refactor(splits): Minor cleanups to periph invocation

* Add strlcpy from public domain version.
* Leverage strlcpy to detect truncation of behavior dev strs, and log.
* Use `offsetof` for cleaner detection on peripheral side.

app/CMakeLists.txt

@@ -23,6 +23,7 @@ zephyr_linker_sources(RODATA include/linker/zmk-events.ld)
 # find_package(Zephyr) which defines the target.
 target_include_directories(app PRIVATE include)
 target_sources_ifdef(CONFIG_ZMK_SLEEP app PRIVATE src/power.c)
+target_sources(app PRIVATE src/stdlib.c)
 target_sources(app PRIVATE src/activity.c)
 target_sources(app PRIVATE src/kscan.c)
 target_sources(app PRIVATE src/matrix_transform.c)

app/include/zmk/stdlib.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <stdlib.h> /* for size_t */
+
+/*
+ * ANSI C version of strlcpy
+ * Based on the NetBSD strlcpy man page.
+ *
+ * Nathan Myers <ncm-nospam@cantrip.org>, 2003/06/03
+ * Placed in the public domain.
+ */
+
+size_t strlcpy(char *dst, const char *src, size_t size);

app/src/split/bluetooth/central.c

@@ -17,6 +17,7 @@
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
+#include <zmk/stdlib.h>
 #include <zmk/ble.h>
 #include <zmk/behavior.h>
 #include <zmk/split/bluetooth/uuid.h>
@@ -549,8 +550,12 @@ int zmk_split_bt_invoke_behavior(uint8_t source, struct zmk_behavior_binding *bi
                                                          .position = event.position,
                                                          .state = state ? 1 : 0,
                                                      }};
-    strncpy(payload.behavior_dev, binding->behavior_dev, ZMK_SPLIT_RUN_BEHAVIOR_DEV_LEN - 1);
-    payload.behavior_dev[ZMK_SPLIT_RUN_BEHAVIOR_DEV_LEN - 1] = '\0';
+    const size_t payload_dev_size = sizeof(payload.behavior_dev);
+    if (strlcpy(payload.behavior_dev, binding->behavior_dev, payload_dev_size) >=
+        payload_dev_size) {
+        LOG_ERR("Truncated behavior label %s to %s before invoking peripheral behavior",
+                log_strdup(binding->behavior_dev), log_strdup(payload.behavior_dev));
+    }
 
     struct zmk_split_run_behavior_payload_wrapper wrapper = {.source = source, .payload = payload};
     return split_bt_invoke_behavior_payload(wrapper);

app/src/split/bluetooth/service.c

@@ -51,8 +51,10 @@ static ssize_t split_svc_run_behavior(struct bt_conn *conn, const struct bt_gatt
     // We run if:
     // 1: We've gotten all the position/state/param data.
     // 2: We have a null terminated string for the behavior device label.
+    const size_t behavior_dev_offset =
+        offsetof(struct zmk_split_run_behavior_payload, behavior_dev);
     if ((end_addr > sizeof(struct zmk_split_run_behavior_data)) &&
-        payload->behavior_dev[end_addr - sizeof(struct zmk_split_run_behavior_data) - 1] == '\0') {
+        payload->behavior_dev[end_addr - behavior_dev_offset - 1] == '\0') {
         struct zmk_behavior_binding binding = {
             .param1 = payload->data.param1,
             .param2 = payload->data.param2,

app/src/stdlib.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zmk/stdlib.h>
+#include <string.h>
+
+/*
+ * ANSI C version of strlcpy
+ * Based on the NetBSD strlcpy man page.
+ *
+ * Nathan Myers <ncm-nospam@cantrip.org>, 2003/06/03
+ * Placed in the public domain.
+ */
+
+size_t strlcpy(char *dst, const char *src, size_t size) {
+    const size_t len = strlen(src);
+    if (size != 0) {
+        memcpy(dst, src, (len > size - 1) ? size - 1 : len);
+        dst[size - 1] = 0;
+    }
+    return len;
+}