diff --git a/TODO.TXT b/TODO.TXT
index 5277752..e42a658 100644
--- a/TODO.TXT
+++ b/TODO.TXT
@@ -12,5 +12,5 @@ X C transpiler for unpacking
 X C transpiler for packing
 X setup tests
 X add bitmanip operators
-  handling of overflown or negative expression
+X handling of overflown or negative expression
   release 1.0.0
diff --git a/c/result.h b/c/result.h
new file mode 100644
index 0000000..fe20140
--- /dev/null
+++ b/c/result.h
@@ -0,0 +1,10 @@
+#pragma once
+
+typedef enum biner_result_t {
+  BINER_RESULT_COMPLETED,
+  BINER_RESULT_CONTINUE,
+
+  BINER_RESULT_ERROR_THRESHOULD_,
+
+  BINER_RESULT_MEMORY_ERROR,
+} biner_result_t;
diff --git a/test/main.c b/test/main.c
index 0fea754..ee44515 100644
--- a/test/main.c
+++ b/test/main.c
@@ -10,8 +10,15 @@
 #include <string.h>
 
 #include "../c/pack.h"
+#include "../c/result.h"
 #include "../c/unpack.h"
-#include "../c/zone.h"
+
+static inline void* biner_malloc_(size_t sz, void* udata) {
+  (void) udata;
+
+  /* returning NULL can abort the unpacking */
+  return malloc(sz);
+}
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wpedantic"
@@ -67,10 +74,11 @@ int main(void) {
     .sex  = SEX_CAT,
     .role = ROLE_STUDENT,
 
-    .property_count = 2,
+    .property_count = 3,
     .properties = (strpair_t[]) {
-      { .key = str_("like"), .value = str_("rat"), },
-      { .key = str_("hate"), .value = str_("dog"), },
+      { .key = str_("like"),  .value = str_("rat"), },
+      { .key = str_("hate"),  .value = str_("dog"), },
+      { .key = str_("empty"), .value = str_(""), },
     },
 
     .student = {
@@ -85,13 +93,18 @@ int main(void) {
   user_t dst = {0};
 
   size_t b = 0;
-  bool completed = false;
-  while (!completed) {
+  biner_result_t result = BINER_RESULT_CONTINUE;
+  while (result == BINER_RESULT_CONTINUE) {
     uint8_t c;
-    completed = user_pack(&pack_ctx, &src, &c);
-    if (user_unpack(&unpack_ctx, &dst, c) != completed) {
+    result = user_pack(&pack_ctx, &src, &c);
+    if (result >= BINER_RESULT_ERROR_THRESHOULD_) {
+      fprintf(stderr, "unexpected error happened\n");
+      return EXIT_FAILURE;
+    }
+
+    if (user_unpack(&unpack_ctx, &dst, c) != result) {
       fprintf(stderr, "fail at the %zu byte: ", b);
-      if (completed) {
+      if (result == BINER_RESULT_COMPLETED) {
         fprintf(stderr, "pack function finsihed but unpack not\n");
       } else {
         fprintf(stderr, "unpack function finsihed but pack not\n");
diff --git a/transpile_c.c b/transpile_c.c
index 96500c2..62e488e 100644
--- a/transpile_c.c
+++ b/transpile_c.c
@@ -290,6 +290,7 @@ static void print_struct_member_context_struct_(
   fprintf(p->dst, "size_t count; ");
   fprintf(p->dst, "size_t count_max; ");
   fprintf(p->dst, "size_t byte; ");
+  fprintf(p->dst, "void*  udata; ");
 
   bool require_subctx = false;
   const biner_tree_struct_member_t* itr = m;
@@ -342,6 +343,8 @@ static void print_struct_member_pack_code_each_(
       print_expr_(
         info->p, (const biner_tree_expr_t*) (info->p->zone+info->t->expr));
       fprintf(info->p->dst, "; ");
+      fprintf(info->p->dst,
+        "if (ctx->count_max <= 0) { ++ctx->step; goto CONTINUE; } ");
       fprintf(info->p->dst, "ctx->count = 0; ");
       break;
     case BINER_TREE_STRUCT_MEMBER_TYPE_QUALIFIER_STATIC_ARRAY:
@@ -353,7 +356,7 @@ static void print_struct_member_pack_code_each_(
     if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
       fprintf(info->p->dst, "ctx->subctx.%s = (", name);
       print_fixed_decl_name_(info->p, dname);
-      fprintf(info->p->dst, CONTEXT_SUFFIX_") {0}; ");
+      fprintf(info->p->dst, CONTEXT_SUFFIX_") { .udata = ctx->udata, }; ");
     }
 
