4.6.1 Calling Convention for yylex

The value that yylex returns must be the positive numeric code for the type of token it has just found; a zero or negative value signifies end-of-input.

When a token is referred to in the grammar rules by a name, that name in the parser implementation 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 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, possibly converted to unsigned char to avoid sign-extension. The null character must not be used this way, because its code is zero and that signifies end-of-input.

Here is an example showing these things:

int
yylex (void)
{
  …
  if (c == EOF)    /* Detect end-of-input. */
    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.