Flex: Use Text File as Input Stream

You have two ways of solving it. The first is to redirect input from standard input with the command prompt < operation:

> parser.exe < some_file.txt

The other solution is to let the program open the file, and tell the lexer what the file is. For more information about it see the Flex manual. The important functions are yy_create_buffer and yy_switch_to_buffer.

Try to add the following code to your *.l file.

int main(int argc, char *argv[])
{
    yyin = fopen(argv[1], "r");
    yylex();
    fclose(yyin);
}

Adding onto the above answer by @Eliko, while using flex with yacc/bison, you can define FILE *yyin; in the global part of your grammar.y file. The definition in the generated lex.yy.c is an extern FILE *yyin by default. Thus, in your grammar.y, do something like this:

/* Some other global definitions */
FILE *yyin;
%%
/* Grammar rules*/
/* Grammar rules*/
%%
void main(int argc, char **argv) {
  /* Process command line args*/
  yyin = fopen("input.c", "r");
  yyparse();
  fclose(yyin);
  return 0;
}