B. Glossary

Backus-Naur Form (BNF)

Formal method of specifying context-free grammars. BNF was first used in the ALGOL-60 report, 1963. See section Languages and Context-Free Grammars.

Context-free grammars

Grammars specified as rules that can be applied regardless of context. Thus, if there is a rule which says that an integer can be used as an expression, integers are allowed anywhere an expression is permitted. See section Languages and Context-Free Grammars.

Dynamic allocation

Allocation of memory that occurs during execution, rather than at compile time or on entry to a function.

Empty string

Analogous to the empty set in set theory, the empty string is a character string of length zero.

Finite-state stack machine

A "machine" that has discrete states in which it is said to exist at each instant in time. As input to the machine is processed, the machine moves from state to state as specified by the logic of the machine. In the case of the parser, the input is the language being parsed, and the states correspond to various stages in the grammar rules. See section The Bison Parser Algorithm .

Grouping

A language construct that is (in general) grammatically divisible; for example, `expression' or `declaration' in C. See section Languages and Context-Free Grammars.

Infix operator

An arithmetic operator that is placed between the operands on which it performs some operation.

Input stream

A continuous flow of data between devices or programs.

Language construct

One of the typical usage schemas of the language. For example, one of the constructs of the C language is the if statement. See section Languages and Context-Free Grammars.

Left associativity

Operators having left associativity are analyzed from left to right: `a+b+c' first computes `a+b' and then combines with `c'. See section Operator Precedence.

Left recursion

A rule whose result symbol is also its first component symbol; for example, `expseq1 : expseq1 ',' exp;'. See section Recursive Rules.

Left-to-right parsing

Parsing a sentence of a language by analyzing it token by token from left to right. See section The Bison Parser Algorithm .

Lexical analyzer (scanner)

A function that reads an input stream and returns tokens one by one. See section The Lexical Analyzer Function yylex.

Lexical tie-in

A flag, set by actions in the grammar rules, which alters the way tokens are parsed. See section 7.2 Lexical Tie-ins.

Literal string token

A token which constists of two or more fixed characters. See section 3.2 Symbols, Terminal and Nonterminal.

Look-ahead token

A token already read but not yet shifted. See section Look-Ahead Tokens.

LALR(1)

The class of context-free grammars that Bison (like most other parser generators) can handle; a subset of LR(1). See section Mysterious Reduce/Reduce Conflicts.

LR(1)

The class of context-free grammars in which at most one token of look-ahead is needed to disambiguate the parsing of any piece of input.

Nonterminal symbol

A grammar symbol standing for a grammatical construct that can be expressed through rules in terms of smaller constructs; in other words, a construct that is not a token. See section 3.2 Symbols, Terminal and Nonterminal.

Parse error

An error encountered during parsing of an input stream due to invalid syntax. See section 6. Error Recovery.

Parser

A function that recognizes valid sentences of a language by analyzing the syntax structure of a set of tokens passed to it from a lexical analyzer.

Postfix operator

An arithmetic operator that is placed after the operands upon which it performs some operation.

Reduction

Replacing a string of nonterminals and/or terminals with a single nonterminal, according to a grammar rule. See section The Bison Parser Algorithm .

Reentrant

A reentrant subprogram is a subprogram which can be in invoked any number of times in parallel, without interference between the various invocations. See section A Pure (Reentrant) Parser.

Reverse polish notation

A language in which all operators are postfix operators.

Right recursion

A rule whose result symbol is also its last component symbol; for example, `expseq1: exp ',' expseq1;'. See section Recursive Rules.

Semantics

In computer languages, the semantics are specified by the actions taken for each instance of the language, i.e., the meaning of each statement. See section Defining Language Semantics.

Shift

A parser is said to shift when it makes the choice of analyzing further input from the stream rather than reducing immediately some already-recognized rule. See section The Bison Parser Algorithm .

Single-character literal

A single character that is recognized and interpreted as is. See section From Formal Rules to Bison Input.

Start symbol

The nonterminal symbol that stands for a complete valid utterance in the language being parsed. The start symbol is usually listed as the first nonterminal symbol in a language specification. See section The Start-Symbol.

Symbol table

A data structure where symbol names and associated data are stored during parsing to allow for recognition and use of existing information in repeated uses of a symbol. See section 2.4 Multi-Function Calculator: mfcalc.

Token

A basic, grammatically indivisible unit of a language. The symbol that describes a token in the grammar is a terminal symbol. The input of the Bison parser is a stream of tokens which comes from the lexical analyzer. See section 3.2 Symbols, Terminal and Nonterminal.

Terminal symbol

A grammar symbol that has no rules in the grammar and therefore is grammatically indivisible. The piece of text it represents is a token. See section Languages and Context-Free Grammars.