diff --git a/kernel/console.c b/kernel/console.c
index 39415d6..05dc526 100644
--- a/kernel/console.c
+++ b/kernel/console.c
@@ -89,7 +89,7 @@ consoleread(int user_dst, uint64 dst, int n)
     // wait until interrupt handler has put some
     // input into cons.buffer.
     while(cons.r == cons.w){
-      if(myproc()->killed){
+      if(killed(myproc())){
         release(&cons.lock);
         return -1;
       }
diff --git a/kernel/defs.h b/kernel/defs.h
index 5cf2de2..95fb94b 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -90,6 +90,8 @@ void            proc_mapstacks(pagetable_t);
 pagetable_t     proc_pagetable(struct proc *);
 void            proc_freepagetable(pagetable_t, uint64);
 int             kill(int);
+int             killed(struct proc*);
+void            setkilled(struct proc*);
 struct cpu*     mycpu(void);
 struct cpu*     getmycpu(void);
 struct proc*    myproc();
@@ -132,9 +134,9 @@ int             strncmp(const char*, const char*, uint);
 char*           strncpy(char*, const char*, int);
 
 // syscall.c
-int             argint(int, int*);
+void            argint(int, int*);
 int             argstr(int, char*, int);
-int             argaddr(int, uint64 *);
+void             argaddr(int, uint64 *);
 int             fetchstr(uint64, char*, int);
 int             fetchaddr(uint64, uint64*);
 void            syscall();
diff --git a/kernel/fs.c b/kernel/fs.c
index 247a86f..b220491 100644
--- a/kernel/fs.c
+++ b/kernel/fs.c
@@ -573,6 +573,7 @@ dirlookup(struct inode *dp, char *name, uint *poff)
 }
 
 // Write a new directory entry (name, inum) into the directory dp.
+// Returns 0 on success, -1 on failure (e.g. out of disk blocks).
 int
 dirlink(struct inode *dp, char *name, uint inum)
 {
@@ -597,7 +598,7 @@ dirlink(struct inode *dp, char *name, uint inum)
   strncpy(de.name, name, DIRSIZ);
   de.inum = inum;
   if(writei(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
-    panic("dirlink");
+    return -1;
 
   return 0;
 }
diff --git a/kernel/pipe.c b/kernel/pipe.c
index b6eefb9..f6b501a 100644
--- a/kernel/pipe.c
+++ b/kernel/pipe.c
@@ -81,7 +81,7 @@ pipewrite(struct pipe *pi, uint64 addr, int n)
 
   acquire(&pi->lock);
   while(i < n){
-    if(pi->readopen == 0 || pr->killed){
+    if(pi->readopen == 0 || killed(pr)){
       release(&pi->lock);
       return -1;
     }
@@ -111,7 +111,7 @@ piperead(struct pipe *pi, uint64 addr, int n)
 
   acquire(&pi->lock);
   while(pi->nread == pi->nwrite && pi->writeopen){  //DOC: pipe-empty
-    if(pr->killed){
+    if(killed(pr)){
       release(&pi->lock);
       return -1;
     }
diff --git a/kernel/proc.c b/kernel/proc.c
index 0fa6a2c..959b778 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -259,7 +259,7 @@ userinit(void)
 int
 growproc(int n)
 {
-  uint sz;
+  uint64 sz;
   struct proc *p = myproc();
 
   sz = p->sz;
@@ -390,7 +390,7 @@ exit(int status)
 int
 wait(uint64 addr)
 {
-  struct proc *np;
+  struct proc *pp;
   int havekids, pid;
   struct proc *p = myproc();
 
