utils: Add safer strnlen function

The function searches for the terminating `NULL` character in the given
string up to a given maximun length.

Author: LukasWoodtli

core/utils.c

@@ -1155,3 +1155,15 @@ lwm2m_data_type_t utils_depthToDatatype(uri_depth_t depth)
 
     return LWM2M_TYPE_UNDEFINED;
 }
+
+size_t utils_strnlen(const char *str, size_t max_size) {
+    if (str == NULL) {
+        return 0;
+    }
+    const char *pos = memchr(str, '\0', max_size);
+    if (pos == NULL) {
+        return max_size;
+    }
+
+    return pos - str;
+}

core/utils.h

@@ -47,5 +47,15 @@ lwm2m_server_t *utils_findBootstrapServer(lwm2m_context_t *contextP, void *fromS
 #if defined(LWM2M_SERVER_MODE) || defined(LWM2M_BOOTSTRAP_SERVER_MODE)
 lwm2m_client_t *utils_findClient(lwm2m_context_t *contextP, void *fromSessionH);
 #endif
+/** A safer version of `strlen`. Finds the number of chars (bytes) of the given string up to a given max. length.
+ *
+ * The behaviour is undefined if the maximum length value is bigger than the actually reserved memory behind the @a str
+ * buffer.
+ *
+ * @param str The null-terminated string to find the length of.
+ * @param max_size Max size of string.
+ * @return The size of the string if it is smaller or equal to max_size. Otherwise max_size.
+ */
+size_t utils_strnlen(const char *str, size_t max_size);
 
 #endif /* WAKAAMA_UTILS_H */

tests/CMakeLists.txt

@@ -15,6 +15,7 @@ set(TEST_SOURCES
     core_logging_tests.c
     core_registration_tests.c
     core_uritests.c
+    core_utils_tests.c
     data_senml_cbor_tests.c
     data_senml_json_tests.c
     data_tlv_json_lwm2m_data_test.c

tests/core_utils_tests.c

@@ -0,0 +1,104 @@
+/*******************************************************************************
+ *
+ * Copyright (c) 2025 GARDENA GmbH
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * and Eclipse Distribution License v1.0 which accompany this distribution.
+ *
+ * The Eclipse Public License is available at
+ *    http://www.eclipse.org/legal/epl-v20.html
+ * The Eclipse Distribution License is available at
+ *    http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * Contributors:
+ *   Lukas Woodtli, GARDENA GmbH - Please refer to git log
+ *
+ *******************************************************************************/
+#include "CUnit/Basic.h"
+#include "tests.h"
+#include "utils.h"
+
+void test_strnlen_null(void) {
+    size_t len = utils_strnlen(NULL, 10);
+    CU_ASSERT_EQUAL(len, 0);
+}
+
+void test_strnlen_0(void) {
+    const char *string = "";
+    size_t len = utils_strnlen(string, 10);
+    CU_ASSERT_EQUAL(len, 0);
+}
+
+void test_strnlen_1(void) {
+    const char *string = "a";
+    size_t len = utils_strnlen(string, 10);
+    CU_ASSERT_EQUAL(len, 1);
+}
+
+void test_strnlen_2(void) {
+    const char *string = "ab";
+    size_t len = utils_strnlen(string, 10);
+    CU_ASSERT_EQUAL(len, 2);
+}
+
+void test_strnlen_max_len_0(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 0);
+    CU_ASSERT_EQUAL(len, 0);
+}
+
+void test_strnlen_max_len_1(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 1);
+    CU_ASSERT_EQUAL(len, 1);
+}
+
+void test_strnlen_max_len_2(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 2);
+    CU_ASSERT_EQUAL(len, 2);
+}
+
+void test_strnlen_max_len_3(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 3);
+    CU_ASSERT_EQUAL(len, 3);
+}
+
+void test_strnlen_max_len_4(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 4);
+    CU_ASSERT_EQUAL(len, 3);
+}
+
+void test_strnlen_max_len_5(void) {
+    const char *string = "abc";
+    size_t len = utils_strnlen(string, 5);
+    CU_ASSERT_EQUAL(len, 3);
+}
+
+static struct TestTable table[] = {
+    {"test_strnlen_null()", test_strnlen_null},
+    {"test_strnlen_0()", test_strnlen_0},
+    {"test_strnlen_1()", test_strnlen_1},
+    {"test_strnlen_2()", test_strnlen_2},
+    {"test_strnlen_max_len_0()", test_strnlen_max_len_0},
+    {"test_strnlen_max_len_1()", test_strnlen_max_len_1},
+    {"test_strnlen_max_len_2()", test_strnlen_max_len_2},
+    {"test_strnlen_max_len_3()", test_strnlen_max_len_3},
+    {"test_strnlen_max_len_4()", test_strnlen_max_len_4},
+    {"test_strnlen_max_len_5()", test_strnlen_max_len_5},
+    {NULL, NULL},
+};
+
+CU_ErrorCode create_utils_suit(void) {
+    CU_pSuite pSuite = NULL;
+
+    pSuite = CU_add_suite("Suite_utils", NULL, NULL);
+    if (NULL == pSuite) {
+        return CU_get_error();
+    }
+
+    return add_tests(pSuite, table);
+}

tests/tests.h

@@ -58,4 +58,5 @@ CU_ErrorCode create_logging_test_suit(void);
 #ifdef LWM2M_SERVER_MODE
 CU_ErrorCode create_registration_test_suit(void);
 #endif
+CU_ErrorCode create_utils_suit(void);
 #endif /* TESTS_H_ */

tests/unittests.c

@@ -107,6 +107,9 @@ int main(void) {
        goto exit;
 #endif
 
+   if (CUE_SUCCESS != create_utils_suit())
+       goto exit;
+
    CU_basic_set_mode(CU_BRM_VERBOSE);
    CU_basic_run_tests();
    CU_basic_show_failures(CU_get_failure_list());