thread */
-  if (!intr_context() && t != NULL && t->priority > thread_get_priority ())
-    thread_yield ();
+  /* Yields the CPU in case the thread that has been woken up has a higher
+     priority that the current running thread, including the case when called
+     within an interrupt handler. */
+  if (thread_unblocked)
+  {
+    if (intr_context ())
+      intr_yield_on_return ();
+    else
+      thread_yield ();
+  }
 }
 
 static void sema_test_helper (void *sema_);
@@ -193,6 +203,47 @@ lock_init (struct lock *lock)
   sema_init (&lock->semaphore, 1);
 }
 
+/* Current thread donates its priority to donee, iteratively
+   propagating the donation in the case of chains in the wait-for graph.
+   Also keeps track of the donation by updating the donors list. Expects
+   interrupts to be disabled. */
+static void
+donate_priority (struct thread *donee) {
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  struct thread *donor = thread_current ();
+  list_push_back (&donee->donors_list, &donor->donor_elem);
+  
+  while (donee != NULL)
+    {
+      /* Stop propagation of donation once a donee is reached that has
+         a higher effective priority (as its donees can't have less
+         priority than that being donated). */
+      if (donor->priority <= donee->priority)
+          break;
+
+      /* Also stop propagation of donation once a donee is reached with
+         no donees of its own (sink node in WFG). */
+      if (donee->waiting_lock == NULL)
+        {
+          /* Only the sink node of the WFG isn't waiting for a lock and
+             could be on the ready list. Thus, as its priority changed,
+             it must be reinserted into the list. */
+          enum intr_level old_level = intr_disable ();
+          donee->priority = donor->priority;
+          ready_list_reinsert (donee);
+          intr_set_level (old_level);
+          
+          donee = NULL;
+        }
+      else
+        {
+          donee->priority = donor->priority;
+          donee = donee->waiting_lock->holder;
+        }
+    }
+}
+
 /* Acquires LOCK, sleeping until it becomes available if
    necessary.  The lock must not already be held by the current
    thread.
@@ -208,8 +259,19 @@ lock_acquire (struct lock *lock)
   ASSERT (!intr_context ());
   ASSERT (!lock_held_by_current_thread (lock));
 
+  struct thread *t = thread_current ();
+
+  enum intr_level old_level = intr_disable ();
+  if (lock->holder != NULL)
+    {
+      t->waiting_lock = lock;
+      donate_priority (lock->holder);
+    }
+  intr_set_level (old_level);
+
   sema_down (&lock->semaphore);
   lock->holder = thread_current ();
+  t->waiting_lock = NULL;
 }
 
 /* Tries to acquires LOCK and returns true if successful or false
@@ -243,6 +305,51 @@ lock_release (struct lock *lock)
   ASSERT (lock != NULL);
   ASSERT (lock_held_by_current_thread (lock));
 
+  struct thread *current_thread = thread_current ();
+  struct thread *max_donor = NULL;
+
+  struct list orphan_list;
+  list_init (&orphan_list);
+
+  enum intr_level old_level = intr_disable ();
+  /* Loop through current thread's donors, removing the ones waiting for the
+     lock being released and keeping track of them (within orphan_list).
+     Also identifies the highest priority donor thread among them. */
+  struct list_elem *tail = list_tail (&current_thread->donors_list);
+  struct list_elem *e = list_begin (&current_thread->donors_list);
+  while (e != tail)
+    {
+      struct thread *donor = list_entry (e, struct thread, donor_elem);
+      struct list_elem *next = list_next (e);
+
+      /* Excludes donors that aren't waiting for the lock being released,
+         and tracks the rest. */
+      if (donor->waiting_lock == lock)
+        {
+          list_remove (e);
+          list_push_back (&orphan_list, e);
+
+          /* Identify highest priority donor. */
+          if (max_donor == NULL || donor->priority > max_donor->priority)
+            max_donor = donor;
+        }
+
+      e = next;
+    }
+
+  /* If there exists a maximum donor thread waiting for this lock to be
+     released, transfer the remaining orphaned donors to its donor list. */
+  if (max_donor != NULL)
+    {
+      while (!list_empty (&orphan_list))
+        list_push_back (&max_donor->donors_list, list_pop_front (&orphan_list));
+    }
+
+  intr_set_level (old_level);
+  /* Removal of donors to this thread may change its effective priority,
+     so recalculate. */
+  thread_recalculate_priority ();
+
   lock->holder = NULL;
   sema_up (&lock->semaphore);
 }
@@ -265,6 +372,19 @@ struct semaphore_elem
     struct semaphore semaphore;         /* This semaphore. */
   };
 
+/* Function that compares the two threads associated with the provided
+   pointers to their 'elem' member. Returns true if the thread associated
+   with a_ has a lower priority than that of b_. */
+static bool
+priority_less (const struct list_elem *a_, const struct list_elem *b_,
+               void *aux UNUSED)
+{
+  struct thread *a = list_entry (a_, struct thread, elem);
+  struct thread *b = list_entry (b_, struct thread, elem);
+
+  return a->priority < b->priority;
+}
+
 /* Function that compares the two *semaphores* associated with the provided
    list_elem structures. [i.e., takes list_elem of semaphore_elem, and]
    Returns true if the thread associated with the semaphore associated with a_
@@ -274,43 +394,25 @@ struct semaphore_elem
     priority of the first semaphore. This is useful when the thread has not been
     sema'd down yet. */
 static bool
-sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
-                   void *aux)
+sema_priority_more(const struct list_elem *a, const struct list_elem *b,
+                   void *inserting_telem)
 {
-  int a_priority, b_priority;
+  struct list_elem *te_a, *te_b;
   
