use effective way to check if file exists

nasal_err.h

@@ -46,10 +46,10 @@ private:
     uint32_t error;
 public:
     nasal_err():error(0){}
-    void err(const char* stage,const std::string& info)
+    void err(const char* stage,const std::string& info,const char end='\n')
     {
         ++error;
-        std::cerr<<"["<<stage<<"] "<<info<<'\n';
+        std::cerr<<"["<<stage<<"] "<<info<<end;
     }
     void err(const char* stage,uint32_t line,uint32_t column,const std::string& info)
     {

nasal_import.h

@@ -66,6 +66,11 @@ nasal_ast nasal_import::file_import(nasal_ast& node)
     // avoid infinite loading loop
     if(check_exist(filename))
         return {0,ast_root};
+    if(access(filename.c_str(),F_OK)==-1)
+    {
+        nerr.err("link","cannot open file <"+filename+">");
+        return {0,ast_root};
+    }
     
     // start importing...
     lex.scan(filename);
@@ -86,17 +91,21 @@ nasal_ast nasal_import::lib_import()
         "stl/lib.nas"
     };
 
-    std::string filename;
+    std::string filename="";
     for(auto& i:libpath)
-    {
+        if(access(i.c_str(),F_OK)!=-1)
-        std::ifstream fin(i);
-        if(!fin.fail())
         {
             filename=i;
-            fin.close();
             break;
         }
-        fin.close();
+    if(!filename.length())
+    {
+        std::string paths="";
+        for(auto& i:libpath)
+            paths+="  "+i+"\n";
+        nerr.err("link","cannot find lib file in these paths:\n"+paths,' ');
+        nerr.chkerr();
+        return {0,ast_root};
     }
 
     // avoid infinite loading loop