Implement tell()

Makefile.build

@@ -73,6 +73,7 @@ userprog_SRC += userprog/syscall_write.c
 userprog_SRC += userprog/file_descriptors_map.c
 userprog_SRC += userprog/syscall_read.c
 userprog_SRC += userprog/syscall_filesize.c
+userprog_SRC += userprog/syscall_tell.c
 
 # No virtual memory code yet.
 #vm_SRC = vm/file.c			# Some file.

userprog/syscall_tell.c

+/*
+ * Returns the current offset into the file that the given file descriptor is at
+ *
+ * Authored by Joshua Saxby
+ */
+#include <stddef.h>
+#include "system_calls.h"
+#include "filesys/file.h"
+
+void syscall_tell(struct intr_frame *f) {
+	// pop off first int argument from interrupt frame
+	int file_descriptor = *((int*)f->esp + 1);
+	struct file *file = get_associated_file_pointer(file_descriptor);
+    /*
+     * tell() return type is unsigned so we can't return a special error code if
+     * file descriptor passed is invalid. Instead, we return 0 silently.
+     */
+	f->eax = (file != NULL) ? file_tell(file) : 0;
+}

userprog/system_calls.h

@@ -69,6 +69,12 @@ void syscall_filesize(struct intr_frame *f);
  */
 void syscall_remove(struct intr_frame *f);
 
+/*
+ * Returns the position of the next byte to be read or written in open file fd,
+ * expressed in bytes from the beginning of the file. 
+ */
+void syscall_tell(struct intr_frame *f);
+
 /*
  * special additional stuff for handling file descriptors because they're annoying
  */