     fprintf(info->p->dst, "body%zu: ", info->m->index);
@@ -377,9 +380,11 @@ static void print_struct_member_pack_code_each_(
   }
 
   if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
-    fprintf(info->p->dst, "if (");
+    fprintf(info->p->dst, "result = ");
     print_fixed_decl_name_(info->p, dname);
-    fprintf(info->p->dst, "_pack(&ctx->subctx.%s, &s->%s%s, c)) { ", name, name, suffix);
+    fprintf(info->p->dst, "_pack(&ctx->subctx.%s, &s->%s%s, c); ", name, name, suffix);
+    fprintf(info->p->dst,
+      "if (result != BINER_RESULT_COMPLETED) { return result; } else { ");
   } else {
     const char* func = struct_member_type_name_meta_map_[info->t->name].func;
     fprintf(info->p->dst,
@@ -409,11 +414,11 @@ static void print_struct_member_pack_code_each_(
     if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
       fprintf(info->p->dst, "ctx->subctx.%s = (", name);
       print_fixed_decl_name_(info->p, dname);
-      fprintf(info->p->dst, CONTEXT_SUFFIX_") {0}; ");
+      fprintf(info->p->dst, CONTEXT_SUFFIX_") { .udata = ctx->udata, }; ");
     }
     fprintf(info->p->dst, "ctx->byte = 0; ");
   }
-  fprintf(info->p->dst, "} return false; ");
+  fprintf(info->p->dst, "} return BINER_RESULT_CONTINUE; ");
 
   if (info->m->condition != 0) {
     fprintf(info->p->dst, "} ");
@@ -445,8 +450,12 @@ static void print_struct_member_unpack_code_each_(
         info->p, (const biner_tree_expr_t*) (info->p->zone+info->t->expr));
       fprintf(info->p->dst, "; ");
       fprintf(info->p->dst,
-        "s->%s = malloc(sizeof(*s->%s)*ctx->count_max); ",
+        "if (ctx->count_max <= 0) { ++ctx->step; goto CONTINUE; } ");
+      fprintf(info->p->dst,
+        "s->%s = biner_malloc_(sizeof(*s->%s)*ctx->count_max, ctx->udata); ",
         name, name);
+      fprintf(info->p->dst,
+        "if (s->%s == NULL) { return BINER_RESULT_MEMORY_ERROR; } ", name);
       fprintf(info->p->dst, "ctx->count = 0; ");
       break;
     case BINER_TREE_STRUCT_MEMBER_TYPE_QUALIFIER_STATIC_ARRAY:
@@ -458,7 +467,7 @@ static void print_struct_member_unpack_code_each_(
     if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
       fprintf(info->p->dst, "ctx->subctx.%s = (", name);
       print_fixed_decl_name_(info->p, dname);
-      fprintf(info->p->dst, CONTEXT_SUFFIX_") {0}; ");
+      fprintf(info->p->dst, CONTEXT_SUFFIX_") { .udata = ctx->udata, }; ");
     }
 
     fprintf(info->p->dst, "body%zu: ", info->m->index);
@@ -482,9 +491,11 @@ static void print_struct_member_unpack_code_each_(
   }
 