-  /* If an aux is provided, then use it as the priority of the first semaphore.
-     Otherwise, get the priority of the first semaphore. */
-  if (aux != NULL)
-    a_priority = *(int *) aux;
+  te_b = list_front (
+    &list_entry (b, struct semaphore_elem, elem)->semaphore.waiters);
+
+  if (inserting_telem == NULL)
+    {
+      te_a = list_front (
+        &list_entry (a, struct semaphore_elem, elem)->semaphore.waiters);
+    }
   else
     {
-      struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
-
-      /* If waiters list is empty, return false (i.e., a has lower priority) */
-      if (list_empty(&a->semaphore.waiters))
-        return false;
-
-      /* Otherwise, get the thread with the highest priority from the waiters
-         list. By design, this is the first one in the list (See sema_down). */
-      struct thread *a_thread =
-        list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
-
-      a_priority = a_thread->priority;
+      te_a = inserting_telem;
     }
 
-  struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
-
-  /* If waiters list is empty, return true (i.e., a has higher priority) */
-  if (list_empty(&b->semaphore.waiters))
-    return true;
-
-  struct thread *b_thread =
-    list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
-
-  b_priority = b_thread->priority;
-
-  return a_priority > b_priority;
+  return priority_more (te_a, te_b, NULL);
 }
 
 /* Initializes condition variable COND.  A condition variable
@@ -355,14 +457,7 @@ cond_wait (struct condition *cond, struct lock *lock)
   ASSERT (lock_held_by_current_thread (lock));
   
   sema_init (&waiter.semaphore, 0);
-
-  /* Insert the semaphore_elem into the waiters list in order of priority.
-     We pass the priority of the current thread as aux to sema_priority_more
-     because the thread has not been sema'd down yet (See sema_priority_more) */
-  int priority = thread_current ()->priority;
-  list_insert_ordered (&cond->waiters, &waiter.elem, sema_priority_more,
-                       &priority);
-
+  list_push_back (&cond->waiters, &waiter.elem);
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);
@@ -384,8 +479,13 @@ cond_signal (struct condition *cond, struct lock *lock UNUSED)
   ASSERT (lock_held_by_current_thread (lock));
 
   if (!list_empty (&cond->waiters))
-    sema_up (&list_entry (list_pop_front (&cond->waiters),
-                          struct semaphore_elem, elem)->semaphore);
+    {
+      /* Enforce wake-up of highest priority thread within the singleton 
+         semaphores waiting for condvar. */
+      struct list_elem *e = list_min (&cond->waiters, sema_priority_more, NULL);
+      list_remove (e);
+      sema_up (&list_entry (e, struct semaphore_elem, elem)->semaphore);
+    }
 }
 
 /* Wakes up all threads, if any, waiting on COND (protected by

src/threads/thread.c

@@ -4,6 +4,8 @@
 #include <random.h>
 #include <stdio.h>
 #include <string.h>
+#include "devices/timer.h"
+#include "threads/fixed-point.h"
 #include "threads/flags.h"
 #include "threads/interrupt.h"
 #include "threads/intr-stubs.h"
@@ -49,9 +51,11 @@ struct kernel_thread_frame
 static long long idle_ticks;    /* # of timer ticks spent idle. */
 static long long kernel_ticks;  /* # of timer ticks in kernel threads. */
 static long long user_ticks;    /* # of timer ticks in user programs. */
+static fp32_t load_avg = { 0 }; /* System load average. */
 
 /* Scheduling. */
 #define TIME_SLICE 4            /* # of timer ticks to give each thread. */
+#define PRI_UPDATE_FREQ 4       /* # of timer ticks to update priorities. */
 static unsigned thread_ticks;   /* # of timer ticks since last yield. */
 
 /* If false (default), use round-robin scheduler.
@@ -64,12 +68,18 @@ static void kernel_thread (thread_func *, void *aux);
 static void idle (void *aux UNUSED);
 static struct thread *running_thread (void);
 static struct thread *next_thread_to_run (void);
-static void init_thread (struct thread *, const char *name, int priority);
+static void init_thread (struct thread *, const char *name, int nice,
+                         int priority, fp32_t recent_cpu);
 static bool is_thread (struct thread *) UNUSED;
 static void *alloc_frame (struct thread *, size_t size);
+static int calculate_bsd_priority (fp32_t recent_cpu, int nice);
+static void update_recent_cpu (struct thread *t, void *aux UNUSED);
+static void recalculate_priority (struct thread *t);
 static void schedule (void);
 void thread_schedule_tail (struct thread *prev);
 static tid_t allocate_tid (void);
+static bool donor_priority_less (const struct list_elem *a_,
+                                 const struct list_elem *b_, void *aux UNUSED);
 
