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4.1 The Parser Function yyparse

You call the function yyparse to cause parsing to occur. This function reads tokens, executes actions, and ultimately returns when it encounters end-of-input or an unrecoverable syntax error. You can also write an action which directs yyparse to return immediately without reading further.

Function: int yyparse (void)

The value returned by yyparse is 0 if parsing was successful (return is due to end-of-input).

The value is 1 if parsing failed because of invalid input, i.e., input that contains a syntax error or that causes YYABORT to be invoked.

The value is 2 if parsing failed due to memory exhaustion.

In an action, you can cause immediate return from yyparse by using these macros:


Return immediately with value 0 (to report success).


Return immediately with value 1 (to report failure).


Return immediately with value 2 (to report memory exhaustion).

If you use a reentrant parser, you can optionally pass additional parameter information to it in a reentrant way. To do so, use the declaration %parse-param:

Directive: %parse-param {argument-declaration} …

Declare that one or more argument-declaration are additional yyparse arguments. The argument-declaration is used when declaring functions or prototypes. The last identifier in argument-declaration must be the argument name.

Here’s an example. Write this in the parser:

%parse-param {int *nastiness} {int *randomness}

Then call the parser like this:

  int nastiness, randomness;
  …  /* Store proper data in nastiness and randomness. */
  value = yyparse (&nastiness, &randomness);

In the grammar actions, use expressions like this to refer to the data:

exp: …    { …; *randomness += 1; … }

Using the following:

%parse-param {int *randomness}

Results in these signatures:

void yyerror (int *randomness, const char *msg);
int  yyparse (int *randomness);

Or, if both %define api.pure full (or just %define api.pure) and %locations are used:

void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
int  yyparse (int *randomness);

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