diff --git a/include/common/structure.h b/include/common/structure.h
index ccc77ba..1268381 100644
--- a/include/common/structure.h
+++ b/include/common/structure.h
@@ -7,8 +7,10 @@ char** string_list_init();
 void string_list_free(char **string_list);
 char* string_list_dump(char **string_list);
 uint32_t string_list_len(char **string_list);
-char** string_list_update(char **base_list, char **update_list);
-char** string_list_append(char **string_list, const char *string);
+//char** string_list_update(char **base_list, char **update_list);
+//char** string_list_append(char **string_list, const char *string);
+void string_list_update(char ***base_list, char **update_list);
+void string_list_append(char ***string_list, const char *string);
 
 uint32_t** uint32_list_init();
 char* uint32_list_dump(uint32_t **int_list);
diff --git a/src/applet/dnsproxy.c b/src/applet/dnsproxy.c
index 0d676d8..47512a0 100644
--- a/src/applet/dnsproxy.c
+++ b/src/applet/dnsproxy.c
@@ -11,15 +11,15 @@ char* dnsproxy_config(dnsproxy *info);
 void dnsproxy_dump(const char *caption, dnsproxy *info);
 
 void dnsproxy_add_primary(dnsproxy *info, const char *server) { // add primary dns server
-    info->primary = string_list_append(info->primary, server);
+    string_list_append(&info->primary, server);
 }
 
 void dnsproxy_add_fallback(dnsproxy *info, const char *server) { // add fallback dns server
-    info->fallback = string_list_append(info->fallback, server);
+    string_list_append(&info->fallback, server);
 }
 
 void dnsproxy_add_bootstrap(dnsproxy *info, const char *server) { // add bootstrap dns server
-    info->bootstrap = string_list_append(info->bootstrap, server);
+    string_list_append(&info->bootstrap, server);
 }
 
 void dnsproxy_free(dnsproxy *info) { // free dnsproxy options
diff --git a/src/cleardns.c b/src/cleardns.c
index 9f1a975..772baec 100644
--- a/src/cleardns.c
+++ b/src/cleardns.c
@@ -10,6 +10,7 @@
 #include "adguard.h"
 #include "system.h"
 #include "sundry.h"
+#include "structure.h"
 
 char* init(int argc, char *argv[]) { // return config file
     char *config = string_init(CONFIG_FILE);
@@ -45,6 +46,27 @@ int main(int argc, char *argv[]) { // ClearDNS service
     LOG_LEVEL = LOG_DEBUG;
     log_info("ClearDNS server start (%s)", VERSION);
 
+    char **test = string_list_init();
+    log_info("len -> %u", string_list_len(test));
+    string_list_append(&test, "test1");
+    string_list_append(&test, "test2");
+    string_list_append(&test, "test3");
+    log_info("len -> %u", string_list_len(test));
+
+    char *temp = string_list_dump(test);
+    log_info("content -> %s", temp);
+    free(temp);
+
+    char **add = string_list_init();
+    string_list_append(&add, "test4");
+    string_list_append(&add, "test5");
+
+    string_list_update(&test, add);
+    log_info("len -> %d", string_list_len(test));
+    temp = string_list_dump(test);
+    log_info("content -> %s", temp);
+    free(temp);
+
 //    process *test = process_init("TEST", "lls");
 //    process *test = process_init("TEST", "ls");
 //    process_add_arg(test, "-al");
@@ -56,11 +78,11 @@ int main(int argc, char *argv[]) { // ClearDNS service
 //    wait(&status);
 //    return 0;
 
-    create_folder(WORK_DIR);
+//    create_folder(WORK_DIR);
 
     // TODO: load assets first
-    load_config(config_file);
-    free(config_file);
+//    load_config(config_file);
+//    free(config_file);
 