 /* Initializes the threading system by transforming the code
    that's currently running into a thread.  This can't work in
@@ -95,7 +105,9 @@ thread_init (void)
 
   /* Set up a thread structure for the running thread. */
   initial_thread = running_thread ();
-  init_thread (initial_thread, "main", PRI_DEFAULT);
+  fp32_t initial_thread_recent_cpu = { 0 };
+  init_thread (initial_thread, "main", NICE_DEFAULT, PRI_DEFAULT,
+               initial_thread_recent_cpu);
   initial_thread->status = THREAD_RUNNING;
   initial_thread->tid = allocate_tid ();
 }
@@ -145,6 +157,35 @@ thread_tick (void)
   else
     kernel_ticks++;
 
+  /* Update system load_avg and all threads recent_cpu every second. */
+  int64_t ticks = timer_ticks ();
+  if (thread_mlfqs)
+    {
+      if (t != idle_thread)
+          t->recent_cpu = fp_add_int (t->recent_cpu, 1);
+
+      if (ticks % TIMER_FREQ == 0)
+        {
+          size_t ready = threads_ready ();
+          if (t != idle_thread)
+            ready++;
+          fp32_t old_coeff = fp_div_int (fp_mul_int (load_avg, 59), 60);
+          fp32_t new_coeff = fp_div_int (fp_from_int (ready), 60);
+          load_avg = fp_add (old_coeff, new_coeff);
+
+          thread_foreach (update_recent_cpu, NULL);
+          /* Priorities have been updated, need to re-sort. */
+          list_sort (&ready_list, priority_more, NULL);
+        }
+
+      /* Recent cpu was updated, update priority. */
+      if (t != idle_thread && ticks % PRI_UPDATE_FREQ == 0)
+        {
+          t->base_priority = calculate_bsd_priority (t->recent_cpu, t->nice);
+          recalculate_priority (t);
+        }
+    }
+
   /* Enforce preemption. */
   if (++thread_ticks >= TIME_SLICE)
     intr_yield_on_return ();
@@ -192,7 +233,8 @@ thread_create (const char *name, int priority,
     return TID_ERROR;
 
   /* Initialize thread. */
-  init_thread (t, name, priority);
+  struct thread *parent_thread = thread_current ();
+  init_thread (t, name, parent_thread->nice, priority, parent_thread->recent_cpu);
   tid = t->tid = allocate_tid ();
 
   /* Prepare thread for first run by initializing its stack.
@@ -220,8 +262,9 @@ thread_create (const char *name, int priority,
   /* Add to run queue. */
   thread_unblock (t);
 
-  /* Yield if the new thread has a higher priority than the current thread. */
-  if (priority > thread_get_priority ())
+  /* Yield if the newly created thread has higher priority than the current
+     thread. */
+  if (t->priority > thread_get_priority ())
     thread_yield ();
 
   return tid;
@@ -262,7 +305,7 @@ thread_unblock (struct thread *t)
   ASSERT (t->status == THREAD_BLOCKED);
 
   /* Insert the thread back into the ready list in priority order. */
-  list_insert_ordered(&ready_list, &t->elem, priority_more, NULL);
+  list_insert_ordered (&ready_list, &t->elem, priority_more, NULL);
 
   t->status = THREAD_READY;
   intr_set_level (old_level);
@@ -334,9 +377,11 @@ thread_yield (void)
 
   old_level = intr_disable ();
 
-  /* Insert the thread back into the ready list in priority order. */
   if (cur != idle_thread)
-    list_insert_ordered(&ready_list, &cur->elem, priority_more, NULL);
+    {
+      /* Insert the thread back into the ready list in priority order. */
+      list_insert_ordered (&ready_list, &cur->elem, priority_more, NULL);
+    }
 
   cur->status = THREAD_READY;
   schedule ();
@@ -361,94 +406,160 @@ thread_foreach (thread_action_func *func, void *aux)
 }
 
 /* Function that compares the two threads associated with the provided
-   list_elem structures. Returns true if the thread associated with a_ has
-   a higher priority than that of b_. */
+   pointers to their 'elem' member. Returns true if the thread associated
+   with a_ has a higher priority than that of b_. */
 bool
 priority_more (const struct list_elem *a_, const struct list_elem *b_,
-                   void *aux UNUSED)
+               void *aux UNUSED)
 {
   struct thread *a = list_entry (a_, struct thread, elem);
   struct thread *b = list_entry (b_, struct thread, elem);
 
   return a->priority > b->priority;
+
 }
 
-/* Sets the current thread's priority to NEW_PRIORITY. */
-void
-thread_set_priority (int new_priority) 
+/* Function that compares the two threads associated with the provided
+   pointers to their 'donor_elem' member. Returns true if the thread associated
+   with a_ has a lower priority than that of b_. */
+static bool
+donor_priority_less (const struct list_elem *a_, const struct list_elem *b_,
+               void *aux UNUSED)
 {
-  ASSERT (new_priority >= PRI_MIN);
-  ASSERT (new_priority <= PRI_MAX);
+  struct thread *a = list_entry (a_, struct thread, donor_elem);
+  struct thread *b = list_entry (b_, struct thread, donor_elem);
 
