Manual

Chapter 11: Frequently Asked Questions 113
}
char *snd = (yylex (), yylval);
printf ("\"%s\", \"%s\"\n", fst, snd);
return 0;
If you compile and run this code, you get:
$ flex -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf ’one\ntwo\n’ | ./split-lines
"one
two", "two"
this is because yytext is a buffer provided for reading in the action, but if you want to
keep it, you have to duplicate it (e.g., using strdup). Note that the output may depend
on how your implementation of Lex handles yytext. For instance, when given the Lex
compatibility option ‘-l’ (which triggers the option ‘%array’) Flex generates a different
behavior:
$ flex -l -osplit-lines.c split-lines.l
$ gcc -osplit-lines split-lines.c -ll
$ printf ’one\ntwo\n’ | ./split-lines
"two", "two"
11.4 Implementing Gotos/Loops
My simple calculator supports variables, assignments, and functions,
but how can I implement gotos, or loops
Although very pedagogical, the examples included in the document blur the distinction
to make between the parser—whose job is to recover the structure of a text and to transmit
it to subsequent modules of the program—and the processing (such as the execution) of this
structure. This works well with so called straight line programs, i.e., precisely those that
have a straightforward execution model: execute simple instructions one after the others.
If you want a richer model, you will probably need to use the parser to construct a tree
that does represent the structure it has recovered; this tree is usually called the abstract
syntax tree, or AST for short. Then, walking through this tree, traversing it in various
ways, will enable treatments such as its execution or its translation, which will result in an
interpreter or a compiler.
This topic is way beyond the scope of this manual, and the reader is invited to consult
the dedicated literature.
11.5 Multiple start-symbols
I have several closely related grammars, and I would like to share their
implementations. In fact, I could use a single grammar but with
multiple entry points.
Bison does not support multiple start-symbols, but there is a very simple means to
simulate them. If foo and bar are the two pseudo start-symbols, then introduce two new
tokens, say START_FOO and START_BAR, and use them as switches from the real start-symbol: