examples: build system: add link-time registration plugin example

Closes https://github.com/espressif/esp-idf/issues/7682

examples/build_system/cmake/plugins/components/plugins/CMakeLists.txt

@@ -0,0 +1,3 @@
+idf_component_register(SRCS plugins.c
+                       INCLUDE_DIRS include
+                       LDFRAGMENTS linker.lf)

examples/build_system/cmake/plugins/components/plugins/include/plugins_api.h

@@ -0,0 +1,48 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* This stucture describes the plugin to the rest of the application */
+typedef struct {
+    /* A pointer to the plugin name */
+    const char* name;
+
+    /* A function which the plugin provides to the application.
+     * In this example, this function prints something to the console
+     * depending on the value of the argument 'arg'.
+     */
+    void (*greet)(const char* arg);
+} example_plugin_desc_t;
+
+/**
+ * @brief Register the plugin with the application
+ * This function is called from each plugin's "constructor" function.
+ * It adds the plugin to the list.
+ * @param plugin_desc  Pointer to the structure which describes the given plugin.
+ */
+void example_plugin_register(const example_plugin_desc_t* plugin_desc);
+
+/**
+ * @brief Print the list of registered plugins to the console.
+ * This function is called from the application.
+ */
+void example_plugins_list(void);
+
+/**
+ * @brief Invoke 'greet' function of each registered plugin with the given argument.
+ * This function is called from the application.
+ * @param arg  argument to pass to plugins' greet functions.
+ */
+void example_plugins_greet(const char* arg);
+
+
+#ifdef __cplusplus
+}
+#endif

examples/build_system/cmake/plugins/components/plugins/linker.lf

@@ -0,0 +1,13 @@
+[sections:plugins_desc]
+entries:
+    .plugins_desc
+
+[scheme:plugins_desc_default]
+entries:
+    plugins_desc -> flash_rodata
+
+[mapping:plugins_desc]
+archive: *
+entries:
+    * (plugins_desc_default);
+        plugins_desc -> flash_rodata KEEP() SORT(name) SURROUND(plugins_array)

examples/build_system/cmake/plugins/components/plugins/plugins.c

@@ -0,0 +1,71 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include <stdio.h>
+#include <string.h>
+#include <sys/queue.h>
+#include "plugins_api.h"
+
+/**
+ * Demonstration of dynamic registration (self-registration):
+ *
+ * - example_plugin_register function is called from "constructor" functions of each plugin.
+ *   Information about the plugin is passed inside 'example_plugin_desc_t' structure.
+ *   This function adds each plugin description into linked list (s_plugins_list).
+ *
+ * - example_plugins_greet function iterates over the linked list.
+ */
+
+struct plugin_record {
+    example_plugin_desc_t plugin_desc;
+    LIST_ENTRY(plugin_record) list_entry;
+};
+
+static LIST_HEAD(plugins_list, plugin_record) s_plugins_list = LIST_HEAD_INITIALIZER(s_plugins_list);
+
+void example_plugin_register(const example_plugin_desc_t* plugin_desc)
+{
+    struct plugin_record *record = (struct plugin_record *) malloc(sizeof(struct plugin_record));
+    if (record == NULL) {
+        abort();
+    }
+    memcpy(&record->plugin_desc, plugin_desc, sizeof(*plugin_desc));
+
+    struct plugin_record *head = LIST_FIRST(&s_plugins_list);
+    if (head == NULL) {
+        LIST_INSERT_HEAD(&s_plugins_list, record, list_entry);
+    } else {
+        LIST_INSERT_BEFORE(head, record, list_entry);
+    }
+    printf("Successfully registered plugin '%s'\n", plugin_desc->name);
+}
+
+void example_plugins_greet(const char* arg)
+{
+    struct plugin_record *it;
+    LIST_FOREACH(it, &s_plugins_list, list_entry) {
+        printf("Calling greet function of plugin '%s'...\n", it->plugin_desc.name);
+        (*it->plugin_desc.greet)(arg);
+        printf("Done with greet function of plugin '%s'.\n", it->plugin_desc.name);
+    }
+}
+
+/**
+ * Demonstration of static registration.
+ * Symbols '_plugins_array_start' and '_plugins_array_end' mark the beginning and end
+ * of the array where 'example_plugin_desc_t' structures are placed by the linker.
+ * The names of these variables are determined by linker.lf in 'plugins' component,
+ * look for 'SURROUND(plugins_array)'.
+ */
+
+void example_plugins_list(void)
+{
+    printf("List of plugins:\n");
+    extern const example_plugin_desc_t _plugins_array_start;
+    extern const example_plugin_desc_t _plugins_array_end;
+    for (const example_plugin_desc_t* it = &_plugins_array_start; it != &_plugins_array_end; ++it) {
+        printf("- Plugin '%s', function greet=%p\n", it->name, it->greet);
+    }
+}