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diff --git a/src/ioq.h b/src/ioq.h
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+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/**
+ * Asynchronous I/O queues.
+ */
+
+#ifndef BFS_IOQ_H
+#define BFS_IOQ_H
+
+#include <stddef.h>
+
+/**
+ * An queue of asynchronous I/O operations.
+ */
+struct ioq;
+
+/**
+ * An I/O queue response.
+ */
+struct ioq_res {
+	/** The opened directory. */
+	struct bfs_dir *dir;
+	/** The error code, if the operation failed. */
+	int error;
+
+	/** Arbitrary user data. */
+	void *ptr;
+};
+
+/**
+ * Create an I/O queue.
+ *
+ * @param depth
+ *         The maximum depth of the queue.
+ * @param threads
+ *         The maximum number of background threads.
+ * @return
+ *         The new I/O queue, or NULL on failure.
+ */
+struct ioq *ioq_create(size_t depth, size_t threads);
+
+/**
+ * Asynchronous bfs_opendir().
+ *
+ * @param ioq
+ *         The I/O queue.
+ * @param dfd
+ *         The base file descriptor.
+ * @param path
+ *         The path to open, relative to dfd.
+ * @param ptr
+ *         An arbitrary pointer to associate with the request.
+ * @return
+ *         0 on success, or -1 on failure.
+ */
+int ioq_opendir(struct ioq *ioq, int dfd, const char *path, void *ptr);
+
+/**
+ * Pop a response from the queue.
+ *
+ * @param ioq
+ *         The I/O queue.
+ * @return
+ *         The next response, or NULL.
+ */
+struct ioq_res *ioq_pop(struct ioq *ioq);
+
+/**
+ * Pop a response from the queue, without blocking.
+ *
+ * @param ioq
+ *         The I/O queue.
+ * @return
+ *         The next response, or NULL.
+ */
+struct ioq_res *ioq_trypop(struct ioq *ioq);
+
+/**
+ * Free a response.
+ *
+ * @param ioq
+ *         The I/O queue.
+ * @param res
+ *         The response to free.
+ */
+void ioq_free(struct ioq *ioq, struct ioq_res *res);
+
+/**
+ * Stop and destroy an I/O queue.
+ */
+void ioq_destroy(struct ioq *ioq);
+
+#endif // BFS_IOQ_H