-  int old_priority = thread_get_priority ();
-  if (new_priority == old_priority)
+  return a->priority < b->priority;
+}
+
+/* Sets the current thread's base priority to new_base_priority.
+   Updates the current thread's effective priority if necessary. */
+void
+thread_set_priority (int new_base_priority)
+{
+
+  if (thread_mlfqs)
     return;
 
-  thread_current ()->priority = new_priority;
+  ASSERT (new_base_priority >= PRI_MIN);
+  ASSERT (new_base_priority <= PRI_MAX);
 
-  enum intr_level old_level = intr_disable ();
+  struct thread *t = thread_current ();
 
-  /* If the thread is in the ready list, the list must be reordered to maintain
-    the priority order. */
-  if (thread_current ()->status == THREAD_READY) {
-    /* Remove from the ready list and reinsert it in priority order. */
-    list_remove (&thread_current ()->elem);
-    list_insert_ordered (&ready_list, &thread_current ()->elem, priority_more,
-                         NULL);
-  }
+  /* If the base priority is unchanged, do nothing. */
+  if (new_base_priority == t->base_priority)
+    return;
 
-  if (new_priority < old_priority && !list_empty (&ready_list)) {
-    /* If the new priority is lower than the old priority, check if the current
-       thread no longer has the highest priority. If it doesn't, yield the CPU.
-     */
+  t->base_priority = new_base_priority;
+  recalculate_priority (t);
 
-    struct thread *next_thread =
-      list_entry (list_front (&ready_list), struct thread, elem);
-
-    if (next_thread->priority > new_priority) {
-      thread_yield();
-    }
-  }
-
-  intr_set_level (old_level);
+  thread_yield ();
 }
 
-/* Returns the current thread's priority. */
+/* Returns the current thread's effective priority. */
 int
 thread_get_priority (void) 
 {
   return thread_current ()->priority;
 }
 
+
+/* Updates recent_cpu for a thread. */
+static void
+update_recent_cpu (struct thread *t, void *aux UNUSED)
+{
+  fp32_t curr_recent_cpu = t->recent_cpu;
+  fp32_t dbl_load_avg = fp_mul_int (load_avg, 2);
+  fp32_t recent_cpu_coeff
+      = fp_div (dbl_load_avg, fp_add_int (dbl_load_avg, 1));
+  t->recent_cpu
+      = fp_add_int (fp_mul (recent_cpu_coeff, curr_recent_cpu), t->nice);
+  // recent_cpu was updated, update priority.
+  t->base_priority = calculate_bsd_priority (t->recent_cpu, t->nice);
+  recalculate_priority (t);
+}
+
+/* Recalculates the effective priority of the current thread. */
+void
+thread_recalculate_priority (void)
+{
+  struct thread *t = thread_current ();
+  recalculate_priority (t);
+}
+
+static void
+recalculate_priority (struct thread *t)
+{
+  enum intr_level old_level = intr_disable ();
+  t->priority = t->base_priority;
+
+  /* If there are no donors to the current thread, then the effective
+     priority is just the base priority. */
+  if (!list_empty (&t->donors_list))
+    {
+      int max_donated_priority =
+        list_entry (list_max (&t->donors_list, donor_priority_less, NULL),
+                    struct thread, donor_elem)->priority;
+
+      /* The effective priority is the max donated priority if this is
+         higher than the base priority. */
+      if (max_donated_priority > t->priority)
+        t->priority = max_donated_priority;
+    }
+  intr_set_level (old_level);
+}
+
 /* Sets the current thread's nice value to NICE. */
 void
-thread_set_nice (int nice UNUSED) 
+thread_set_nice (int nice)
 {
-  /* Not yet implemented. */
+  ASSERT (NICE_MIN <= nice && nice <= NICE_MAX);
+
+  struct thread *t = thread_current ();
+  t->nice = nice;
+  t->base_priority = calculate_bsd_priority (t->recent_cpu, t->nice);
+  recalculate_priority (t);
+  struct thread *next_t
+      = list_entry (list_begin (&ready_list), struct thread, elem);
+  if (t->priority < next_t->priority)
+    thread_yield ();
 }
 
 /* Returns the current thread's nice value. */
 int
 thread_get_nice (void) 
 {
-  /* Not yet implemented. */
-  return 0;
+  return thread_current ()->nice;
 }
 
 /* Returns 100 times the system load average. */
 int
 thread_get_load_avg (void) 
 {
-  /* Not yet implemented. */
-  return 0;
+  return fp_round (fp_mul_int (load_avg, 100));
 }
 
 /* Returns 100 times the current thread's recent_cpu value. */
 int
 thread_get_recent_cpu (void) 
 {
-  /* Not yet implemented. */
-  return 0;
+  return fp_round (fp_mul_int (thread_current ()->recent_cpu, 100));
+}
+
+/* Reinsert thread t into the ready list at its correct position
+   in descending order of priority. Used when this thread's priority
+   may have changed. Must be called with interrupts disabled. */
+void
+ready_list_reinsert (struct thread *t)
+{
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  /* If the thread isn't ready to run, do nothing. */
+  if (t->status != THREAD_READY)
+    return;
+
+  list_remove (&t->elem);
+  list_insert_ordered (&ready_list, &t->elem, priority_more, NULL);
 }
 
