Implement syscall_read for console input w/ S.

src/userprog/syscall.c

@@ -1,5 +1,6 @@
 #include "userprog/syscall.h"
 #include "devices/shutdown.h"
+#include "devices/input.h"
 #include "threads/vaddr.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
@@ -150,10 +151,29 @@ syscall_filesize (int fd UNUSED)
 }
 
 static int
-syscall_read (int fd UNUSED, void *buffer UNUSED, unsigned size UNUSED)
+syscall_read (int fd, void *buffer, unsigned size)
 {
-  //TODO
-  return 0;
+  /* 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