 //    process_list_init();
 //    process_list_append(dnsproxy_load("Domestic", loader.domestic, "domestic.json"));
diff --git a/src/common/json.c b/src/common/json.c
index 623958d..faccd76 100644
--- a/src/common/json.c
+++ b/src/common/json.c
@@ -92,14 +92,14 @@ char* json_string_value(char* key, cJSON *json) { // json string value -> string
 
 char** json_string_list_value(char *key, cJSON *json, char **string_list) { // json string array -> string list
     if (cJSON_IsString(json)) {
-        string_list = string_list_append(string_list, json->valuestring);
+        string_list_append(&string_list, json->valuestring);
     } else if (cJSON_IsArray(json)) {
         json = json->child;
         while (json != NULL) {
             if (!cJSON_IsString(json)) {
                 log_fatal("`%s` must be string array", key);
             }
-            string_list = string_list_append(string_list, json->valuestring);
+            string_list_append(&string_list, json->valuestring);
             json = json->next; // next key
         }
     } else if (!cJSON_IsNull(json)) { // allow null -> empty string list
diff --git a/src/common/structure.c b/src/common/structure.c
index 90b89cb..d0bf9b2 100644
--- a/src/common/structure.c
+++ b/src/common/structure.c
@@ -4,6 +4,7 @@
 #include <string.h>
 #include "sundry.h"
 #include "constant.h"
+#include "structure.h"
 
 char** string_list_init() { // init string list
     char **string_list = (char **)malloc(sizeof(char *));
@@ -18,19 +19,36 @@ uint32_t string_list_len(char **string_list) { // get len of string list
 }
 
 // TODO: use char *** with void return
-char** string_list_append(char **string_list, const char *string) { // add new string at the end of list
-    uint32_t len = string_list_len(string_list);
-    string_list = (char **)realloc(string_list, sizeof(char *) * (len + 2)); // extend string list
-    string_list[len] = string_init(string);
-    string_list[len + 1] = NULL; // list end sign
-    return string_list;
+//char** string_list_append(char **string_list, const char *string) { // add new string at the end of list
+//    uint32_t len = string_list_len(string_list);
+//    string_list = (char **)realloc(string_list, sizeof(char *) * (len + 2)); // extend string list
+//    string_list[len] = string_init(string);
+//    string_list[len + 1] = NULL; // list end sign
+//    return string_list;
+//}
+
+void string_list_append(char ***string_list, const char *string) {
+    uint32_t len = string_list_len(*string_list);
+    *string_list = (char **)realloc(*string_list, sizeof(char *) * (len + 2)); // extend string list
+    (*string_list)[len] = string_init(string);
+    (*string_list)[len + 1] = NULL; // list end sign
+//    return string_list;
 }
 
-char** string_list_update(char **base_list, char **update_list) { // combine two string list
+//char** string_list_update(char **base_list, char **update_list) { // combine two string list
+//    for (char **string = update_list; *string != NULL; ++string) {
+////        base_list = string_list_append(base_list, *string);
+//        string_list_append(&base_list, *string);
+//    }
+//    return base_list;
+//}
+
+void string_list_update(char ***base_list, char **update_list) { // combine two string list
     for (char **string = update_list; *string != NULL; ++string) {
-        base_list = string_list_append(base_list, *string);
+//        base_list = string_list_append(base_list, *string);
+        string_list_append(base_list, *string);
     }
-    return base_list;
+//    return base_list;
 }
 
 void string_list_free(char **string_list) { // free string list
diff --git a/src/common/system.c b/src/common/system.c
index 8799993..703695c 100644
--- a/src/common/system.c
+++ b/src/common/system.c
@@ -7,16 +7,17 @@
 #include "sundry.h"
 #include "constant.h"
 
-int run_command(const char *command) { // running command with system shell
+int run_command(const char *command) { // running command under system shell
     log_debug("Run command -> `%s`", command);
-    int ret = system(command) / 256;
-    if (ret != 0) {
-        log_warn("Command `%s` return non-zero code -> %d", command, ret);
+    int ret_code = system(command) / 256;
+    if (ret_code != 0) {
+        log_warn("Command `%s` return non-zero code %d", command, ret_code);
     }
-    return ret;
+    return ret_code;
 }
 