 /* Idle thread.  Executes when no other thread is ready to run.
@@ -524,7 +635,8 @@ is_thread (struct thread *t)
 /* Does basic initialization of T as a blocked thread named
    NAME. */
 static void
-init_thread (struct thread *t, const char *name, int priority)
+init_thread (struct thread *t, const char *name, int nice, int priority,
+             fp32_t recent_cpu)
 {
   enum intr_level old_level;
 
@@ -536,9 +648,19 @@ init_thread (struct thread *t, const char *name, int priority)
   t->status = THREAD_BLOCKED;
   strlcpy (t->name, name, sizeof t->name);
   t->stack = (uint8_t *) t + PGSIZE;
-  t->priority = priority;
   t->magic = THREAD_MAGIC;
 
+  t->base_priority
+      = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
+  list_init (&t->donors_list);
+  t->waiting_lock = NULL;
+
+  t->nice = nice;
+  t->recent_cpu = recent_cpu;
+  t->priority = t->base_priority;
+
+  t->exit_status = -1;
+
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);
@@ -617,6 +739,21 @@ thread_schedule_tail (struct thread *prev)
     }
 }
 
+/* Calculates BSD priority for a thread */
+static int
+calculate_bsd_priority (fp32_t recent_cpu, int nice)
+{
+
+  ASSERT (thread_mlfqs);
+
+  int priority = PRI_MAX - (fp_round (recent_cpu) / 4) - (nice * 2);
+  if (priority < PRI_MIN)
+    return PRI_MIN;
+  if (priority > PRI_MAX)
+    return PRI_MAX;
+  return priority;
+}
+
 /* Schedules a new process.  At entry, interrupts must be off and
    the running process's state must have been changed from
    running to some other state.  This function finds another

src/threads/thread.h

@@ -4,6 +4,7 @@
 #include <debug.h>
 #include <list.h>
 #include <stdint.h>
+#include "threads/fixed-point.h"
 
 /* States in a thread's life cycle. */
 enum thread_status
@@ -24,6 +25,10 @@ typedef int tid_t;
 #define PRI_DEFAULT 31                  /* Default priority. */
 #define PRI_MAX 63                      /* Highest priority. */
 
+#define NICE_MIN -20   /* Lowest niceness. */
+#define NICE_DEFAULT 0 /* Default niceness. */
+#define NICE_MAX 20    /* Highest niceness. */
+
 /* A kernel thread or user process.
 
    Each thread structure is stored in its own 4 kB page.  The
@@ -90,9 +95,24 @@ struct thread
     int priority;                       /* Priority. */
     struct list_elem allelem;           /* List element for all threads list. */
 
+    /* Donation Related */
+    int base_priority;                  /* Base priority of the thread. */
+    struct list donors_list;            /* List of threads that have donated
+                                           to this thread. */
+    struct lock *waiting_lock;          /* The lock that the current thread is
+                                           waiting for. */
+    struct list_elem donor_elem;        /* List element so that thread can be
+                                           enlisted in other donors list. */
+
+    /* MLFQS items */
+    int nice;                           /* Nice value for this thread */
+    fp32_t recent_cpu;                  /* Amount of time this process received */
+
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
+    int exit_status;                    /* Exit Status: 0 = successful exit. */
+
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
@@ -135,10 +155,13 @@ bool priority_more (const struct list_elem *a_, const struct list_elem *b_,
                     void *aux UNUSED);
 int thread_get_priority (void);
 void thread_set_priority (int);
+void thread_recalculate_priority (void);
 
 int thread_get_nice (void);
 void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
 
+void ready_list_reinsert (struct thread *t);
+
 #endif /* threads/thread.h */

src/userprog/process.c

@@ -14,57 +14,127 @@
 #include "threads/flags.h"
 #include "threads/init.h"
 #include "threads/interrupt.h"
+#include "threads/synch.h"
 #include "threads/palloc.h"
+#include "threads/malloc.h"
 #include "threads/thread.h"
 #include "threads/vaddr.h"
+#include "threads/synch.h"
+#include "devices/timer.h"
+
+/* Defines the native number of bytes processed by the processor
+   (for the purposes of alignment). */
+#define WORD_SIZE 4
+
+/* Keeps track of the position of pointers to user program arguments
+   within a linked list. */
+struct arg_elem
+  {
+    char* arg;
+    struct list_elem elem;
+  };
+
+/* Holds the data required to be passed from a kernel thread to a thread
+   that executes process_start for the purpose of starting a user process. */
+struct process_start_data
+  {
+    char *cmd; /* Pointer to a copy of the command used to execute the process.
+                  Allocated a page that must be freed by process_start. */
+    char *cmd_saveptr; /* Value pointed to by 'saveptr' argument used by
+                          successive calls to strtok_r to split 'cmd' into
+                          tokens while maintaining state. */
+    char file_name[FNAME_MAX_LEN + 1]; /* Name of the file of the process to
+                                      be started. */
+  };
 
 static thread_func start_process NO_RETURN;
 static bool load (const char *cmdline, void (**eip) (void), void **esp);
 