@@ -399,32 +399,32 @@ wait(uint64 addr)
   for(;;){
     // Scan through table looking for exited children.
     havekids = 0;
-    for(np = proc; np < &proc[NPROC]; np++){
-      if(np->parent == p){
+    for(pp = proc; pp < &proc[NPROC]; pp++){
+      if(pp->parent == p){
         // make sure the child isn't still in exit() or swtch().
-        acquire(&np->lock);
+        acquire(&pp->lock);
 
         havekids = 1;
-        if(np->state == ZOMBIE){
+        if(pp->state == ZOMBIE){
           // Found one.
-          pid = np->pid;
-          if(addr != 0 && copyout(p->pagetable, addr, (char *)&np->xstate,
-                                  sizeof(np->xstate)) < 0) {
-            release(&np->lock);
+          pid = pp->pid;
+          if(addr != 0 && copyout(p->pagetable, addr, (char *)&pp->xstate,
+                                  sizeof(pp->xstate)) < 0) {
+            release(&pp->lock);
             release(&wait_lock);
             return -1;
           }
-          freeproc(np);
-          release(&np->lock);
+          freeproc(pp);
+          release(&pp->lock);
           release(&wait_lock);
           return pid;
         }
-        release(&np->lock);
+        release(&pp->lock);
       }
     }
 
     // No point waiting if we don't have any children.
-    if(!havekids || p->killed){
+    if(!havekids || killed(p)){
       release(&wait_lock);
       return -1;
     }
@@ -603,6 +603,25 @@ kill(int pid)
   return -1;
 }
 
+void
+setkilled(struct proc *p)
+{
+  acquire(&p->lock);
+  p->killed = 1;
+  release(&p->lock);
+}
+
+int
+killed(struct proc *p)
+{
+  int k;
+  
+  acquire(&p->lock);
+  k = p->killed;
+  release(&p->lock);
+  return k;
+}
+
 // Copy to either a user address, or kernel address,
 // depending on usr_dst.
 // Returns 0 on success, -1 on error.
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 95b9f70..ee94696 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -12,7 +12,7 @@ int
 fetchaddr(uint64 addr, uint64 *ip)
 {
   struct proc *p = myproc();
-  if(addr >= p->sz || addr+sizeof(uint64) > p->sz)
+  if(addr >= p->sz || addr+sizeof(uint64) > p->sz) // both tests needed, in case of overflow
     return -1;
   if(copyin(p->pagetable, (char *)ip, addr, sizeof(*ip)) != 0)
     return -1;
@@ -25,9 +25,8 @@ int
 fetchstr(uint64 addr, char *buf, int max)
 {
   struct proc *p = myproc();
-  int err = copyinstr(p->pagetable, buf, addr, max);
-  if(err < 0)
-    return err;
+  if(copyinstr(p->pagetable, buf, addr, max) < 0)
+    return -1;
   return strlen(buf);
 }
 
@@ -54,21 +53,19 @@ argraw(int n)
 }
 
 // Fetch the nth 32-bit system call argument.
-int
+void
 argint(int n, int *ip)
 {
   *ip = argraw(n);
-  return 0;
 }
 
 // Retrieve an argument as a pointer.
 // Doesn't check for legality, since
 // copyin/copyout will do that.
-int
+void
 argaddr(int n, uint64 *ip)
 {
   *ip = argraw(n);
-  return 0;
 }
 
 // Fetch the nth word-sized system call argument as a null-terminated string.
@@ -78,8 +75,7 @@ int
 argstr(int n, char *buf, int max)
 {
   uint64 addr;
-  if(argaddr(n, &addr) < 0)
-    return -1;
+  argaddr(n, &addr);
   return fetchstr(addr, buf, max);
 }
 
diff --git a/kernel/sysfile.c b/kernel/sysfile.c
index 5dc453b..d8a6fca 100644
--- a/kernel/sysfile.c
+++ b/kernel/sysfile.c
@@ -24,8 +24,7 @@ argfd(int n, int *pfd, struct file **pf)
   int fd;
   struct file *f;
 
-  if(argint(n, &fd) < 0)
-    return -1;
+  argint(n, &fd);
   if(fd < 0 || fd >= NOFILE || (f=myproc()->ofile[fd]) == 0)
     return -1;
   if(pfd)
@@ -73,7 +72,9 @@ sys_read(void)
   int n;
   uint64 p;
 
