library: Add facility to register custom init/deinit functions

These can be linked into the application to do initialization/cleanup
without having to modify the source code.

src/libstrongswan/library.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2009-2018 Tobias Brunner
+ * Copyright (C) 2009-2025 Tobias Brunner
  * Copyright (C) 2008 Martin Willi
  *
  * Copyright (C) secunet Security Networks AG
@@ -95,6 +95,30 @@ void library_add_namespace(char *ns)
 	}
 }
 
+#define MAX_LIBSTRONGSWAN_INIT_FUNCTIONS 10
+
+/**
+ * Static array for init function registration using __attribute__((constructor))
+ */
+static library_init_t init_functions[MAX_LIBSTRONGSWAN_INIT_FUNCTIONS];
+static int init_function_count;
+
+/**
+ * Described in header
+ */
+void library_init_register(library_init_t init)
+{
+	if (init_function_count < MAX_LIBSTRONGSWAN_INIT_FUNCTIONS - 1)
+	{
+		init_functions[init_function_count++] = init;
+	}
+	else
+	{
+		fprintf(stderr, "failed to register init function, please increase "
+				"MAX_LIBSTRONGSWAN_INIT_FUNCTIONS");
+	}
+}
+
 /**
  * Register plugins if built statically
  */
@@ -149,6 +173,7 @@ void library_deinit()
 {
 	private_library_t *this = (private_library_t*)lib;
 	bool detailed;
+	int i;
 
 	if (!this || !ref_put(&this->ref))
 	{	/* have more users */
@@ -161,6 +186,11 @@ void library_deinit()
 	/* make sure the cache is clear before unloading plugins */
 	lib->credmgr->flush_cache(lib->credmgr, CERT_ANY);
 
+	for (i = 0; i < init_function_count; ++i)
+	{
+		init_functions[i](FALSE);
+	}
+
 	key_exchange_deinit();
 
 	this->public.streams->destroy(this->public.streams);
@@ -441,5 +471,12 @@ bool library_init(char *settings, const char *namespace)
 
 	key_exchange_init();
 
+	for (i = 0; i < init_function_count; ++i)
+	{
+		if (!init_functions[i](TRUE))
+		{
+			this->init_failed = TRUE;
+		}
+	}
 	return !this->init_failed;
 }

src/libstrongswan/library.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2010-2018 Tobias Brunner
+ * Copyright (C) 2010-2025 Tobias Brunner
  * Copyright (C) 2008 Martin Willi
  *
  * Copyright (C) secunet Security Networks AG
@@ -285,6 +285,24 @@ bool library_init(char *settings, const char *namespace);
  */
 void library_deinit();
 
+/**
+ * Custom function called during init/deinit of a library.
+ *
+ * @param init			TRUE during init, FALSE during deinit
+ * @return				FALSE if an error occurred and init should fail
+ */
+typedef bool (*library_init_t)(bool init);
+
+/**
+ * Register a custom init function that's called at the end of library_init()
+ * and the start of library_deinit().
+ *
+ * To be called from __attribute__((constructor)) functions.
+ *
+ * @param init				init function
+ */
+void library_init_register(library_init_t init);
+
 /**
  * Library instance, set after library_init() and before library_deinit() calls.
  */