Added dispatch_once and a private Once struct.

Author: SSheldon

src/ffi.rs

@@ -7,7 +7,7 @@ pub type dispatch_function_t = extern fn(*mut c_void);
 // dispatch_semaphore_t
 pub type dispatch_group_t = *mut ();
 pub type dispatch_object_t = *mut ();
-// dispatch_once_t
+pub type dispatch_once_t = c_long;
 pub type dispatch_queue_t = *mut ();
 pub type dispatch_time_t = u64;
 // dispatch_source_type_t
@@ -42,7 +42,7 @@ extern {
     // void dispatch_apply ( size_t iterations, dispatch_queue_t queue, void (^block)(size_t) );
     pub fn dispatch_apply_f(iterations: size_t, queue: dispatch_queue_t, context: *mut c_void, work: extern fn(*mut c_void, size_t));
     // void dispatch_once ( dispatch_once_t *predicate, dispatch_block_t block );
-    // void dispatch_once_f ( dispatch_once_t *predicate, void *context, dispatch_function_t function );
+    pub fn dispatch_once_f(predicate: *mut dispatch_once_t, context: *mut c_void, function: dispatch_function_t);
 
     // void dispatch_group_async ( dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block );
     pub fn dispatch_group_async_f(group: dispatch_group_t, queue: dispatch_queue_t, context: *mut c_void, work: dispatch_function_t);

src/lib.rs

@@ -44,6 +44,7 @@ assert!(nums[0] == "2");
 
 extern crate libc;
 
+use std::cell::UnsafeCell;
 use std::ffi::{CStr, CString};
 use std::mem;
 use std::ptr;
@@ -450,6 +451,33 @@ impl Drop for GroupGuard {
     }
 }
 
+#[allow(dead_code)]
+struct Once {
+    predicate: UnsafeCell<dispatch_once_t>,
+}
+
+#[allow(dead_code)]
+impl Once {
+    // TODO: make this a const fn when the feature is stable
+    pub fn new() -> Once {
+        Once { predicate: UnsafeCell::new(0) }
+    }
+
+    #[inline(always)]
+    pub fn call_once<F>(&'static self, work: F) where F: FnOnce() {
+        let predicate = unsafe { *self.predicate.get() };
+        if predicate != !0 {
+            let mut work = Some(work);
+            let (context, work) = context_and_sync_function(&mut work);
+            unsafe {
+                dispatch_once_f(self.predicate.get(), context, work);
+            }
+        }
+    }
+}
+
+unsafe impl Sync for Once { }
+
 #[cfg(test)]
 mod tests {
     use std::sync::{Arc, Mutex};