Yacc turns the specification file into a C program, which
parses the input according to the specification given.
The algorithm used to go from the
specification to the parser is complex, and will not be discussed
the references for more information).
The parser itself, however, is relatively simple,
and understanding how it works, while
not strictly necessary, will nevertheless make
treatment of error recovery and ambiguities much more
The parser produced by Yacc consists
of a finite state machine with a stack.
The parser is also capable of reading and remembering the next
is always the one on the top of the stack.
The states of the finite state machine are given
small integer labels; initially, the machine is in state 0,
the stack contains only state 0, and no lookahead token has been read.
The machine has only four actions available to it, called
A move of the parser is done as follows:
Based on its current state, the parser decides
whether it needs a lookahead token to decide
what action should be done; if it needs one, and does
to obtain the next token.
Using the current state, and the lookahead token
if needed, the parser decides on its next action, and
This may result in states being pushed onto the stack, or popped off of
the stack, and in the lookahead token being processed
action is the most common action the parser takes.
Whenever a shift action is taken, there is always
For example, in state 56 there may be
which says, in state 56, if the lookahead token is IF,
the current state (56) is pushed down on the stack,
and state 34 becomes the current state (on the
The lookahead token is cleared.
action keeps the stack from growing without
Reduce actions are appropriate when the parser has seen
the right hand side of a grammar rule,
and is prepared to announce that it has seen
an instance of the rule, replacing the right hand side
It may be necessary to consult the lookahead token
to decide whether to reduce, but
usually it is not; in fact, the default
action (represented by a ``.'') is often a reduce action.
Reduce actions are associated with individual grammar rules.
Grammar rules are also given small integer
numbers, leading to some confusion.
Suppose the rule being reduced is
The reduce action depends on the
left hand symbol (A in this case), and the number of
symbols on the right hand side (three in this case).
To reduce, first pop off the top three states
(In general, the number of states popped equals the number of symbols on the
In effect, these states were the ones
put on the stack while recognizing
and no longer serve any useful purpose.
After popping these states, a state is uncovered
which was the state the parser was in before beginning to
Using this uncovered state, and the symbol
on the left side of the rule, perform what is in
A new state is obtained, pushed onto the stack, and parsing continues.
There are significant differences between the processing of
the left hand symbol and an ordinary shift of a token,
however, so this action is called a
In particular, the lookahead token is cleared by a shift, and
is not affected by a goto.
In any case, the uncovered state contains an entry such as:
causing state 20 to be pushed
onto the stack, and become the current state.
In effect, the reduce action ``turns back the clock'' in the parse,
popping the states off the stack to go back to the
state where the right hand side of the rule was first seen.
The parser then behaves as if it had seen the left side at that time.
If the right hand side of the rule is empty,
no states are popped off of the stack: the uncovered state
is in fact the current state.
The reduce action is also important in the treatment of user-supplied
When a rule is reduced, the code supplied with the rule is executed
before the stack is adjusted.
In addition to the stack holding the states, another stack,
running in parallel with it, holds the values returned
from the lexical analyzer and the actions.
When a shift takes place, the external variable
is copied onto the value stack.
After the return from the user code, the reduction is carried out.
action is done, the external variable
is copied onto the value stack.
The pseudo-variables $1, $2, etc., refer to the value stack.
The other two parser actions are conceptually much simpler.
action indicates that the entire input has been seen and
that it matches the specification.
This action appears only when the lookahead token is
the endmarker, and indicates that the parser has successfully
action, on the other hand, represents a place where the parser
can no longer continue parsing according to the specification.
The input tokens it has seen, together with the lookahead token,
cannot be followed by anything that would result
The parser reports an error, and attempts to recover the situation and
resume parsing: the error recovery (as opposed to the detection of error)
will be covered in Section 7.
It is time for an example!
Consider the specification
When Yacc is invoked with the
is produced, with a human-readable description of the parser.
file corresponding to the above grammar (with some statistics
stripped off the end) is:
rhyme : sound place\_ (1)
Notice that, in addition to the actions for each state, there is a
description of the parsing rules being processed in each
state. The \_ character is used to indicate
what has been seen, and what is yet to come, in each rule.
It is instructive to follow the steps of the parser while
Initially, the current state is state 0.
The parser needs to refer to the input in order to decide
between the actions available in state 0, so
is read, becoming the lookahead token.
is ``shift 3'', so state 3 is pushed onto the stack,
and the lookahead token is cleared.
State 3 becomes the current state.
is read, becoming the lookahead token.
The action in state 3 on the token
so state 6 is pushed onto the stack, and the lookahead is cleared.
The stack now contains 0, 3, and 6.
In state 6, without even consulting the lookahead,
the parser reduces by rule 2.
This rule has two symbols on the right hand side, so
two states, 6 and 3, are popped off of the stack, uncovering state 0.
Consulting the description of state 0, looking for a goto on
is obtained; thus state 2 is pushed onto the stack,
becoming the current state.
In state 2, the next token,
The action is ``shift 5'', so state 5 is pushed onto the stack, which now has
0, 2, and 5 on it, and the lookahead token is cleared.
In state 5, the only action is to reduce by rule 3.
This has one symbol on the right hand side, so one state, 5,
is popped off, and state 2 is uncovered.
Now, the stack contains 0, 2, and 4.
In state 4, the only action is to reduce by rule 1.
There are two symbols on the right, so the top two states are popped off,
uncovering state 0 again.
In state 0, there is a goto on
causing the parser to enter state 1.
In state 1, the input is read; the endmarker is obtained,
indicated by ``$end'' in the
The action in state 1 when the endmarker is seen is to accept,
successfully ending the parse.
The reader is urged to consider how the parser works
when confronted with such incorrect strings as
.I "DING DONG DELL DELL" ,
A few minutes spend with this and other simple examples will
probably be repaid when problems arise in more complicated contexts.