How do I execute code only during initialization (only before the first scan)?

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You can specify an initial action by defining the macro YY_USER_INIT (though note that yyout may not be available at the time this macro is executed). Or you can add to the beginning of your rules section:

     %%
         /* Must be indented! */
         static int did_init = 0;
     
         if ( ! did_init ){
     do_my_init();
             did_init = 1;
         }