-  if(argfd(0, 0, &f) < 0 || argint(2, &n) < 0 || argaddr(1, &p) < 0)
+  argaddr(1, &p);
+  argint(2, &n);
+  if(argfd(0, 0, &f) < 0)
     return -1;
   return fileread(f, p, n);
 }
@@ -84,8 +85,10 @@ sys_write(void)
   struct file *f;
   int n;
   uint64 p;
-
-  if(argfd(0, 0, &f) < 0 || argint(2, &n) < 0 || argaddr(1, &p) < 0)
+  
+  argaddr(1, &p);
+  argint(2, &n);
+  if(argfd(0, 0, &f) < 0)
     return -1;
 
   return filewrite(f, p, n);
@@ -110,7 +113,8 @@ sys_fstat(void)
   struct file *f;
   uint64 st; // user pointer to struct stat
 
-  if(argfd(0, 0, &f) < 0 || argaddr(1, &st) < 0)
+  argaddr(1, &st);
+  if(argfd(0, 0, &f) < 0)
     return -1;
   return filestat(f, st);
 }
@@ -268,19 +272,31 @@ create(char *path, short type, short major, short minor)
   iupdate(ip);
 
   if(type == T_DIR){  // Create . and .. entries.
-    dp->nlink++;  // for ".."
-    iupdate(dp);
     // No ip->nlink++ for ".": avoid cyclic ref count.
     if(dirlink(ip, ".", ip->inum) < 0 || dirlink(ip, "..", dp->inum) < 0)
-      panic("create dots");
+      goto fail;
   }
 
   if(dirlink(dp, name, ip->inum) < 0)
-    panic("create: dirlink");
+    goto fail;
+
+  if(type == T_DIR){
+    // now that success is guaranteed:
+    dp->nlink++;  // for ".."
+    iupdate(dp);
+  }
 
   iunlockput(dp);
 
   return ip;
+
+ fail:
+  // something went wrong. de-allocate ip.
+  ip->nlink = 0;
+  iupdate(ip);
+  iunlockput(ip);
+  iunlockput(dp);
+  return 0;
 }
 
 uint64
@@ -292,7 +308,8 @@ sys_open(void)
   struct inode *ip;
   int n;
 
-  if((n = argstr(0, path, MAXPATH)) < 0 || argint(1, &omode) < 0)
+  argint(1, &omode);
+  if((n = argstr(0, path, MAXPATH)) < 0)
     return -1;
 
   begin_op();
@@ -375,9 +392,9 @@ sys_mknod(void)
   int major, minor;
 
   begin_op();
+  argint(1, &major);
+  argint(2, &minor);
   if((argstr(0, path, MAXPATH)) < 0 ||
-     argint(1, &major) < 0 ||
-     argint(2, &minor) < 0 ||
      (ip = create(path, T_DEVICE, major, minor)) == 0){
     end_op();
     return -1;
@@ -419,7 +436,8 @@ sys_exec(void)
   int i;
   uint64 uargv, uarg;
 
-  if(argstr(0, path, MAXPATH) < 0 || argaddr(1, &uargv) < 0){
+  argaddr(1, &uargv);
+  if(argstr(0, path, MAXPATH) < 0) {
     return -1;
   }
   memset(argv, 0, sizeof(argv));
@@ -462,8 +480,7 @@ sys_pipe(void)
   int fd0, fd1;
   struct proc *p = myproc();
 
-  if(argaddr(0, &fdarray) < 0)
-    return -1;
+  argaddr(0, &fdarray);
   if(pipealloc(&rf, &wf) < 0)
     return -1;
   fd0 = -1;
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index 8ca45ba..1de184e 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -10,8 +10,7 @@ uint64
 sys_exit(void)
 {
   int n;
-  if(argint(0, &n) < 0)
-    return -1;
+  argint(0, &n);
   exit(n);
   return 0;  // not reached
 }
@@ -32,19 +31,17 @@ uint64
 sys_wait(void)
 {
   uint64 p;
-  if(argaddr(0, &p) < 0)
-    return -1;
+  argaddr(0, &p);
   return wait(p);
 }
 
 uint64
 sys_sbrk(void)
 {
-  int addr;
+  uint64 addr;
   int n;
 
