json: support parsing and serializing of strings with char arrays

Support parsing and serializing of struct fields that are defined as a
char array.

Use the token JSON_TOK_STRING_BUF to parse and serialize a string for a
char array, for example:

struct foo {
  const char *str;
  char str_buf[30];
};

struct json_obj_descr foo[] = {
  JSON_OBJ_DESCR_PRIM(struct foo, str, JSON_TOK_STRING),
  JSON_OBJ_DESCR_PRIM(struct foo, str_buf, JSON_TOK_STRING_BUF),
};

The struct 'json_obj_descr' now has an additional union member 'field'
to store the size of the struct field, which is used with the token
'JSON_TOK_STRING_BUF' to determine the element size.

Fixes: #65200
Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>

Author: cjwinklhofer

include/zephyr/data/json.h

@@ -35,6 +35,7 @@ enum json_tokens {
 	JSON_TOK_ARRAY_START = '[',
 	JSON_TOK_ARRAY_END = ']',
 	JSON_TOK_STRING = '"',
+	JSON_TOK_STRING_BUF = 's',
 	JSON_TOK_COLON = ':',
 	JSON_TOK_COMMA = ',',
 	JSON_TOK_NUMBER = '0',
@@ -108,6 +109,9 @@ struct json_obj_descr {
 			const struct json_obj_descr *element_descr;
 			size_t n_elements;
 		} array;
+		struct {
+			size_t size;
+		} field;
 	};
 };
 
@@ -157,6 +161,9 @@ typedef int (*json_append_bytes_t)(const char *bytes, size_t len,
 		.field_name_len = sizeof(#field_name_) - 1, \
 		.type = type_, \
 		.offset = offsetof(struct_, field_name_), \
+		.field = { \
+			.size = SIZEOF_FIELD(struct_, field_name_) \
+		}, \
 	}
 
 /**

lib/utils/json.c

@@ -307,6 +307,7 @@ static int element_token(enum json_tokens token)
 	case JSON_TOK_OBJECT_START:
 	case JSON_TOK_ARRAY_START:
 	case JSON_TOK_STRING:
+	case JSON_TOK_STRING_BUF:
 	case JSON_TOK_NUMBER:
 	case JSON_TOK_INT64:
 	case JSON_TOK_UINT64:
@@ -419,6 +420,20 @@ static int skip_field(struct json_obj *obj, struct json_obj_key_value *kv)
 	return 0;
 }
 
+static int decode_string_buf(const struct json_token *token, char *str, size_t size)
+{
+	size_t len = token->end - token->start;
+
+	if (size <= len) {
+		return -EINVAL;
+	}
+
+	strncpy(str, token->start, len);
+	str[len] = '\0';
+
+	return 0;
+}
+
 static int decode_num(const struct json_token *token, int32_t *num)
 {
 	/* FIXME: strtod() is not available in newlib/minimal libc,
@@ -516,6 +531,10 @@ static bool equivalent_types(enum json_tokens type1, enum json_tokens type2)
 		return true;
 	}
 
+	if (type1 == JSON_TOK_STRING && type2 == JSON_TOK_STRING_BUF) {
+		return true;
+	}
+
 	if (type1 == JSON_TOK_ARRAY_START && type2 == JSON_TOK_OBJ_ARRAY) {
 		return true;
 	}
@@ -595,6 +614,11 @@ static int64_t decode_value(struct json_obj *obj,
 
 		return 0;
 	}
+	case JSON_TOK_STRING_BUF: {
+		char *str = field;
+
+		return decode_string_buf(value, str, descr->field.size);
+	}
 	default:
 		return -EINVAL;
 	}
@@ -615,6 +639,8 @@ static ptrdiff_t get_elem_size(const struct json_obj_descr *descr)
 		return sizeof(struct json_obj_token);
 	case JSON_TOK_STRING:
 		return sizeof(char *);
+	case JSON_TOK_STRING_BUF:
+		return descr->field.size;
 	case JSON_TOK_TRUE:
 	case JSON_TOK_FALSE:
 		return sizeof(bool);
@@ -1023,7 +1049,7 @@ static int arr_encode(const struct json_obj_descr *elem_descr,
 	return append_bytes("]", 1, data);
 }
 
-static int str_encode(const char **str, json_append_bytes_t append_bytes,
+static int str_encode(const char *str, json_append_bytes_t append_bytes,
 		      void *data)
 {
 	int ret;
@@ -1033,7 +1059,7 @@ static int str_encode(const char **str, json_append_bytes_t append_bytes,
 		return ret;
 	}
 
-	ret = json_escape_internal(*str, append_bytes, data);
+	ret = json_escape_internal(str, append_bytes, data);
 	if (!ret) {
 		return append_bytes("\"", 1, data);
 	}
@@ -1144,7 +1170,12 @@ static int encode(const struct json_obj_descr *descr, const void *val,
 	case JSON_TOK_TRUE:
 		return bool_encode(ptr, append_bytes, data);
 	case JSON_TOK_STRING:
+		const char **str = ptr;
+
+		return str_encode(*str, append_bytes, data);
+	case JSON_TOK_STRING_BUF:
 		return str_encode(ptr, append_bytes, data);
+
 	case JSON_TOK_ARRAY_START:
 		return arr_encode(descr->array.element_descr, ptr,
 				  val, append_bytes, data);
@@ -1182,8 +1213,7 @@ int json_obj_encode(const struct json_obj_descr *descr, size_t descr_len,
 	}
 
 	for (i = 0; i < descr_len; i++) {
-		ret = str_encode((const char **)&descr[i].field_name,
-				 append_bytes, data);
+		ret = str_encode(descr[i].field_name, append_bytes, data);
 		if (ret < 0) {
 			return ret;
 		}