5 files changed, 90 insertions, 3 deletions
diff --git a/libcr_ipc/CMakeLists.txt b/libcr_ipc/CMakeLists.txt
index 3388fcd..957b638 100644
--- a/libcr_ipc/CMakeLists.txt
+++ b/libcr_ipc/CMakeLists.txt
@@ -7,6 +7,7 @@ add_library(libcr_ipc INTERFACE)
 target_include_directories(libcr_ipc SYSTEM INTERFACE ${CMAKE_CURRENT_LIST_DIR}/include)
 target_sources(libcr_ipc INTERFACE
 	sema.c
+	mutex.c
 )
 target_link_libraries(libcr_ipc INTERFACE
 	libcr
diff --git a/libcr_ipc/include/libcr_ipc/mutex.h b/libcr_ipc/include/libcr_ipc/mutex.h
new file mode 100644
index 0000000..38ebde0
--- /dev/null
+++ b/libcr_ipc/include/libcr_ipc/mutex.h
@@ -0,0 +1,46 @@
+/* libcr_ipc/mutex.h - Simple mutexes for libcr (header file)
+ *
+ * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-Licence-Identifier: AGPL-3.0-or-later
+ */
+
+#ifndef _COROUTINE_MUTEX_H_
+#define _COROUTINE_MUTEX_H_
+
+#include <stdbool.h> /* for bool */
+
+#include <libcr/coroutine.h> /* for cid_t */
+
+struct _cr_mutex_cid_list {
+	cid_t                            val;
+	struct _cr_mutex_cid_list        *next;
+};
+
+/**
+ * A cr_mutex_t is a fair mutex.
+ */
+typedef struct {
+	bool                             locked;
+	struct _cr_mutex_cid_list        *head, **tail;
+} cr_mutex_t;
+
+/**
+ * Lock the mutex.  Blocks if it is already locked.
+ *
+ * @blocks maybe
+ * @yields maybe
+ * @run_in coroutine
+ */
+void cr_mutex_lock(cr_mutex_t *);
+
+/**
+ * Unlock the mutex.  Unblocks a coroutine that is blocked on
+ * cr_mutex_lock().
+ *
+ * @blocks never
+ * @yields never
+ * @may_run_in coroutine
+ */
+void cr_mutex_unluck(cr_mutex_t *);
+
+#endif /* _COROUTINE_MUTEX_H_ */
diff --git a/libcr_ipc/include/libcr_ipc/sema.h b/libcr_ipc/include/libcr_ipc/sema.h
index a2a176d..454eb5d 100644
--- a/libcr_ipc/include/libcr_ipc/sema.h
+++ b/libcr_ipc/include/libcr_ipc/sema.h
@@ -1,4 +1,4 @@
-/* coroutine_sema.h - Simple semaphores for coroutine.{h,c}
+/* libcr_ipc/sema.h - Simple semaphores for libcr (header file)
  *
  * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
  * SPDX-Licence-Identifier: AGPL-3.0-or-later
@@ -7,6 +7,10 @@
 #ifndef _COROUTINE_SEMA_H_
 #define _COROUTINE_SEMA_H_
 
+#include <stdbool.h> /* for bool */
+
+#include <libcr/coroutine.h> /* for cid_t */
+
 struct _cr_sema_cid_list {
 	cid_t                            val;
 	struct _cr_sema_cid_list        *next;
diff --git a/libcr_ipc/mutex.c b/libcr_ipc/mutex.c
new file mode 100644
index 0000000..c5cea96
--- /dev/null
+++ b/libcr_ipc/mutex.c
@@ -0,0 +1,37 @@
+/* libcr_ipc/mutex.c - Simple mutexes for libcr (implementation file)
+ *
+ * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-Licence-Identifier: AGPL-3.0-or-later
+ */
+
+#include <libcr_ipc/mutex.h>
+
+void cr_mutex_lock(cr_mutex_t *mu) {
+	assert(mu);
+	if (!mu->tail)
+		mu->tail = &mu->head;
+	if (!mu->locked) {
+		mu->locked = true;
+		return;
+	}
+	struct _cr_mutex_cid_list self = {
+		.val = cr_getcid(),
+		.next = NULL,
+	};
+	*(mu->tail) = &self;
+	mu->tail = &(self.next);
+	cr_pause_and_yield();
+}
+
+void cr_mutex_unlock(cr_mutex_t *mu) {
+	assert(mu);
+	assert(mu->tail);
+	assert(mu->locked);
+	if (mu->head) {
+		cr_unpause(mu->head->val);
+		mu->head = mu->head->next;
+		if (!mu->head)
+			mu->tail = &mu->head;
+	} else
+		mu->locked = false;
+}
diff --git a/libcr_ipc/sema.c b/libcr_ipc/sema.c
index a3b2ca0..9645a41 100644
--- a/libcr_ipc/sema.c
+++ b/libcr_ipc/sema.c
@@ -1,4 +1,4 @@
-/* coroutine_sema.h - Simple semaphores for coroutine.{h,c}
+/* libcr_ipc/sema.c - Simple semaphores for libcr (implementation file)
  *
  * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
  * SPDX-Licence-Identifier: AGPL-3.0-or-later
@@ -6,7 +6,6 @@
 
 #include <assert.h>
 
-#include <libcr/coroutine.h>
 #include <libcr_ipc/sema.h>
 
 /** Drain the sema->{head,tail} list.  Returns true if cr_getcid() was drained.  */