diff --git a/src/libstd/sys/unix/sync.rs b/src/libstd/sys/unix/sync.rs
index aabeb41a369d2..954bfbb6b18b7 100644
--- a/src/libstd/sys/unix/sync.rs
+++ b/src/libstd/sys/unix/sync.rs
@@ -254,53 +254,61 @@ mod os {
 mod os {
     use libc;
 
-    // size of the type minus width of the magic int field
+    // size of the type minus width of the magic and alignment field
     #[cfg(target_arch = "x86_64")]
-    const __PTHREAD_MUTEX_SIZE__: usize = 48 - 4;
+    const __PTHREAD_MUTEX_SIZE__: usize = 48 - 4 - 8;
 
     #[cfg(target_arch = "x86_64")]
-    const __PTHREAD_COND_SIZE__: usize = 40 - 4;
+    const __PTHREAD_MUTEXATTR_SIZE__: usize = 16 - 8; // no magic field
 
     #[cfg(target_arch = "x86_64")]
-    const __PTHREAD_RWLOCK_SIZE__: usize = 64 - 4;
+    const __PTHREAD_COND_SIZE__: usize = 40 - 4 - 8;
+
+    #[cfg(target_arch = "x86_64")]
+    const __PTHREAD_RWLOCK_SIZE__: usize = 64 - 4 - 8;
 
     const _PTHREAD_MUTEX_MAGIC_INIT: libc::c_uint = 0x33330003;
     const _PTHREAD_COND_MAGIC_INIT: libc::c_uint = 0x55550005;
     const _PTHREAD_RWLOCK_MAGIC_INIT: libc::c_uint = 0x99990009;
 
-    // note the actual structs are smaller
-
-    #[repr(C, packed)]
+    #[repr(C)]
     pub struct pthread_mutex_t {
         __magic: libc::c_uint,
         __opaque: [u8; __PTHREAD_MUTEX_SIZE__],
+        __align: libc::c_longlong,
     }
-    #[repr(C, packed)]
+    #[repr(C)]
     pub struct pthread_mutexattr_t {
-        __opaque: [u8; 16],
+        __opaque: [u8; __PTHREAD_MUTEXATTR_SIZE__],
+        __align: libc::c_longlong,
     }
-    #[repr(C, packed)]
+    #[repr(C)]
     pub struct pthread_cond_t {
         __magic: libc::c_uint,
         __opaque: [u8; __PTHREAD_COND_SIZE__],
+        __align: libc::c_longlong,
     }
-    #[repr(C, packed)]
+    #[repr(C)]
     pub struct pthread_rwlock_t {
         __magic: libc::c_uint,
         __opaque: [u8; __PTHREAD_RWLOCK_SIZE__],
+        __align: libc::c_longlong,
     }
 
     pub const PTHREAD_MUTEX_INITIALIZER: pthread_mutex_t = pthread_mutex_t {
         __magic: _PTHREAD_MUTEX_MAGIC_INIT,
         __opaque: [0; __PTHREAD_MUTEX_SIZE__],
+        __align: 0,
     };
     pub const PTHREAD_COND_INITIALIZER: pthread_cond_t = pthread_cond_t {
         __magic: _PTHREAD_COND_MAGIC_INIT,
         __opaque: [0; __PTHREAD_COND_SIZE__],
+        __align: 0,
     };
     pub const PTHREAD_RWLOCK_INITIALIZER: pthread_rwlock_t = pthread_rwlock_t {
         __magic: _PTHREAD_RWLOCK_MAGIC_INIT,
         __opaque: [0; __PTHREAD_RWLOCK_SIZE__],
+        __align: 0,
     };
 
     pub const PTHREAD_MUTEX_RECURSIVE: libc::c_int = 2;