diff --git a/README.md b/README.md
index b9fdf6f..e38c9c7 100644
--- a/README.md
+++ b/README.md
@@ -631,7 +631,7 @@ nasal_ref builtin_keys(nasal_ref* local,nasal_gc& gc)
     if(hash.type!=vm_hash)
         return builtin_err("keys","\"hash\" must be hash");
     // push vector into local scope to avoid being sweeped
-    if(gc.top+1>=gc.stack+STACK_MAX_DEPTH-1)
+    if(gc.top+1>=gc.stack+nasal_gc::stack_depth-1)
         return builtin_err("keys","expand temporary space error:stackoverflow");
     (++gc.top)[0]=gc.alloc(vm_vec);
     auto& vec=gc.top[0].vec().elems;
diff --git a/nasal_builtin.h b/nasal_builtin.h
index f3b73b3..8b39648 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -293,14 +293,14 @@ nasal_ref builtin_split(nasal_ref* local,nasal_gc& gc)
     size_t source_len=source.length();
 
     // push it to local scope to avoid being sweeped
-    if(gc.top+1>=gc.stack+STACK_MAX_DEPTH-1)
+    if(gc.top+1>=gc.stack+nasal_gc::stack_depth-1)
         return builtin_err("split","expand temporary space error:stackoverflow");
     (++gc.top)[0]=gc.alloc(vm_vec);
 
     std::vector<nasal_ref>& vec=gc.top[0].vec().elems;
     if(!delimeter_len)
     {
-        for(int i=0;i<source_len;++i)
+        for(size_t i=0;i<source_len;++i)
         {
             vec.push_back(gc.alloc(vm_str));
             vec.back().str()=source[i];
@@ -310,11 +310,11 @@ nasal_ref builtin_split(nasal_ref* local,nasal_gc& gc)
     }
 
     std::string tmp="";
-    for(int i=0;i<source_len;++i)
+    for(size_t i=0;i<source_len;++i)
     {
         bool check_delimeter=false;
         if(source[i]==delimeter[0])
-            for(int j=0;j<delimeter_len;++j)
+            for(size_t j=0;j<delimeter_len;++j)
             {
                 if(i+j>=source_len || source[i+j]!=delimeter[j])
                     break;
@@ -598,7 +598,7 @@ nasal_ref builtin_keys(nasal_ref* local,nasal_gc& gc)
     if(hash.type!=vm_hash)
         return builtin_err("keys","\"hash\" must be hash");
     // push vector into local scope to avoid being sweeped
-    if(gc.top+1>=gc.stack+STACK_MAX_DEPTH-1)
+    if(gc.top+1>=gc.stack+nasal_gc::stack_depth-1)
         return builtin_err("keys","expand temporary space error:stackoverflow");
     (++gc.top)[0]=gc.alloc(vm_vec);
     auto& vec=gc.top[0].vec().elems;
@@ -1010,8 +1010,8 @@ nasal_ref builtin_sleep(nasal_ref* local,nasal_gc& gc)
 }
 nasal_ref builtin_pipe(nasal_ref* local,nasal_gc& gc)
 {
-    int fd[2];
 #ifndef _WIN32
+    int fd[2];
     nasal_ref res=gc.alloc(vm_vec);
     if(pipe(fd)==-1)
         return builtin_err("pipe","failed to create pipe");
@@ -1099,7 +1099,7 @@ nasal_ref builtin_chdir(nasal_ref* local,nasal_gc& gc)
 nasal_ref builtin_environ(nasal_ref* local,nasal_gc& gc)
 {
     char** env=environ;
-    if(gc.top+1>=gc.stack+STACK_MAX_DEPTH-1)
+    if(gc.top+1>=gc.stack+nasal_gc::stack_depth-1)
         return builtin_err("environ","expand temporary space error:stackoverflow");
     (++gc.top)[0]=gc.alloc(vm_vec);
     auto& vec=gc.top[0].vec().elems;
diff --git a/nasal_codegen.h b/nasal_codegen.h
index 6a5b8a7..9d95492 100644
--- a/nasal_codegen.h
+++ b/nasal_codegen.h
@@ -431,7 +431,7 @@ void nasal_codegen::func_gen(const nasal_ast& ast)
     block_gen(block);
     in_iterloop.pop();
     code[local_label].num=local.back().size();
-    if(local.back().size()>=STACK_MAX_DEPTH)
+    if(local.back().size()>=nasal_gc::stack_depth)
         die("too many local variants: "+std::to_string(local.back().size())+".",block.line());
     local.pop_back();
 
@@ -549,7 +549,7 @@ void nasal_codegen::mcall(const nasal_ast& ast)
         return;
     }
     calc_gen(ast[0]);
-    for(int i=1;i<ast.size()-1;++i)
+    for(size_t i=1;i<ast.size()-1;++i)
     {
         const nasal_ast& tmp=ast[i];
         switch(tmp.type())
@@ -1134,7 +1134,7 @@ void nasal_codegen::compile(const nasal_parse& parse,const nasal_import& import)
     gen(op_intg,global.size(),0);
     block_gen(parse.ast()); // generate main block
     gen(op_exit,0,0);
-    if(global.size()>=STACK_MAX_DEPTH)
+    if(global.size()>=nasal_gc::stack_depth)
         die("too many global variants: "+std::to_string(global.size())+".",0);
     nerr.chkerr();
 }
diff --git a/nasal_gc.h b/nasal_gc.h
index 9181ab0..a0af763 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -370,14 +370,15 @@ inline nasal_func&  nasal_ref::func (){return *value.gcobj->ptr.func; }
 inline nasal_upval& nasal_ref::upval(){return *value.gcobj->ptr.upval;}
 inline nasal_obj&   nasal_ref::obj  (){return *value.gcobj->ptr.obj;  }
 
-const uint32_t STACK_MAX_DEPTH=8191;
 const nasal_ref zero={vm_num,(double)0};
 const nasal_ref one ={vm_num,(double)1};
 const nasal_ref nil ={vm_nil,(double)0};
+
 struct nasal_gc
 {
+    static const uint32_t   stack_depth=8192;        // depth of value stack
     nasal_ref               funcr;                   // function register
-    nasal_ref               stack[STACK_MAX_DEPTH+1];// 1 reserved to avoid stack overflow
+    nasal_ref               stack[stack_depth];      // the last one is reserved to avoid stack overflow
     nasal_ref*              top;                     // stack top
     std::vector<nasal_ref>  strs;                    // reserved address for const vm_str
     std::vector<nasal_val*> memory;                  // gc memory
diff --git a/nasal_vm.h b/nasal_vm.h
index 8336b7d..dfbca83 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -135,7 +135,7 @@ void nasal_vm::init(
     files=filenames.data();
     files_size=filenames.size();
     /* set canary and program counter */
-    canary=gc.stack+STACK_MAX_DEPTH-1;
+    canary=gc.stack+nasal_gc::stack_depth-1; // gc.stack[nasal_gc::stack_depth-1]
     mem_addr=nullptr;
     pc=0;
     localr=nullptr;