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Many sites offer tutorials on how to use things like flex, bison, and LLVM. However, these sites give code examples, with no links to the source documentation. Below I explain what each of these tools does, and explain how to access the documentation (which, with the exception of LLVM, is fairly well written).


Documentation can be accessed via “info flex” on a system with the flex-doc package installed. On ubuntu, run:

sudo apt install flex-doc
info flex

To get this documentation

A fast lexer, flex ( reads an input stream and breaks into tokens. Tokens have a identifier, and optionally a value. For example, the “if” keyword in C is a represented as the “IF” token. A string literal in C (“Hello world”) is represented as something like a LITERAL token, with a value of “Hello world”.

Notable things:

  1. yytext stores a pointer to either a character string or an array; can be selected via special directives
  2. yylen (maybe yyleng?) stores the length of the character string or array


Documentation: Or, this can be found using the info command, much like flex:

apt install bison-doc info bison

Bison is a LALR(1) grammar parser; much like yacc. It reads in a list of tokens (for example, generated from flex), and does stuff to them. For most compilers, you will generate a triple tree from the grammar; that is, a tree which represents the commands and the arguments. When using LLVM, this is where you would create the IR.

Bison tips

This all applies to the C++ Bison scaffolding.

Enable debugging

In our .yy file, place these things near the top:

%code requires
    #define YYDEBUG 1

Then in the class where you define your parser, do something like this:


Content © 2022 Charles Hathaway