-  if(argint(0, &n) < 0)
-    return -1;
+  argint(0, &n);
   addr = myproc()->sz;
   if(growproc(n) < 0)
     return -1;
@@ -57,12 +54,11 @@ sys_sleep(void)
   int n;
   uint ticks0;
 
-  if(argint(0, &n) < 0)
-    return -1;
+  argint(0, &n);
   acquire(&tickslock);
   ticks0 = ticks;
   while(ticks - ticks0 < n){
-    if(myproc()->killed){
+    if(killed(myproc())){
       release(&tickslock);
       return -1;
     }
@@ -77,8 +73,7 @@ sys_kill(void)
 {
   int pid;
 
-  if(argint(0, &pid) < 0)
-    return -1;
+  argint(0, &pid);
   return kill(pid);
 }
 
diff --git a/kernel/trampoline.S b/kernel/trampoline.S
index 2ce4886..0aaa413 100644
--- a/kernel/trampoline.S
+++ b/kernel/trampoline.S
@@ -1,18 +1,19 @@
-	#
-        # code to switch between user and kernel space.
         #
-        # this code is mapped at the same virtual address
-        # (TRAMPOLINE) in user and kernel space so that
-        # it continues to work when it switches page tables.
-	#
-	# kernel.ld causes this to be aligned
-        # to a page boundary.
+        # low-level code to handle traps from user space into
+        # the kernel, and returns from kernel to user.
+        #
+        # the kernel maps the page holding this code
+        # at the same virtual address (TRAMPOLINE)
+        # in user and kernel space so that it continues
+        # to work when it switches page tables.
+        # kernel.ld causes this code to start at 
+        # a page boundary.
         #
 
 #include "riscv.h"
 #include "memlayout.h"
 
-	.section trampsec
+.section trampsec
 .globl trampoline
 trampoline:
 .align 4
@@ -31,7 +32,7 @@ uservec:
 
         # each process has a separate p->trapframe memory area,
         # but it's mapped to the same virtual address
-        # (TRAPFRAME) in every process.
+        # (TRAPFRAME) in every process's user page table.
         li a0, TRAPFRAME
         
         # save the user registers in TRAPFRAME
@@ -70,29 +71,27 @@ uservec:
         csrr t0, sscratch
         sd t0, 112(a0)
 
-        # restore kernel stack pointer from p->trapframe->kernel_sp
+        # initialize kernel stack pointer, from p->trapframe->kernel_sp
         ld sp, 8(a0)
 
         # make tp hold the current hartid, from p->trapframe->kernel_hartid
         ld tp, 32(a0)
 
-        # load the address of usertrap(), p->trapframe->kernel_trap
+        # load the address of usertrap(), from p->trapframe->kernel_trap
         ld t0, 16(a0)
 
-        # restore kernel page table from p->trapframe->kernel_satp
+        # load the kernel page table, from p->trapframe->kernel_satp
         ld t1, 0(a0)
         csrw satp, t1
         sfence.vma zero, zero
 
-        # a0 is no longer valid, since the kernel page
-        # table does not specially map p->tf.
-
         # jump to usertrap(), which does not return
         jr t0
 
 .globl userret
 userret:
         # userret(pagetable)
+        # called by usertrapret() in trap.c to
         # switch from kernel to user.
         # a0: user page table, for satp.
 
diff --git a/kernel/trap.c b/kernel/trap.c
index 75fb3ec..512c850 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -53,15 +53,15 @@ usertrap(void)
   if(r_scause() == 8){
     // system call
 
-    if(p->killed)
+    if(killed(p))
       exit(-1);
 
     // sepc points to the ecall instruction,
     // but we want to return to the next instruction.
     p->trapframe->epc += 4;
 
-    // an interrupt will change sstatus &c registers,
-    // so don't enable until done with those registers.
+    // an interrupt will change sepc, scause, and sstatus,
+    // so enable only now that we're done with those registers.
     intr_on();
 
     syscall();
@@ -70,10 +70,10 @@ usertrap(void)
   } else {
     printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
     printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
-    p->killed = 1;
+    setkilled(p);
   }
 
-  if(p->killed)
+  if(killed(p))
     exit(-1);
 
