13. End-of-file rules

The special rule "<<EOF>>" indicates actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., indicates no further files to process). The action must finish by doing one of four things:

- assigning yyin to a new input file (in previous versions of flex, after doing the assignment you had to call the special action YY_NEW_FILE; this is no longer necessary);
- executing a return statement;
- executing the special ‘yyterminate()’ action;
- or, switching to a new buffer using ‘yy_switch_to_buffer()’ as shown in the example above.

<<EOF>> rules may not be used with other patterns; they may only be qualified with a list of start conditions. If an unqualified <<EOF>> rule is given, it applies to all start conditions which do not already have <<EOF>> actions. To specify an <<EOF>> rule for only the initial start condition, use

<INITIAL><<EOF>>

These rules are useful for catching things like unclosed comments. An example:

%x quote
%%

…other rules for dealing with quotes…

<quote><<EOF>>   {
         error( "unterminated quote" );
         yyterminate();
         }
<<EOF>>  {
         if ( *++filelist )
             yyin = fopen( *filelist, "r" );
         else
            yyterminate();
         }

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