diff --git a/kernel/context/context.rs b/kernel/context/context.rs
index 4df574f..02ddb0c 100644
--- a/kernel/context/context.rs
+++ b/kernel/context/context.rs
@@ -9,6 +9,10 @@ use context::memory::{Grant, Memory, SharedMemory, Tls};
 use syscall::data::Event;
 use sync::{WaitMap, WaitQueue};
 
+/// Unique identifier for a context (i.e. `pid`).
+use ::core::sync::atomic::AtomicUsize;
+int_like!(ContextId, AtomicContextId, usize, AtomicUsize);
+
 /// The status of a context - used for scheduling
 /// See syscall::process::waitpid and the sync module for examples of usage
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -22,9 +26,9 @@ pub enum Status {
 #[derive(Debug)]
 pub struct Context {
     /// The ID of this context
-    pub id: usize,
+    pub id: ContextId,
     /// The ID of the parent context
-    pub ppid: usize,
+    pub ppid: ContextId,
     /// The real user id
     pub ruid: u32,
     /// The real group id
@@ -42,7 +46,7 @@ pub struct Context {
     /// Context is halting parent
     pub vfork: bool,
     /// Context is being waited on
-    pub waitpid: Arc<WaitMap<usize, usize>>,
+    pub waitpid: Arc<WaitMap<ContextId, usize>>,
     /// Context should wake up at specified time
     pub wake: Option<(u64, u64)>,
     /// The architecture specific context
@@ -75,10 +79,10 @@ pub struct Context {
 
 impl Context {
     /// Create a new context
-    pub fn new(id: usize) -> Context {
+    pub fn new(id: ContextId) -> Context {
         Context {
             id: id,
-            ppid: 0,
+            ppid: ContextId::from(0),
             ruid: 0,
             rgid: 0,
             euid: 0,
diff --git a/kernel/context/event.rs b/kernel/context/event.rs
index b41a692..40f9efa 100644
--- a/kernel/context/event.rs
+++ b/kernel/context/event.rs
@@ -17,7 +17,7 @@ pub struct RegKey {
 
 #[derive(PartialEq, Eq, PartialOrd, Ord)]
 pub struct ProcessKey {
-    context_id: usize,
+    context_id: context::context::ContextId,
     fd: usize,
 }
 
diff --git a/kernel/context/list.rs b/kernel/context/list.rs
index 5541efa..11c25c2 100644
--- a/kernel/context/list.rs
+++ b/kernel/context/list.rs
@@ -7,11 +7,11 @@ use spin::RwLock;
 
 use arch;
 use syscall::error::{Result, Error, EAGAIN};
-use super::context::Context;
+use super::context::{Context, ContextId};
 
 /// Context list type
 pub struct ContextList {
-    map: BTreeMap<usize, Arc<RwLock<Context>>>,
+    map: BTreeMap<ContextId, Arc<RwLock<Context>>>,
     next_id: usize
 }
 
@@ -25,7 +25,7 @@ impl ContextList {
     }
 
     /// Get the nth context.
-    pub fn get(&self, id: usize) -> Option<&Arc<RwLock<Context>>> {
+    pub fn get(&self, id: ContextId) -> Option<&Arc<RwLock<Context>>> {
         self.map.get(&id)
     }
 
@@ -34,7 +34,7 @@ impl ContextList {
         self.map.get(&super::CONTEXT_ID.load(Ordering::SeqCst))
     }
 
-    pub fn iter(&self) -> ::collections::btree_map::Iter<usize, Arc<RwLock<Context>>> {
+    pub fn iter(&self) -> ::collections::btree_map::Iter<ContextId, Arc<RwLock<Context>>> {
         self.map.iter()
     }
 
@@ -44,7 +44,7 @@ impl ContextList {
             self.next_id = 1;
         }
 
-        while self.map.contains_key(&self.next_id) {
+        while self.map.contains_key(&ContextId::from(self.next_id)) {
             self.next_id += 1;
         }
 
@@ -52,7 +52,7 @@ impl ContextList {
             return Err(Error::new(EAGAIN));
         }
 
