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Calling Convention for `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. Note: Symbols.
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:
if (c == EOF) /* Detect end of file. */
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'.
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