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flex-lexer

What does code block in flex-lexer rule section do?

I’m learning flex and met a issue about code block in rule section.

In flex’s manual http://westes.github.io/flex/manual/Comments-in-the-Input.html#Comments-in-the-Input, there’s a code block in rule section:

%{
/* code block */
%}

/* Definitions Section */
%x STATE_X

%%
    /* Rules Section */
ruleA   /* after regex */ { /* code block */ } /* after code block */
        /* Rules Section (indented) */
<STATE_X>{
ruleC   ECHO;
ruleD   ECHO;
%{
/* code block */
%}
}
%%
/* User Code Section */

You could see there’s a second code block between two %%, I have two questions:

  1. when will this code execute?
  2. what’s the difference between this and YY_USER_ACTION?
Solution

  • flex manual

    A code block in the rules section has unpredictable results unless:

    • It occurs before the first pattern, or

    • It contains nothing other than white space or comments.

    This particular code block consists only of white space and a comment. So the question of when it executes is pretty zen. (In the "sound of one hand clapping" sense.) It does nothing. When? Well, whenever. Nothing is hard to observe.

    YY_USER_ACTION happens just after the pattern is recognised, before the rule action (even if that action is empty). If you don't define YY_USER_ACTION, it also does nothing so I suppose there is no difference from a comment. But normally it's defined to do something, and it is inserted in every rule, not just one place. So that's completely different.