Simulink model cannot read from TXT file after being built into executable.

I used s-function block in my simulink model calling a C file, in which it should read from a TXT file. Here's the code of: static void mdlOutputs(SimStruct *S, int_T tid)

   const real_T *u = (const real_T*) ssGetInputPortSignal(S,0);
   real_T       *y = ssGetOutputPortSignal(S,0);

   char line[80];
   fr = fopen ("C:\\filename.txt", "r"); 

   while(fgets(line, 80, fr) != NULL)
   {
   scanf ("%c",line);
   }

   y[0] = (int)line[0];
   y[1] = (int)line[1];
   y[2] = (int)line[2];
   y[3] = (int)line[3];
   y[4] = (int)line[4];

   fclose(fr);

It runs with no problem as a model. However, after I compiled it into executable using real-time workshop(matlab coder), it cannnot read from that file anymore. How should I fix that?