-        let id = self.next_id;
+        let id = ContextId::from(self.next_id);
         self.next_id += 1;
 
         assert!(self.map.insert(id, Arc::new(RwLock::new(Context::new(id)))).is_none());
@@ -85,7 +85,7 @@ impl ContextList {
         Ok(context_lock)
     }
 
-    pub fn remove(&mut self, id: usize) -> Option<Arc<RwLock<Context>>> {
+    pub fn remove(&mut self, id: ContextId) -> Option<Arc<RwLock<Context>>> {
         self.map.remove(&id)
     }
 }
diff --git a/kernel/context/mod.rs b/kernel/context/mod.rs
index d787eff..2b1214c 100644
--- a/kernel/context/mod.rs
+++ b/kernel/context/mod.rs
@@ -1,11 +1,12 @@
 //! Context management
 use alloc::boxed::Box;
-use core::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
+use core::sync::atomic::Ordering;
 use spin::{Once, RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 pub use self::context::{Context, Status};
 pub use self::list::ContextList;
 pub use self::switch::switch;
+pub use context::context::ContextId;
 
 /// Context struct
 mod context;
@@ -35,7 +36,7 @@ pub const CONTEXT_MAX_FILES: usize = 65536;
 static CONTEXTS: Once<RwLock<ContextList>> = Once::new();
 
 #[thread_local]
-static CONTEXT_ID: AtomicUsize = ATOMIC_USIZE_INIT;
+static CONTEXT_ID: context::AtomicContextId = context::AtomicContextId::default();
 
 pub fn init() {
     let mut contexts = contexts_mut();
@@ -69,6 +70,6 @@ pub fn contexts_mut() -> RwLockWriteGuard<'static, ContextList> {
     CONTEXTS.call_once(init_contexts).write()
 }
 
-pub fn context_id() -> usize {
+pub fn context_id() -> context::ContextId {
     CONTEXT_ID.load(Ordering::SeqCst)
 }
diff --git a/kernel/lib.rs b/kernel/lib.rs
index fadacba..db6a4aa 100644
--- a/kernel/lib.rs
+++ b/kernel/lib.rs
@@ -44,13 +44,13 @@ extern crate spin;
 
 use core::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
 
-/// Context management
-pub mod context;
-
 #[macro_use]
+#[macro_export]
 /// Shared data structures
 pub mod common;
 
+/// Context management
+pub mod context;
 
 /// ELF file parsing
 #[cfg(all(not(test), target_arch = "x86_64"))]
@@ -105,7 +105,7 @@ pub extern fn userspace_init() {
 /// Allow exception handlers to send signal to arch-independant kernel
 #[no_mangle]
 pub extern fn ksignal(signal: usize) {
-    println!("SIGNAL {}, CPU {}, PID {}", signal, cpu_id(), context::context_id());
+    println!("SIGNAL {}, CPU {}, PID {:?}", signal, cpu_id(), context::context_id());
     {
         let contexts = context::contexts();
         if let Some(context_lock) = contexts.current() {
diff --git a/kernel/scheme/sys/context.rs b/kernel/scheme/sys/context.rs
index 75b5e4f..3f179b3 100644
--- a/kernel/scheme/sys/context.rs
+++ b/kernel/scheme/sys/context.rs
@@ -83,8 +83,8 @@ pub fn resource() -> Result<Vec<u8>> {
             let name = str::from_utf8(&name_bytes).unwrap_or("");
 
             string.push_str(&format!("{:<6}{:<6}{:<6}{:<6}{:<6}{:<6}{:<8}{}\n",
-                               context.id,
-                               context.ppid,
+                               context.id.into(),
+                               context.ppid.into(),
                                context.euid,
                                context.egid,
                                stat_string,
diff --git a/kernel/scheme/user.rs b/kernel/scheme/user.rs
index c918d9e..3a8c4af 100644
--- a/kernel/scheme/user.rs
+++ b/kernel/scheme/user.rs
@@ -53,7 +53,7 @@ impl UserInner {
 