-/* Starts a new thread running a user program loaded from
-   FILENAME.  The new thread may be scheduled (and may even exit)
+/* Starts a new thread running a user program executed via
+   CMD.  The new thread may be scheduled (and may even exit)
    before process_execute() returns.  Returns the new process's
    thread id, or TID_ERROR if the thread cannot be created. */
 tid_t
-process_execute (const char *file_name) 
+process_execute (const char *cmd) 
 {
-  char *fn_copy;
+  char *cmd_copy;
   tid_t tid;
   
-  /* Make a copy of FILE_NAME.
-     Otherwise there's a race between the caller and load(). */
-  fn_copy = palloc_get_page (0);
-  if (fn_copy == NULL)
-    return TID_ERROR;
-  strlcpy (fn_copy, file_name, PGSIZE);
+  struct process_start_data *data = malloc (sizeof (struct process_start_data));
+  if (data == NULL)
+    {
+      return TID_ERROR;
+    }
 
-  /* Create a new thread to execute FILE_NAME. */
-  tid = thread_create (file_name, PRI_DEFAULT, start_process, fn_copy);
+  /* Make a copy of command.
+     Otherwise there's a race between the caller and load(). */
+  cmd_copy = palloc_get_page (0);
+  if (cmd_copy == NULL)
+    return TID_ERROR;
+
+  /* Imposing implicit limit that the command line arguments
+    including the user program name fit within a single page. */
+  strlcpy (cmd_copy, cmd, PGSIZE);
+
+  /* Retrieve first argument of command, which is the file name
+     of the process. */
+  char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
+
+  /* Create a new thread to execute the command, by initializing
+     it running the function 'start_process' with the appropriate
+     arguments. For details of arguments, see 'start_process'. */
+  data->cmd = cmd_copy;
+  strlcpy (data->file_name, file_name, FNAME_MAX_LEN + 1);
+
+  tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
   if (tid == TID_ERROR)
-    palloc_free_page (fn_copy); 
+    palloc_free_page (cmd_copy); 
   return tid;
 }
 
-/* A thread function that loads a user process and starts it
-   running. */
+static bool install_page (void *upage, void *kpage, bool writable);
+static bool process_init_stack (char *cmd_saveptr, void **esp, char *file_name);
+static void *push_to_stack (void **esp, void *data, size_t data_size);
+#define push_var_to_stack(esp, var) (push_to_stack (esp, &var, sizeof (var)))
+
+/* Make the current thread execute 'cmd', passing in a copy of the
+   command string used for processing, the saveptr used by strtok_r
+   (in order to further tokenize the same command and retrieve its
+   arguments), as well as the name of the file being executed. This
+   involves loading the specified file and starting it running. */
 static void
-start_process (void *file_name_)
+start_process (void *proc_start_data)
 {
-  char *file_name = file_name_;
   struct intr_frame if_;
   bool success;
 
+  struct process_start_data *data = proc_start_data;
+
   /* Initialize interrupt frame and load executable. */
   memset (&if_, 0, sizeof if_);
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
-  success = load (file_name, &if_.eip, &if_.esp);
+  success = load (data->file_name, &if_.eip, &if_.esp);
 
   /* If load failed, quit. */
-  palloc_free_page (file_name);
   if (!success) 
-    thread_exit ();
+    {
+      palloc_free_page (data->cmd);
+      goto fail;
+    }
+
+  /* Initialize user process stack and free page used to store the
+     command that executed the process. */
+  success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
+  palloc_free_page (data->cmd);
+  
+  /* If stack initialization failed, free resources and quit. */
+  if (!success)
+    {
+      process_exit ();
+      goto fail;
+    }
 
