Make explicit that open doesn't return fd 0 or 1.

Make independence of fds across processes explicit.
Thanks to Ryan Seth Propper <rpropper@stanford.edu> for question.

diff --git a/doc/userprog.texi b/doc/userprog.texi
index ff98074f8de198ea6465271848f573e7d5c73d7d..59b198abdda810745fd6c5d950a5ceb8723c6378 100644 (file)
--- a/doc/userprog.texi
+++ b/doc/userprog.texi
@@ -661,8 +661,12 @@ when the process exits or is terminated.
 
 File descriptors numbered 0 and 1 are reserved for the console: fd 0
 is standard input (@code{stdin}), fd 1 is standard output
-(@code{stdout}).  These special file descriptors are valid as system
-call arguments only as explicitly described below.
+(@code{stdout}).  The @code{open} system call will never return either
+of these file descriptors, which are valid as system call arguments only
+as explicitly described below.
+
+Each process has an independent set of file descriptors.  File
+descriptors are not inherited by child processes.
 
 Consider implementing this function in terms of @func{filesys_open}.
 @end deftypefn