         self.call_inner(Packet {
             id: self.next_id.fetch_add(1, Ordering::SeqCst),
-            pid: pid,
+            pid: pid.into(),
             uid: uid,
             gid: gid,
             a: a,
@@ -297,7 +297,7 @@ impl Scheme for UserScheme {
 
         inner.call_inner(Packet {
             id: id,
-            pid: pid,
+            pid: pid.into(),
             uid: uid,
             gid: gid,
             a: SYS_FMAP,
diff --git a/kernel/syscall/fs.rs b/kernel/syscall/fs.rs
index 9aae871..d0ba198 100644
--- a/kernel/syscall/fs.rs
+++ b/kernel/syscall/fs.rs
@@ -25,7 +25,7 @@ pub fn file_op(a: usize, fd: usize, c: usize, d: usize) -> Result<usize> {
 
     let mut packet = Packet {
         id: 0,
-        pid: pid,
+        pid: pid.into(),
         uid: uid,
         gid: gid,
         a: a,
diff --git a/kernel/syscall/mod.rs b/kernel/syscall/mod.rs
index 952ccfa..2780655 100644
--- a/kernel/syscall/mod.rs
+++ b/kernel/syscall/mod.rs
@@ -14,6 +14,8 @@ use self::data::TimeSpec;
 use self::error::{Error, Result, ENOSYS};
 use self::number::*;
 
+use context::ContextId;
+
 /// Filesystem syscalls
 pub mod fs;
 
@@ -54,13 +56,13 @@ pub extern fn syscall(a: usize, b: usize, c: usize, d: usize, e: usize, f: usize
             },
             _ => match a {
                 SYS_EXIT => exit(b),
-                SYS_WAITPID => waitpid(b, c, d),
+                SYS_WAITPID => waitpid(ContextId::from(b), c, d).map(ContextId::into),
                 SYS_EXECVE => exec(validate_slice(b as *const u8, c)?, validate_slice(d as *const [usize; 2], e)?),
                 SYS_CHDIR => chdir(validate_slice(b as *const u8, c)?),
-                SYS_GETPID => getpid(),
+                SYS_GETPID => getpid().map(ContextId::into),
                 SYS_BRK => brk(b),
                 SYS_IOPL => iopl(b),
-                SYS_CLONE => clone(b, stack),
+                SYS_CLONE => clone(b, stack).map(ContextId::into),
                 SYS_YIELD => sched_yield(),
                 SYS_NANOSLEEP => nanosleep(validate_slice(b as *const TimeSpec, 1).map(|req| &req[0])?, validate_slice_mut(c as *mut TimeSpec, 1).ok().map(|rem| &mut rem[0])),
                 SYS_GETCWD => getcwd(validate_slice_mut(b as *mut u8, c)?),
diff --git a/kernel/syscall/process.rs b/kernel/syscall/process.rs
index 9a65f6b..34ab02e 100644
--- a/kernel/syscall/process.rs
+++ b/kernel/syscall/process.rs
@@ -13,6 +13,7 @@ use arch::paging::{ActivePageTable, InactivePageTable, Page, PhysicalAddress, Vi
 use arch::paging::temporary_page::TemporaryPage;
 use arch::start::usermode;
 use context;
+use context::ContextId;
 use context::memory::Grant;
 use elf::{self, program_header};
 use scheme;
@@ -54,7 +55,7 @@ pub fn brk(address: usize) -> Result<usize> {
     }
 }
 
