Link dispatch instead of System outside of OSX/iOS.

Enable travis tests for linux to verify.
This fixes #4.

.travis.yml

@@ -2,7 +2,9 @@ language: rust
 rust:
   - stable
   - nightly
-os: osx
+os:
+  - osx
+  - linux
 env:
   - IOS_ARCHS=""
 matrix:
@@ -14,3 +16,7 @@ matrix:
 sudo: false
 install: ./travis_install.sh
 script: ./travis_test.sh
+addons:
+  apt:
+    packages:
+      - libdispatch-dev

src/ffi.rs

@@ -24,7 +24,10 @@ pub type dispatch_time_t = u64;
 // dispatch_io_interval_flags_t
 pub type dispatch_queue_attr_t = *const dispatch_object_s;
 
-#[link(name = "System", kind = "dylib")]
+#[cfg_attr(any(target_os = "macos", target_os = "ios"),
+           link(name = "System", kind = "dylib"))]
+#[cfg_attr(not(any(target_os = "macos", target_os = "ios")),
+           link(name = "dispatch", kind = "dylib"))]
 extern {
     static _dispatch_main_q: dispatch_object_s;
     static _dispatch_queue_attr_concurrent: dispatch_object_s;

src/lib.rs

@@ -66,12 +66,21 @@ pub enum QueueAttribute {
 }
 
 impl QueueAttribute {
+    #[cfg(not(all(test, target_os = "linux")))]
     fn as_raw(&self) -> dispatch_queue_attr_t {
         match *self {
             QueueAttribute::Serial => DISPATCH_QUEUE_SERIAL,
             QueueAttribute::Concurrent => DISPATCH_QUEUE_CONCURRENT,
         }
     }
+
+    #[cfg(all(test, target_os = "linux"))]
+    fn as_raw(&self) -> dispatch_queue_attr_t {
+        // The Linux tests use Ubuntu's libdispatch-dev package, which is
+        // apparently really old from before OSX 10.7.
+        // Back then, the attr for dispatch_queue_create must be NULL.
+        ptr::null()
+    }
 }
 
 /// The priority of a global concurrent queue.

travis_test.sh

@@ -2,9 +2,14 @@
 
 set -eu
 
-if [ -z "$IOS_ARCHS" ]; then
+if [ -n "$IOS_ARCHS" ]; then
+    ./rust-test-ios
+elif [ "$TRAVIS_OS_NAME" = "linux" ]; then
     cargo build --verbose
-    cargo test --verbose
+    # The Ubuntu libdispatch doesn't seem to be in great shape,
+    # so just run a quick smoke test of the basic cases.
+    cargo test --verbose test_serial_queue
 else
-    ./rust-test-ios
+    cargo build --verbose
+    cargo test --verbose
 fi