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diff --git a/coroutine_chan.h b/coroutine_chan.h
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+/* coroutine_chan.h - Simple request/response system for coroutine.{h,c}
+ *
+ * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-Licence-Identifier: AGPL-3.0-or-later
+ */
+
+/**
+ * The cr_chan_* macros form a simple request/response channel
+ * mechanism built on top of the cr_pause_and_yeild() and cr_unpause()
+ * primitives.
+ */
+#ifndef _COROUTINE_CHAN_H_
+#define _COROUTINE_CHAN_H_
+
+#include "coroutine.h"
+
+/**
+ * cr_chan_t(req_t, resp_t) returns the type definition for a channel
+ * on which the requester submits a value of type `req_t` and the
+ * responder responds with a value of type `resp_t`.
+ */
+#define cr_chan_t(req_t, resp_t) struct { \
+		cid_t requester;          \
+		cid_t responder;          \
+		req_t  req;               \
+		resp_t resp;              \
+	}
+
+/* These are "functions" are preprocessor macros instead of real C
+ * functions so that the compiler can do type-checking instead of
+ * having these functions take `void*`.  */ 
+
+/**
+ * ch_chan_req(cr_chan_t(req_t, resp_t) *ch, resp_t *resp, req_t req)
+ * submits the `req` request to `ch` puts the response in `*resp_p`.
+ *
+ * Blocks until the responder has called both cr_chan_recv_req() and
+ * cr_chan_send_resp().
+ */
+#define cr_chan_req(ch, _resp_p, _req) do {                             \
+		(ch)->requester = cr_getcid();                          \
+		(ch)->req = (_req);                                     \
+		if ((ch)->responder != 0)                               \
+			cr_unpause((ch)->responder);                    \
+		cr_pause_and_yield();                                   \
+		if ((typeof(&(ch)->resp))(_resp_p))                     \
+			*((typeof(&(ch)->resp))(_resp_p)) = (ch)->resp; \
+	} while (0)
+
+/**
+ * cr_chan_have_req(cr_chan_t(req_t, resp_t) *ch) allows a responder
+ * to check whether or not there is a request waiting to be received
+ * (with cr_chan_recv_req()) without blocking if there is not.
+ *
+ * Never blocks.
+ */
+#define cr_chan_have_req(ch) ((ch)->requester != 0)
+
+/**
+ * cr_chan_recv_req(cr_chan_t(req_t, resp_t) *ch, req_t *req_p) reads
+ * a request from ch into `*req_p`.
+ *
+ * If there is not a pending request on `ch`, blocks until there is.
+ */
+#define cr_chan_recv_req(ch, _req_p) do {      \
+		(ch)->responder = cr_getcid(); \
+		if ((ch)->requester == 0)      \
+			cr_pause_and_yield();  \
+		*(_req_p) = (ch)->req;         \
+	} while (0)
+
+/**
+ * cr_chan_send_resp(cr_chan_t(req_t, resp_t) *ch, resp_t resp) sends
+ * the reply to the most-recently-read request.
+ *
+ * Never blocks.
+ */
+#define cr_chan_send_resp(ch, _resp) do {    \
+		cr_unpause((ch)->requester); \
+		(ch)->responder = 0;         \
+		(ch)->requester = 0;         \
+		(ch)->resp = (_resp);        \
+	} while (0)
+
+#endif /* _COROUTINE_CHAN_H_ */