from Pogo, Walt Kelly
Racoon is shooting configuration bugs.
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The NTP configuration process is driven by a phrase-structure grammar which is used to specify the format of the configuration commands and the actions needed to build an abstract syntax tree (AST). The grammar is fed to a parser generator (Bison) which produces a parser for the configuration file.
The generated parser is used to parse an NTP configuration file and check it for syntax and semantic errors. The result of the parse is an AST, which contains a representation of the various commands and options. This AST is then traversed to set up the NTP daemon to the correct configuration.
This document is intended for developers who wish to modify the configuration code and/or add configuration commands and options. It contains a description of the files used in the configuration process as well as guidelines on how to construct them.
A brief description of the files used by the configuration code is given below:
| File | Description |
|---|---|
| ntp_config.y | This file is a Bison source file that contains the phrase-structure grammar and the actions that need to be performed to generate an AST. |
| ntp_config.c | This file contains the major chunk of the configuration code. It contains all the functions that are called for building the AST as well as the functions that are needed for traversing the AST. |
| ntp_config.h | This file is the header file for ntp_config.c. It mainly contains the structure definitions needed to build the AST. |
| ntp_scanner.c | This file contains the code for a simple lexical analyzer. This file is directly included into the ntp_config.c file since this code is only used by the configuration code. The most important function in this file is yylex, which is called by the generated parser to get the next token on the input line. |
| ntp_data_structures.c | This file contains a generic implementation of a priority queue and a simple queue. This code can be used to create a queue for any structure. |
| ntp_data_structures.h | This header file contains the structure declarations and function prototypes needed to use the data structures defined in ntp_data_structures.c. This file forms the public interface of the data structures. |
| ntp_config.tab.c | This file is generated by Bison from the ntp_config.y file. This file is also included directly into the configuration code. |
A high-level description of the configuration process showing where all the files fit in is given below:
The scanner reads in an NTP configuration file and converts it into tokens. The Bison generated parser reads these tokens and converts them into an AST. The AST traverser consists of a set of functions that configure parts of NTP on the basis of what is on the tree. A more detailed description of these parts and the files used is given below: