%{

#define YY_USER_INIT open_input_file()

extern FILE *yyin;

void open_input_file(void)

{

  char *file_name,buffer[1024];

  yyin      = NULL; 

  while(yyin == NULL){

    printf("Input file: ");

    file_name = fgets(buffer,1024,stdin);

    if(file_name){

      file_name[strlen(file_name)-1] = '\0';

      yyin = fopen(file_name,"r");

      if(yyin == NULL){

        printf("Unable to open \"%s\"\n",file_name);

      }

    } else {

      printf("stdin\n");

      yyin = stdin;

      break;

    }

  }

}

%}

%%