-pub fn clone(flags: usize, stack_base: usize) -> Result<usize> {
+pub fn clone(flags: usize, stack_base: usize) -> Result<ContextId> {
     let ppid;
     let pid;
     {
@@ -665,10 +666,10 @@ pub fn exec(path: &[u8], arg_ptrs: &[[usize; 2]]) -> Result<usize> {
                     if let Some(context_lock) = contexts.get(ppid) {
                         let mut context = context_lock.write();
                         if ! context.unblock() {
-                            println!("{} not blocked for exec vfork unblock", ppid);
+                            println!("{:?} not blocked for exec vfork unblock", ppid);
                         }
                     } else {
-                        println!("{} not found for exec vfork unblock", ppid);
+                        println!("{:?} not found for exec vfork unblock", ppid);
                     }
                 }
             },
@@ -756,7 +757,7 @@ pub fn exit(status: usize) -> ! {
                     let mut parent = parent_lock.write();
                     if vfork {
                         if ! parent.unblock() {
-                            println!("{} not blocked for exit vfork unblock", ppid);
+                            println!("{:?} not blocked for exit vfork unblock", ppid);
                         }
                     }
                     parent.waitpid.clone()
@@ -767,7 +768,7 @@ pub fn exit(status: usize) -> ! {
                 }
                 waitpid.send(pid, status);
             } else {
-                println!("{} not found for exit vfork unblock", ppid);
+                println!("{:?} not found for exit vfork unblock", ppid);
             }
         }
     }
@@ -798,7 +799,7 @@ pub fn getgid() -> Result<usize> {
     Ok(context.rgid as usize)
 }
 
-pub fn getpid() -> Result<usize> {
+pub fn getpid() -> Result<ContextId> {
     let contexts = context::contexts();
     let context_lock = contexts.current().ok_or(Error::new(ESRCH))?;
     let context = context_lock.read();
@@ -817,7 +818,7 @@ pub fn iopl(_level: usize) -> Result<usize> {
     Ok(0)
 }
 
-pub fn kill(pid: usize, sig: usize) -> Result<usize> {
+pub fn kill(pid: ContextId, sig: usize) -> Result<usize> {
     use syscall::flag::*;
 
     let _context_lock = {
@@ -827,19 +828,19 @@ pub fn kill(pid: usize, sig: usize) -> Result<usize> {
     };
 
     let term = || {
-        println!("Terminate {}", pid);
+        println!("Terminate {:?}", pid);
     };
 
     let core = || {
-        println!("Core {}", pid);
+        println!("Core {:?}", pid);
     };
 
     let stop = || {
-        println!("Stop {}", pid);
+        println!("Stop {:?}", pid);
     };
 
     let cont = || {
-        println!("Continue {}", pid);
+        println!("Continue {:?}", pid);
     };
 
     match sig {
@@ -1001,7 +1002,7 @@ pub fn virttophys(virtual_address: usize) -> Result<usize> {
     }
 }
 
-fn reap(pid: usize) -> Result<usize> {
+fn reap(pid: ContextId) -> Result<ContextId> {
     // Spin until not running
     let mut running = false;
     while running {
@@ -1019,7 +1020,7 @@ fn reap(pid: usize) -> Result<usize> {
     contexts.remove(pid).ok_or(Error::new(ESRCH)).and(Ok(pid))
 }
 
-pub fn waitpid(pid: usize, status_ptr: usize, flags: usize) -> Result<usize> {
+pub fn waitpid(pid: ContextId, status_ptr: usize, flags: usize) -> Result<ContextId> {
     let waitpid = {
         let contexts = context::contexts();
         let context_lock = contexts.current().ok_or(Error::new(ESRCH))?;
@@ -1034,13 +1035,13 @@ pub fn waitpid(pid: usize, status_ptr: usize, flags: usize) -> Result<usize> {
         &mut tmp
     };
 
-    if pid == 0 {
+    if pid.into() == 0 {
         if flags & WNOHANG == WNOHANG {
             if let Some((w_pid, status)) = waitpid.receive_any_nonblock() {
                 status_slice[0] = status;
                 reap(w_pid)
             } else {
-                Ok(0)
+                Ok(ContextId::from(0))
             }
         } else {
             let (w_pid, status) = waitpid.receive_any();
@@ -1053,7 +1054,7 @@ pub fn waitpid(pid: usize, status_ptr: usize, flags: usize) -> Result<usize> {
                 status_slice[0] = status;
                 reap(pid)
             } else {
-                Ok(0)
+                Ok(ContextId::from(0))
             }
         } else {
             let status = waitpid.receive(&pid);