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diff --git a/libcr_ipc/coroutine_rpc.h b/libcr_ipc/coroutine_rpc.h
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+/* coroutine_rpc.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_rpc_* macros form a simple request/response mechanism built
+ * on top of the cr_pause_and_yeild() and cr_unpause() primitives.
+ */
+#ifndef _COROUTINE_RPC_H_
+#define _COROUTINE_RPC_H_
+
+#include <assert.h>
+
+#include "coroutine.h"
+
+/**
+ * cr_rpc_t(req_t, resp_t) returns the type definition for a
+ * rcp-channel on which the requester submits a value of type `req_t`
+ * and the responder responds with a value of type `resp_t`.
+ *
+ * There may be multiple concurrent requesters, but only one
+ * concurrent responder.  If you need multiple concurrent responders,
+ * set up a system of coroutine_chan.h channels.
+ */
+#define cr_rpc_t(req_t, resp_t) struct {            \
+		cid_t   requester, *tail_requester; \
+		cid_t   responder;                  \
+		req_t  *req_p;                      \
+		resp_t *resp_p;                     \
+	}
+
+/* These "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_rpc_req(cr_rpc_t(req_t, resp_t) *ch, resp_t *resp, req_t req)
+ * submits the `req` request to `ch` puts the response in `*resp_p`.
+ *
+ * Blocking: Always; until the responder has called both
+ * cr_rpc_recv_req() and cr_rpc_send_resp().
+ */
+#define cr_rpc_req(ch, _resp_p, _req) do {                    \
+		cid_t next = 0;                               \
+		if ((ch)->requester) {                        \
+			*((ch)->tail_requester = cr_getcid(); \
+			(ch)->tail_requester = &next;         \
+			cr_pause_and_yield();                 \
+		} else {                                      \
+			(ch)->requester = cr_getcid();        \
+			(ch)->tail_requester = &next;         \
+		}                                             \
+		assert((ch)->requester == cr_getcid());       \
+		(ch)->req_p = &(_req);                        \
+		(ch)->resp_p = (_resp_p;                      \
+		if ((ch)->responder != 0)                     \
+			cr_unpause((ch)->responder);          \
+		cr_pause_and_yield();                         \
+		if (next) {                                   \
+			(ch)->requester = next;               \
+			cr_unpause(next);                     \
+		} else {                                      \
+			(ch)->requester = 0;                  \
+			(ch)->tail_requester = NULL;          \
+		}
+	} while (0)
+
+/**
+ * cr_rpc_have_req(cr_rpc_t(req_t, resp_t) *ch) allows a responder
+ * to check whether or not there is a request waiting to be received
+ * (with cr_rpc_recv_req()) without blocking if there is not.
+ *
+ * Blocking: Never.
+ */
+#define cr_rpc_have_req(ch) ((ch)->req_p != NULL)
+
+/**
+ * cr_rpc_recv_req(cr_rpc_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.
+ *
+ * Blocking: Maybe.
+ */
+#define cr_rpc_recv_req(ch, _req_p) do {       \
+		(ch)->responder = cr_getcid(); \
+		if ((ch)->requester == 0)      \
+			cr_pause_and_yield();  \
+		*(_req_p) = *((ch)->req_p);    \
+		(ch)->req_p = NULL;            \
+	} while (0)
+
+/**
+ * cr_rpc_send_resp(cr_rpc_t(req_t, resp_t) *ch, resp_t resp) sends
+ * the reply to the most-recently-read request.
+ *
+ * Blocking: Never.
+ */
+#define cr_rpc_send_resp(ch, _resp) do {       \
+		(ch)->responder = 0;           \
+		*((ch)->resp_p) = (_resp);     \
+		(ch)->resp_p = NULL;           \
+		cr_unpause((ch)->requester);   \
+	} while (0)
+
+#endif /* _COROUTINE_RPC_H_ */