   if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
-    fprintf(info->p->dst, "if (");
+    fprintf(info->p->dst, "result = ");
     print_fixed_decl_name_(info->p, dname);
-    fprintf(info->p->dst, "_unpack(&ctx->subctx.%s, &s->%s%s, c)) { ", name, name, suffix);
+    fprintf(info->p->dst, "_unpack(&ctx->subctx.%s, &s->%s%s, c); ", name, name, suffix);
+    fprintf(info->p->dst,
+      "if (result != BINER_RESULT_COMPLETED) { return result; } else { ");
   } else {
     const char* func = struct_member_type_name_meta_map_[info->t->name].func;
     fprintf(info->p->dst,
@@ -514,11 +525,11 @@ static void print_struct_member_unpack_code_each_(
     if (info->t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
       fprintf(info->p->dst, "ctx->subctx.%s = (", name);
       print_fixed_decl_name_(info->p, dname);
-      fprintf(info->p->dst, CONTEXT_SUFFIX_") {0}; ");
+      fprintf(info->p->dst, CONTEXT_SUFFIX_") { .udata = ctx->udata, }; ");
     }
     fprintf(info->p->dst, "ctx->byte = 0; ");
   }
-  fprintf(info->p->dst, "} return false; ");
+  fprintf(info->p->dst, "} return BINER_RESULT_CONTINUE; ");
 
   if (info->m->condition != 0) {
     fprintf(info->p->dst, "} ");
@@ -536,20 +547,45 @@ static inline void print_struct_member_iteration_code_(
   assert(m != NULL);
   assert(f != NULL);
 
+  bool require_continue   = false;
+  bool require_result_var = false;
+  const biner_tree_struct_member_t* itr = m;
+  while ((uintptr_t) itr != (uintptr_t) p->zone) {
+    const biner_tree_struct_member_type_t* t =
+      (const biner_tree_struct_member_type_t*) (p->zone+itr->type);
+    if (t->qualifier == BINER_TREE_STRUCT_MEMBER_TYPE_QUALIFIER_DYNAMIC_ARRAY) {
+      require_continue = true;
+    }
+    if (t->name == BINER_TREE_STRUCT_MEMBER_TYPE_NAME_USER_DECL) {
+      require_result_var = true;
+    }
+    itr = (const biner_tree_struct_member_t*) (p->zone+itr->prev);
+  }
+
   fprintf(p->dst, "static const void* const steps_[] = { ");
   for (size_t i = 0; i <= m->index; ++i) {
     fprintf(p->dst, "&&init%zu, &&body%zu, ", i, i);
   }
   fprintf(p->dst, "}; ");
 
+  if (require_result_var) {
+    fprintf(p->dst, "biner_result_t result; ");
+  }
+  if (require_continue) {
+    fprintf(p->dst, "CONTINUE: ");
+  }
   fprintf(p->dst,
-    "if (ctx->step >= sizeof(steps_)/sizeof(steps_[0])) return true; ");
+    "if (ctx->step >= sizeof(steps_)/sizeof(steps_[0]))"
+    " return BINER_RESULT_COMPLETED; ");
   fprintf(p->dst, "goto *steps_[ctx->step]; ");
 
   struct_member_each_(p, m, f, SIZE_MAX);
 
   fprintf(p->dst, "NEXT: ");
-  fprintf(p->dst, "return ++ctx->step >= sizeof(steps_)/sizeof(steps_[0]); ");
+  fprintf(p->dst,
+    "return (++ctx->step >= sizeof(steps_)/sizeof(steps_[0]))?"
+    " BINER_RESULT_COMPLETED:"
+    " BINER_RESULT_CONTINUE; ");
 }
 
 static void print_decls_(
@@ -597,7 +633,7 @@ static void print_decls_(
     print_struct_member_context_struct_(p, body, CONTEXT_SUFFIX_);
     print_typedef_footer_(p, name, CONTEXT_SUFFIX_);
 
-    print_func_header_(p, "bool", name, "_pack");
+    print_func_header_(p, "biner_result_t", name, "_pack");
     print_fixed_decl_name_(p, name);
     fprintf(p->dst, CONTEXT_SUFFIX_"* ctx, ");
     fprintf(p->dst, "const ");
@@ -608,7 +644,7 @@ static void print_decls_(
       p, body, &print_struct_member_pack_code_each_);
     fprintf(p->dst, "}\n");
 
-    print_func_header_(p, "bool", name, "_unpack");
+    print_func_header_(p, "biner_result_t", name, "_unpack");
     print_fixed_decl_name_(p, name);
     fprintf(p->dst, CONTEXT_SUFFIX_"* ctx, ");
     print_fixed_decl_name_(p, name);