Detecting and skipping line comments with Flex

Question

How can I detect one line comments like // in Flex and skip those lines?

Also, for /* comments, will the following snippet be enough?

Answer 1

For // you can read until you find the end of line \n or EOF, in case if the comment was at the end of file, for example:

static void
skip_single_line_comment(void)
{
  int c;

  /* Read until we find \n or EOF */
  while((c = input()) != '\n' && c != EOF)
    ;

  /* Maybe you want to place back EOF? */
  if(c == EOF)
    unput(c);
}

as for multiple lines comments /* */, you can read until you see * and peek the next character, if it's / this means this is the end of comment, if not just skip it with any other character. You shouldn't expect EOF, means unclosed comment:

static void
skip_multiple_line_comment(void)
{
  int c;

  for(;;)
  {
    switch(input())
    {
      /* We expect ending the comment first before EOF */
      case EOF:
        fprintf(stderr, "Error unclosed comment, expect */\n");
        exit(-1);
        goto done;
      break;
      /* Is it the end of comment? */
      case '*':
        if((c = input()) == '/')
          goto done;
        unput(c);
        break;
      default:
        /* skip this character */
        break;
    }
  }

done:
  /* exit entry */ ;
}

Complete file:

%{
#include 

static void skip_single_line_comment(void);
static void skip_multiple_line_comment(void);

%}

%option noyywrap

%%
"//"              { puts("short comment was skipped ");
                    skip_single_line_comment();}

"/*"              { puts("long comment begins ");
                    skip_multiple_line_comment();
                    puts("long comment ends");}

" "               { /* empty */ }
[\n|\r\n\t]       { /* empty */ }
.                 { fprintf(stderr, "Tokenizing error: '%c'\n", *yytext);
                    yyterminate(); }
%%

static void
skip_single_line_comment(void)
{
  int c;

  /* Read until we find \n or EOF */
  while((c = input()) != '\n' && c != EOF)
    ;

  /* Maybe you want to place back EOF? */
  if(c == EOF)
    unput(c);
}

static void
skip_multiple_line_comment(void)
{
  int c;

  for(;;)
  {
    switch(input())
    {
      /* We expect ending the comment first before EOF */
      case EOF:
        fprintf(stderr, "Error unclosed comment, expect */\n");
        exit(-1);
        goto done;
      break;
      /* Is it the end of comment? */
      case '*':
        if((c = input()) == '/')
          goto done;
        unput(c);
        break;
      default:
        /* skip this character */
        break;
    }
  }

done:
  /* exit entry */ ;
}

int main(int argc, char **argv)
{
  yylex();
  return 0;
}