%{
#include <stdio.h>
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_pos = 0;
FILE *tmp_file_ptr = NULL;
%}
%option main
%option yywrap
%x INCL
%%
^"#include"[ \t]+   BEGIN(INCL);
.|\n                ECHO;
<INCL>.*\n          { if(include_stack_pos >= MAX_INCLUDE_DEPTH) {
                        fprintf(stderr,"Includes nested too deeply!\n");
                        exit(1);
                      }
                      include_stack[include_stack_pos++] =
                        YY_CURRENT_BUFFER;
                      yytext[strlen(yytext)-1] = '\0'; /* remove '\n' */
                      if(!(tmp_file_ptr = fopen(yytext, "r"))) {
                        fprintf(stderr,
                          "Cannot open include file '%s'!\n", yytext);
                        exit(1);
                      }
                      yy_switch_to_buffer(
                        yy_create_buffer(tmp_file_ptr, YY_BUF_SIZE));
                    BEGIN(INITIAL);
                  }
%%
int yywrap() {
    if(--include_stack_pos < 0)
        return 1;
    else {
        yy_delete_buffer(YY_CURRENT_BUFFER);
        yy_switch_to_buffer(
           include_stack[include_stack_pos]);
        return 0;
    }
}