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
@@ -74,6 +144,109 @@ start_process (void *file_name_)
      and jump to it. */
   asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
   NOT_REACHED ();
+
+/* If starting the process failed, free its common resources and exit. */
+ fail:
+  free (data);
+  thread_exit ();
+}
+
+/* Helper function that initializes the stack of a newly created 
+   user process. Returns true if successful, false otherwise. */
+static bool
+process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
+  {
+    /* Load command line argument *data* to user process stack.
+       This can't cause overflow due to enforcing that the size of
+       command line input must fit in a page. Also keep track
+       of pointers to the argument data within a linked list. */
+    struct list arg_list;
+    list_init (&arg_list);
+
+    char *arg = file_name;
+    int arg_count = 0;
+    while (arg != NULL)
+      {
+        push_to_stack (esp, arg, (strlen (arg) + 1) * sizeof (char));
+        
+        struct arg_elem *arg_elem = malloc (sizeof (struct arg_elem));
+        if (arg_elem == NULL)
+          {
+            printf("ERROR: Couldn't allocate argument pointer memory for %s!\n",
+                   thread_current ()->name);
+            return false;
+          }
+
+        arg_elem->arg = *esp;
+        list_push_front (&arg_list, &arg_elem->elem);
+
+        arg_count++;
+        arg = strtok_r (NULL, " ", &cmd_saveptr);
+      }
+
+    /* Calculate the remaining number of bytes that need to be written
+       to the user process stack in order to check for possible overflow. */
+    size_t align_size = ((unsigned int) *esp % WORD_SIZE) * sizeof (uint8_t);
+    size_t argv_data_size = (arg_count + 1) * sizeof (char *);
+    size_t argv_size = sizeof (char **);
+    size_t argc_size = sizeof (int);
+    size_t return_addr_size = sizeof (void *);
+    size_t remaining_size = align_size + argv_data_size + argv_size + argc_size
+                            + return_addr_size;
+
+    /* If pushing the rest of the data required for the stack would cause
+       overflow, allocate an extra page that is contiguous within the
+       virtual address space (below the current address range). */ 
+    if (PHYS_BASE - *esp + remaining_size > PGSIZE)
+      {
+        uint8_t *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
+        if (!install_page (((uint8_t *) PHYS_BASE) - PGSIZE * 2, kpage, true))
+          return false;
+      }
+
+    /* Align stack pointer to word size before pushing argv elements for
+       performance. */
+    *esp -= align_size;
+    
+    /* Push a null pointer sentinel inside argv. */
+    char *null_sentinel = NULL;
+    push_var_to_stack (esp, null_sentinel); 
+
+    /* Push pointers to process arguments from argument linked list */
+    struct list_elem *e = list_begin (&arg_list);
+    struct list_elem *tail = list_tail (&arg_list);
+    while (e != tail)
+      {
+        struct arg_elem *arg_elem = list_entry (e, struct arg_elem, elem);
+        
+        push_var_to_stack(esp, arg_elem->arg);
+        
+        e = list_next (e);
+        free (arg_elem);
+      }
+
+    /* Push pointer to the start of argv array. */
+    char **argv = *esp;
+    push_var_to_stack(esp, argv);
+
+    /* Push the number of arguments to the stack. */
+    push_var_to_stack(esp, arg_count);
+
+    /* Push fake return address (null pointer). */
+    push_var_to_stack (esp, null_sentinel); 
+
+    return true;
+ }
+
+/* Helper function that pushes the first 'data_size' bytes stored
+   in the address '*data' into the stack given a pointer to the
+   stack pointer '**esp'. */
+static void *
+push_to_stack (void **esp, void *data, size_t data_size)
+{
+  *esp -= data_size;
+  memcpy (*esp, data, data_size);
+  return *esp;
 }
 
 /* Waits for thread TID to die and returns its exit status. 
@@ -88,7 +261,12 @@ start_process (void *file_name_)
 int
 process_wait (tid_t child_tid UNUSED) 
 {
-  return -1;
+  /* As a temporary wait, waiting will just put the thread to sleep for one
+     second (TIMER_FREQ = 100 ticks ~ 1 second). */
+  /* TODO: Implement process_wait () correctly. Remove the next line. */
+  timer_sleep (TIMER_FREQ);
+
+  return 0; /* TODO: Change this too */
 }
 
 /* Free the current process's resources. */
@@ -98,6 +276,8 @@ process_exit (void)
   struct thread *cur = thread_current ();
   uint32_t *pd;
 
+  printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
+
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
   pd = cur->pagedir;
@@ -318,8 +498,6 @@ load (const char *file_name, void (**eip) (void), void **esp)
 
 /* load() helpers. */
 
-static bool install_page (void *upage, void *kpage, bool writable);
-
 /* Checks whether PHDR describes a valid, loadable segment in
    FILE and returns true if so, false otherwise. */
 static bool

src/userprog/syscall.c

@@ -1,11 +1,68 @@
 #include "userprog/syscall.h"
-#include <stdio.h>
-#include <syscall-nr.h>
+#include "devices/shutdown.h"
+#include "devices/input.h"
+#include "threads/vaddr.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
+#include "userprog/process.h"
+#include <stdio.h>
+#include <syscall-nr.h>
 
 static void syscall_handler (struct intr_frame *);
 
+/* A syscall_function is a function that receives up to 3 arguments, the
+   arguments to the functions are either ints or pointers taking up to 32 bits
+   in size. */
+typedef uintptr_t (*syscall_function) (uintptr_t, uintptr_t, uintptr_t);
+
+/* System call function prototypes */
+static void syscall_halt (void);
+static void syscall_exit (int status);
+static pid_t syscall_exec (const char *cmd_line);
+static int syscall_wait (pid_t pid);
+static bool syscall_create (const char *file, unsigned initial_size);
+static bool syscall_remove (const char *file);
+static int syscall_open (const char *file);
+static int syscall_filesize (int fd);
+static int syscall_read (int fd, void *buffer, unsigned size);
+static int syscall_write (int fd, const void *buffer, unsigned size);
+static void syscall_seek (int fd, unsigned position);
+static unsigned syscall_tell (int fd);
+static void syscall_close (int fd);
+
+static void *validate_user_pointer (const void *ptr, size_t size);
+
+/* A struct defining a syscall_function pointer along with its arity. */
+typedef struct
+  {
+    syscall_function function;    /* Function pointer. */
+    int arity;                    /* Number of arguments of the function. */
+  } syscall_arguments;
+
+/* A look-up table mapping numbers to system call functions with their number of
+   arguments. */
+static const syscall_arguments syscall_lookup[] =
+{
+  [SYS_HALT] = {(syscall_function) syscall_halt, 0},
+  [SYS_EXIT] = {(syscall_function) syscall_exit, 1},
+  [SYS_EXEC] = {(syscall_function) syscall_exec, 1},
+  [SYS_WAIT] = {(syscall_function) syscall_wait, 1},
+  [SYS_CREATE] = {(syscall_function) syscall_create, 2},
+  [SYS_REMOVE] = {(syscall_function) syscall_remove, 1},
+  [SYS_OPEN] = {(syscall_function) syscall_open, 1},
+  [SYS_FILESIZE] = {(syscall_function) syscall_filesize, 1},
+  [SYS_READ] = {(syscall_function) syscall_read, 3},
+  [SYS_WRITE] = {(syscall_function) syscall_write, 3},
+  [SYS_SEEK] = {(syscall_function) syscall_seek, 2},
+  [SYS_TELL] = {(syscall_function) syscall_tell, 1},
+  [SYS_CLOSE] = {(syscall_function) syscall_close, 1},
+};
+
+/* The number of syscall functions (i.e, number of elements) within the
+   syscall_lookup table. */
+static const int LOOKUP_SIZE
+  = sizeof (syscall_lookup) / sizeof (syscall_arguments);
+
 void
 syscall_init (void) 
 {
@@ -13,8 +70,165 @@ syscall_init (void)
 }
 
