diff --git a/README b/README
index 5b711b1..ed8bba5 100644
--- a/README
+++ b/README
@@ -6,7 +6,7 @@ ACKNOWLEDGMENTS
 
 xv6 is inspired by John Lions's Commentary on UNIX 6th Edition (Peer
 to Peer Communications; ISBN: 1-57398-013-7; 1st edition (June 14,
-2000)).  See also https://pdos.csail.mit.edu/6.828/, which provides
+2000)).  See also https://pdos.csail.mit.edu/6.1810/, which provides
 pointers to on-line resources for v6.
 
 The following people have made contributions: Russ Cox (context switching,
@@ -14,29 +14,31 @@ locking), Cliff Frey (MP), Xiao Yu (MP), Nickolai Zeldovich, and Austin
 Clements.
 
 We are also grateful for the bug reports and patches contributed by
-Takahiro Aoyagi, Silas Boyd-Wickizer, Anton Burtsev, Ian Chen, Dan
-Cross, Cody Cutler, Mike CAT, Tej Chajed, Asami Doi, eyalz800, Nelson
-Elhage, Saar Ettinger, Alice Ferrazzi, Nathaniel Filardo, flespark,
-Peter Froehlich, Yakir Goaron, Shivam Handa, Matt Harvey, Bryan Henry,
-jaichenhengjie, Jim Huang, Matúš Jókay, Alexander Kapshuk, Anders
-Kaseorg, kehao95, Wolfgang Keller, Jungwoo Kim, Jonathan Kimmitt,
-Eddie Kohler, Vadim Kolontsov, Austin Liew, l0stman, Pavan
-Maddamsetti, Imbar Marinescu, Yandong Mao, Matan Shabtay, Hitoshi
-Mitake, Carmi Merimovich, Mark Morrissey, mtasm, Joel Nider,
-OptimisticSide, Greg Price, Jude Rich, Ayan Shafqat, Eldar Sehayek,
-Yongming Shen, Fumiya Shigemitsu, Cam Tenny, tyfkda, Warren Toomey,
-Stephen Tu, Rafael Ubal, Amane Uehara, Pablo Ventura, Xi Wang, Keiichi
-Watanabe, Nicolas Wolovick, wxdao, Grant Wu, Jindong Zhang, Icenowy
-Zheng, ZhUyU1997, and Zou Chang Wei.
+Takahiro Aoyagi, Silas Boyd-Wickizer, Anton Burtsev, carlclone, Ian
+Chen, Dan Cross, Cody Cutler, Mike CAT, Tej Chajed, Asami Doi,
+eyalz800, Nelson Elhage, Saar Ettinger, Alice Ferrazzi, Nathaniel
+Filardo, flespark, Peter Froehlich, Yakir Goaron, Shivam Handa, Matt
+Harvey, Bryan Henry, jaichenhengjie, Jim Huang, Matúš Jókay, John
+Jolly, Alexander Kapshuk, Anders Kaseorg, kehao95, Wolfgang Keller,
+Jungwoo Kim, Jonathan Kimmitt, Eddie Kohler, Vadim Kolontsov, Austin
+Liew, l0stman, Pavan Maddamsetti, Imbar Marinescu, Yandong Mao, Matan
+Shabtay, Hitoshi Mitake, Carmi Merimovich, Mark Morrissey, mtasm, Joel
+Nider, Hayato Ohhashi, OptimisticSide, Harry Porter, Greg Price, Jude
+Rich, segfault, Ayan Shafqat, Eldar Sehayek, Yongming Shen, Fumiya
+Shigemitsu, Cam Tenny, tyfkda, Warren Toomey, Stephen Tu, Rafael Ubal,
+Amane Uehara, Pablo Ventura, Xi Wang, WaheedHafez, Keiichi Watanabe,
+Nicolas Wolovick, wxdao, Grant Wu, Jindong Zhang, Icenowy Zheng,
+ZhUyU1997, and Zou Chang Wei.
+
 
 The code in the files that constitute xv6 is
-Copyright 2006-2020 Frans Kaashoek, Robert Morris, and Russ Cox.
+Copyright 2006-2022 Frans Kaashoek, Robert Morris, and Russ Cox.
 
 ERROR REPORTS
 
 Please send errors and suggestions to Frans Kaashoek and Robert Morris
 (kaashoek,rtm@mit.edu).  The main purpose of xv6 is as a teaching
-operating system for MIT's 6.S081, so we are more interested in
+operating system for MIT's 6.1810, so we are more interested in
 simplifications and clarifications than new features.
 
 BUILDING AND RUNNING XV6
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index 1de184e..3b4d5bd 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -55,6 +55,8 @@ sys_sleep(void)
   uint ticks0;
 
   argint(0, &n);
+  if(n < 0)
+    n = 0;
   acquire(&tickslock);
   ticks0 = ticks;
   while(ticks - ticks0 < n){
diff --git a/user/cat.c b/user/cat.c
index 598f005..520dc8b 100644
--- a/user/cat.c
+++ b/user/cat.c
@@ -1,5 +1,6 @@
 #include "kernel/types.h"
 #include "kernel/stat.h"
+#include "kernel/fcntl.h"
 #include "user/user.h"
 
 char buf[512];
@@ -32,7 +33,7 @@ main(int argc, char *argv[])
   }
 
   for(i = 1; i < argc; i++){
-    if((fd = open(argv[i], 0)) < 0){
+    if((fd = open(argv[i], O_RDONLY)) < 0){
       fprintf(2, "cat: cannot open %s\n", argv[i]);
       exit(1);
     }
diff --git a/user/grep.c b/user/grep.c
index 2315a0c..6c33766 100644
--- a/user/grep.c
+++ b/user/grep.c
@@ -2,6 +2,7 @@
 
 #include "kernel/types.h"
 #include "kernel/stat.h"
+#include "kernel/fcntl.h"
 #include "user/user.h"
 
 char buf[1024];
@@ -51,7 +52,7 @@ main(int argc, char *argv[])
   }
 
   for(i = 2; i < argc; i++){
-    if((fd = open(argv[i], 0)) < 0){
+    if((fd = open(argv[i], O_RDONLY)) < 0){
       printf("grep: cannot open %s\n", argv[i]);
       exit(1);
     }
diff --git a/user/ls.c b/user/ls.c
index c67b84b..b32c200 100644
--- a/user/ls.c
+++ b/user/ls.c
@@ -2,6 +2,7 @@
 #include "kernel/stat.h"
 #include "user/user.h"
 #include "kernel/fs.h"
+#include "kernel/fcntl.h"
 
 char*
 fmtname(char *path)
@@ -30,7 +31,7 @@ ls(char *path)
   struct dirent de;
   struct stat st;
 
-  if((fd = open(path, 0)) < 0){
+  if((fd = open(path, O_RDONLY)) < 0){
     fprintf(2, "ls: cannot open %s\n", path);
     return;
   }
diff --git a/user/wc.c b/user/wc.c
index 6a851ca..d8f3b2a 100644
--- a/user/wc.c
+++ b/user/wc.c
@@ -1,5 +1,6 @@
 #include "kernel/types.h"
 #include "kernel/stat.h"
+#include "kernel/fcntl.h"
 #include "user/user.h"
 
 char buf[512];
@@ -43,7 +44,7 @@ main(int argc, char *argv[])
   }
 
   for(i = 1; i < argc; i++){
-    if((fd = open(argv[i], 0)) < 0){
+    if((fd = open(argv[i], O_RDONLY)) < 0){
       printf("wc: cannot open %s\n", argv[i]);
       exit(1);
     }