yylex
The value that yylex
returns must be the numeric code for the type
of token it has just found, or 0 for end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
numeric code for that token type. So yylex
can use the name
to indicate that type. See section Symbols, Terminal and Nonterminal.
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
So yylex
can simply return that character code. The null character
must not be used this way, because its code is zero and that is what
signifies end-of-input.
Here is an example showing these things:
yylex () { ... if (c == EOF) /* Detect end of file. */ return 0; ... if (c == '+' || c == '-') return c; /* Assume token type for `+' is '+'. */ ... return INT; /* Return the type of the token. */ ... }
This interface has been designed so that the output from the lex
utility can be used without change as the definition of yylex
.