Fix both the close() and seek() system calls

Makefile.build

@@ -75,6 +75,7 @@ userprog_SRC += userprog/syscall_read.c
 userprog_SRC += userprog/syscall_filesize.c
 userprog_SRC += userprog/syscall_tell.c
 userprog_SRC += userprog/syscall_seek.c
+userprog_SRC += userprog/syscall_close.c
 
 # No virtual memory code yet.
 #vm_SRC = vm/file.c			# Some file.

userprog/syscall.c

@@ -91,6 +91,9 @@ syscall_handler (struct intr_frame *f UNUSED)
   case SYSCALL_SEEK:
 	  syscall_seek(f);
 	  break;
+  case SYSCALL_CLOSE:
+    syscall_close(f);
+    break;
   default:
     printf ("WARNING: Invalid Syscall (%d)\n", syscall_number);
     thread_exit ();

userprog/syscall_close.c

@@ -14,7 +14,7 @@
 
 void syscall_close(struct intr_frame *f) {
 	// pop off first int argument from interrupt frame
-	int* file_descriptor = (void*)(*(int*)f->esp + 1);
+	int file_descriptor = *((int*)f->esp + 1);
 	// Get file associated with file_descriptor
 	struct file *file = get_associated_file_pointer(file_descriptor);
 	if (file == NULL) { // Checking if file is empty or non-existent

userprog/syscall_seek.c

@@ -13,9 +13,9 @@
 
 void syscall_seek(struct intr_frame *f) {
 	// pop off first int argument from interrupt frame
-	int* file_descriptor = (void*)*((int*)f->esp + 1);
+	int file_descriptor = *((int*)f->esp + 1);
 	// pop off second int argument from interrupt frame
-	int* file_pos = (void*)(*(int*)f->esp + 2);
+	unsigned file_pos = *((unsigned*)((int*)f->esp + 2));
 	struct file *file = get_associated_file_pointer(file_descriptor);
 	// Seek through the file
 	file_seek(file, file_pos);

userprog/system_calls.h

@@ -82,6 +82,12 @@ void syscall_tell(struct intr_frame *f);
 */
 void syscall_seek(struct intr_frame *f);
 
+/*
+ * Closes file descriptor fd. Exiting or terminating a process implicitly closes
+ * all its open file descriptors, as if by calling this function for each one.
+ */
+void syscall_close(struct intr_frame *f);
+
 /*
  * special additional stuff for handling file descriptors because they're annoying
  */