   // give up the CPU if this is a timer interrupt.
@@ -101,7 +101,7 @@ usertrapret(void)
   w_stvec(trampoline_uservec);
 
   // set up trapframe values that uservec will need when
-  // the process next re-enters the kernel.
+  // the process next traps into the kernel.
   p->trapframe->kernel_satp = r_satp();         // kernel page table
   p->trapframe->kernel_sp = p->kstack + PGSIZE; // process's kernel stack
   p->trapframe->kernel_trap = (uint64)usertrap;
diff --git a/kernel/uart.c b/kernel/uart.c
index af571b1..e3b3b8a 100644
--- a/kernel/uart.c
+++ b/kernel/uart.c
@@ -92,22 +92,18 @@ uartputc(int c)
     for(;;)
       ;
   }
-
-  while(1){
-    if(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
-      // buffer is full.
-      // wait for uartstart() to open up space in the buffer.
-      sleep(&uart_tx_r, &uart_tx_lock);
-    } else {
-      uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
-      uart_tx_w += 1;
-      uartstart();
-      release(&uart_tx_lock);
-      return;
-    }
+  while(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
+    // buffer is full.
+    // wait for uartstart() to open up space in the buffer.
+    sleep(&uart_tx_r, &uart_tx_lock);
   }
+  uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
+  uart_tx_w += 1;
+  uartstart();
+  release(&uart_tx_lock);
 }
 
+
 // alternate version of uartputc() that doesn't 
 // use interrupts, for use by kernel printf() and
 // to echo characters. it spins waiting for the uart's
diff --git a/user/grep.c b/user/grep.c
index 19882b9..2315a0c 100644
--- a/user/grep.c
+++ b/user/grep.c
@@ -62,7 +62,8 @@ main(int argc, char *argv[])
 }
 
 // Regexp matcher from Kernighan & Pike,
-// The Practice of Programming, Chapter 9.
+// The Practice of Programming, Chapter 9, or
+// https://www.cs.princeton.edu/courses/archive/spr09/cos333/beautiful.html
 
 int matchhere(char*, char*);
 int matchstar(int, char*, char*);
diff --git a/user/user.h b/user/user.h
index 8ac6395..4d398d5 100644
--- a/user/user.h
+++ b/user/user.h
@@ -9,7 +9,7 @@ int write(int, const void*, int);
 int read(int, void*, int);
 int close(int);
 int kill(int);
-int exec(char*, char**);
+int exec(const char*, char**);
 int open(const char*, int);
 int mknod(const char*, short, short);
 int unlink(const char*);
diff --git a/user/usertests.c b/user/usertests.c
index 0a84ef9..7c31013 100644
--- a/user/usertests.c
+++ b/user/usertests.c
@@ -2737,6 +2737,8 @@ diskfull(char *s)
 {
   int fi;
   int done = 0;
+
+  unlink("diskfulldir");
   
   for(fi = 0; done == 0; fi++){
     char name[32];
@@ -2763,6 +2765,39 @@ diskfull(char *s)
     close(fd);
   }
 
+  // now that there are no free blocks, test that dirlink()
+  // merely fails (doesn't panic) if it can't extend
+  // directory content.
+  int nzz = 128;
+  for(int i = 0; i < nzz; i++){
+    char name[32];
+    name[0] = 'z';
+    name[1] = 'z';
+    name[2] = '0' + (i / 32);
+    name[3] = '0' + (i % 32);
+    name[4] = '\0';
+    unlink(name);
+    int fd = open(name, O_CREATE|O_RDWR|O_TRUNC);
+    if(fd < 0){
+      printf("%s: could not create file %s\n", s, name);
+      break;
+    }
+    close(fd);
+  }
+
+  mkdir("diskfulldir");
+  unlink("diskfulldir");
+
+  for(int i = 0; i < nzz; i++){
+    char name[32];
+    name[0] = 'z';
+    name[1] = 'z';
+    name[2] = '0' + (i / 32);
+    name[3] = '0' + (i % 32);
+    name[4] = '\0';
+    unlink(name);
+  }
+
   for(int i = 0; i < fi; i++){
     char name[32];
     name[0] = 'b';