-void remove_file(const char *file) {
+void remove_file(const char *file) { // delete file
+    // TODO: use system interface (not `rm` command)
     char *remove_cmd = string_join("rm -f ", file);
     run_command(remove_cmd);
     free(remove_cmd);
@@ -32,6 +33,7 @@ void create_folder(const char *folder) { // create folder
         return;
     }
     log_debug("Create folder -> %s", folder);
+    // TODO: use system command (not `mkdir` command)
     char *command = string_join("mkdir -p ", folder);
     system(command);
     free(command);
diff --git a/src/loader/loader.c b/src/loader/loader.c
index 8b58ad4..1c3ae82 100644
--- a/src/loader/loader.c
+++ b/src/loader/loader.c
@@ -35,9 +35,9 @@ dnsproxy* load_domestic(cleardns_config *config) {
         domestic->cache = config->cache.size;
         domestic->optimistic = config->cache.optimistic;
     }
-    domestic->bootstrap = string_list_update(domestic->bootstrap, config->domestic.bootstrap);
-    domestic->fallback = string_list_update(domestic->fallback, config->domestic.fallback);
-    domestic->primary = string_list_update(domestic->primary, config->domestic.primary);
+    string_list_update(&domestic->bootstrap, config->domestic.bootstrap);
+    string_list_update(&domestic->fallback, config->domestic.fallback);
+    string_list_update(&domestic->primary, config->domestic.primary);
     return domestic;
 }
 
@@ -49,9 +49,9 @@ dnsproxy* load_foreign(cleardns_config *config) {
         foreign->cache = config->cache.size;
         foreign->optimistic = config->cache.optimistic;
     }
-    foreign->bootstrap = string_list_update(foreign->bootstrap, config->foreign.bootstrap);
-    foreign->fallback = string_list_update(foreign->fallback, config->foreign.fallback);
-    foreign->primary = string_list_update(foreign->primary, config->foreign.primary);
+    string_list_update(&foreign->bootstrap, config->foreign.bootstrap);
+    string_list_update(&foreign->fallback, config->foreign.fallback);
+    string_list_update(&foreign->primary, config->foreign.primary);
     return foreign;
 }
 
@@ -119,8 +119,8 @@ void load_config(const char *config_file) {
     config_parser(config, config_file); // configure parser
 
     // insert code (remove after test)
-    config->assets.update_file = string_list_append(config->assets.update_file, "geoip.dat");
-    config->assets.update_url = string_list_append(config->assets.update_url, "https://test.net/geoip.dat");
+    string_list_append(&config->assets.update_file, "geoip.dat");
+    string_list_append(&config->assets.update_url, "https://test.net/geoip.dat");
     config_dump(config);
 
     log_info("Loading configure options");
@@ -136,6 +136,6 @@ void load_config(const char *config_file) {
     loader.filter = load_filter(config);
     log_debug("Filter options parser success");
     loader.script = string_list_init();
-    loader.script = string_list_update(loader.script, config->script);
+    string_list_update(&loader.script, config->script);
     config_free(config);
 }
diff --git a/src/utils/process.c b/src/utils/process.c
index e1496a2..9e1d61e 100644
--- a/src/utils/process.c
+++ b/src/utils/process.c
@@ -15,7 +15,8 @@ process **process_list;
 process* process_init(const char *caption, const char *bin) { // init process struct
     process *proc = (process *)malloc(sizeof(process));
     proc->name = string_init(caption); // process caption
-    proc->cmd = string_list_append(string_list_init(), bin); // argv[0] normally be process file name
+    proc->cmd = string_list_init();
+    string_list_append(&proc->cmd, bin); // argv[0] normally be process file name
     proc->env = string_list_init(); // empty environment variable
     proc->cwd = WORK_DIR; // current working directory
     proc->is_group = FALSE; // create new process group
@@ -23,7 +24,7 @@ process* process_init(const char *caption, const char *bin) { // init process st
 }
 
 void process_add_arg(process *proc, const char *arg) { // add argument for process
-    proc->cmd = string_list_append(proc->cmd, arg);
+    string_list_append(&proc->cmd, arg);
 }
 
 void process_list_init() { // init process list