Vala async/await for GLib I/O

Learn how to write concise, non‑blocking I/O in Vala with async/await, inspect the generated C, and avoid common pitfalls like reference‑counting and GError handling.

Problem: Blocking the main loop when doing I/O

GLib‑based Vala programs often need to read or write files, download data, or perform other I/O without freezing the user interface or a long‑running main loop. The classic GLib solution is to call an asynchronous API that takes a GAsyncReadyCallback and a GAsyncResult. Writing and wiring those callbacks is verbose and easy to get wrong.

Desired outcome

Use Vala’s async and await keywords to write a single method that performs a file read asynchronously, then call it from a top‑level async function. The resulting binary must still expose a C‑compatible interface, and the program should remain responsive while the I/O is in progress.

Prerequisites

valac 0.56 or newer. The async/await syntax was stabilized in this release.
GLib 2.74 or newer. The target GLib version is specified with --target-glib and determines which async APIs are used.
Gio and GLib development packages installed on the build machine.

Procedure: Writing and invoking an async method

Declare the async method. The async keyword appears before the return type. Vala automatically generates a GAsyncReadyCallback wrapper when the method is compiled.

async string read_file_contents (string path) throws Error
    {
        var file = Gio.File.new_for_path (path);
        // Begin the async operation, which returns a GAsyncResult.
        var result = file.load_contents_async.begin (null, null);
        // Wait for completion and retrieve the result.
        var (success, contents, error) = file.load_contents_async.end (result);
        if (!success)
        {
            throw error;
        }
        return contents;
    }

Call the async method from another async context. Use await to suspend the current async scope until the file read finishes.

async void main ()
    {
        try
        {
            var contents = await read_file_contents ("/etc/hosts");
            print ("File contents:\n%s\n", contents);
        }
        catch (Error e)
        {
            print ("Failed to read file: %s\n", e.message);
        }
    }

Compile the program. The --target-glib flag ensures the generated C uses the correct GLib API. The --async flag is optional on recent compilers but harmless if supplied.
```
valac --target-glib=2.74 example.vala -o example
```
Inspect the generated C. Use -C to emit the C file and search for the GAsyncReadyCallback that the compiler inserted.
```
valac -C example.vala -o example.c
    # grep -n "GAsyncReadyCallback" example.c
```
The callback function will contain calls to g_async_result_is_tagged and g_async_result_complete, confirming that the async machinery is in place.

Run the program and verify non‑blocking behavior. Add a short timeout to the main loop and observe that it fires while the file read is still pending.

public static int main (string[] args)
    {
        // Install a timeout that fires after 0.1 s.
        GLib.timeout_add_seconds (1, () => {
            print ("Main loop still running…\n");
            return false; // remove the handler
        });
        // Kick off the async work.
        main.begin (null, null);
        // Start the default main loop.
        GLib.MainLoop.default ().run ();
        return 0;
    }

If the timeout prints before the file read finishes, the main loop was not blocked.

Expected checks

Compilation success. The compiler should emit a C file without errors and the binary should run without segfaults.
Callback presence. The generated C file must contain a GAsyncReadyCallback that completes the async result.
Non‑blocking main loop. The timeout handler should fire while the file read is still in progress. If it does not, the async call likely did not suspend the scope.

Limitations & Recovery Options

Reference counting. GLib objects used inside an async call must stay alive until the callback runs. Dropping the last reference can cause the object to be finalized, corrupting the callback. Keep a strong reference (e.g., store the Gio.File instance in a field or a local variable that outlives the async chain).
GError handling. If the async operation fails, the compiler will throw a Vala Error. Failing to catch it will terminate the program. Always surround await calls with try/catch.
Compatibility. The async/await feature is fully supported on Vala 0.56+ and GLib 2.74+. Older GLib releases may not expose the required async APIs, leading to compilation errors.

Practical verification checklist

Run valac --version and glib-compile-resources --version to confirm the compiler and GLib versions.
Compile with --target-glib=2.74 and verify the generated C contains a GAsyncReadyCallback.
Execute the binary and watch the console for the timeout message interleaved with the file contents.
If the timeout never fires, add print ("Starting async…"); before await and print ("Async finished"); after it to confirm the scope suspends.

By following this guide you can leverage Vala’s async/await syntax to write clean, non‑blocking code that still compiles to a C‑compatible binary, while being mindful of reference counting and error handling.

Using Vala async/await for Non‑Blocking GLib I/O