 static void
-syscall_handler (struct intr_frame *f UNUSED) 
+syscall_handler (struct intr_frame *f)
 {
-  printf ("system call!\n");
+  /* First, read the system call number from the stack. */
+  validate_user_pointer (f->esp, 1);
+  unsigned syscall_number = *(int *) f->esp;
+
+  /* Ensures the number corresponds to a system call that can be handled. */
+  if (syscall_number >= LOOKUP_SIZE)
+    thread_exit ();
+
+  syscall_arguments syscall = syscall_lookup[syscall_number];
+
+  /* Next, read and copy the arguments from the stack pointer. */
+  validate_user_pointer (f->esp + sizeof (uintptr_t),
+                         syscall.arity * sizeof (uintptr_t));
+  uintptr_t args[3] = {0};
+  for (int i=0; i < syscall.arity; i++)
+      args[i] = *(uintptr_t *) (f->esp + sizeof (uintptr_t) * (i + 1));
+
+  /* Call the function that handles this system call with the arguments. When
+     there is a return value it is stored in f->eax. */
+  f->eax = syscall.function (args[0], args[1], args[2]);
+}
+
+static void
+syscall_halt (void)
+{
+  shutdown_power_off ();
+}
+
+static void
+syscall_exit (int status)
+{
+  /* Sets exit_status of the thread to status. thread_exit () will call
+     process_exit () if user programs are allowed. */
+  thread_current ()->exit_status = status;
   thread_exit ();
 }
+
+static pid_t
+syscall_exec (const char *cmd_line UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_wait (pid_t pid)
+{
+  return process_wait (pid);
+}
+
+static bool
+syscall_create (const char *file UNUSED, unsigned initial_size UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static bool
+syscall_remove (const char *file UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_open (const char *file UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_filesize (int fd UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static int
+syscall_read (int fd, void *buffer, unsigned size)
+{
+  /* Only console (fd = 0) or other files, not including STDOUT, (fd > 1) are
+     allowed. */
+  if (fd < 0 && fd != STDOUT_FILENO)
+    return -1;
+
+  validate_user_pointer (buffer, size);
+
+  if (fd == STDIN_FILENO)
+    {
+      /* Reading from the console. */
+      char *write_buffer = buffer;
+      for (int i = 0; i < size; i++)
+        write_buffer[i] = input_getc ();
+
+      return size;
+    }
+  else
+    {
+      /* Reading from a file. */
+      return 0; // TODO: Implement Write to Files
+    }
+}
+
+static int
+syscall_write (int fd, const void *buffer, unsigned size)
+{
+  /* Only console (fd = 1) or other files, not including STDIN, (fd > 1) are
+     allowed. */
+  if (fd <= 0)
+    return 0;
+
+  validate_user_pointer (buffer, size);
+
+  if (fd == STDOUT_FILENO)
+    {
+      /* Writing to the console. */
+      putbuf (buffer, size);
+      return size;
+    }
+  else
+    {
+      /* Writing to a file. */
+      return 0; // TODO: Implement Write to Files
+    }
+}
+
+static void
+syscall_seek (int fd UNUSED, unsigned position UNUSED)
+{
+  //TODO
+}
+
+static unsigned
+syscall_tell (int fd UNUSED)
+{
+  //TODO
+  return 0;
+}
+
+static void
+syscall_close (int fd UNUSED)
+{
+  //TODO
+}
+
+/* Validates if a block of memory starting at PTR and of size SIZE bytes is
+   fully contained within user virtual memory. Kills the thread (by calling
+   thread_exit) if the memory is invalid. Otherwise, returns the PTR given.
+   If the size is 0, the function does no checks and returns PTR.*/
+static void *
+validate_user_pointer (const void *ptr, size_t size)
+{
+  if (size > 0 && (ptr == NULL ||
+                   !is_user_vaddr (ptr) ||
+                   !is_user_vaddr (ptr + size - 1)))
+    thread_exit ();
+
+  return ptr;
+}

src/userprog/syscall.h

@@ -1,6 +1,8 @@
 #ifndef USERPROG_SYSCALL_H
 #define USERPROG_SYSCALL_H
 
+typedef int pid_t;
+
 void syscall_init (void);
 
 #endif /* userprog/syscall.h */