[Mapserver-users] maplexer and mapparser

[Steve Lime]

Hi Andrew. The files maplexer.c and mapparser.c are generated
automatically by the tools flex and bison respectively, and in turn are
based on the configuration files maplexer.l and mapparser.y. I've never
looked at the C source for those files, nor would I want to. Any changes
made there would be lost anyway. Here's what each file is used for:

  - maplexer.l contains regular expressions and C snippets to break a
map file, certain CGI parameters and MapServer expressions into tokens.
These tokens are then handed off to other sections of the code to be
used. The lexer keeps track of where it is in a file or string and the
function msyylex simply gets the next token. The C code generated by
flex uses lots of global variables (and is not thread safe) to hold
various pieces of information about a token such as it's string
representation.

  - mapparser.y defines the grammar, the rules, for MapServer logical
expressions. It has nothing to do with parsing map files, only
expressions. It's cryptic to look at but once you figure it out it's not
bad. The grammar consists of things like setting operator precedance
(i.e evaluate * before +) and sets the rules for doing things like
adding 2 numbers, or comparing strings. The parser takes tokens from the
lexer and input. There's a function called msyyparse that evaluates any
particular string against the grammar.

Does this help?

Steve 

>>> "Andrew Arace" <AArace at geonetics.com> 12/18/2003 4:59:28 PM >>>
I'm currently working on some modifications to the MapServer source
code, and I've built the whole project in windows 2000, using the .NET
IDE.
I've got the exe, the necessary functions run great.
 
However, my current modifications require me to understand the
maplexer.c and mapparser.c.
I'm a pretty smart guy. But I'm a Unix noobie. (I know, some will
think
that's a contradiction...)
 
Anyway, I have NO IDEA what is going on in those files. No clue.
People
have told me what yacc is, and bison, but I don't have a very good
understanding, and these files are the root of all my confusion.
 
Basically, what are these used for. what is actually going on when the
mapfile parser calls switch(msyylex()) {...} (mapfile.c,
loadMapInternal())
 
What are the crazy int arrays in the maplexer? such as yy_accept[2174]

?  What are the maplexer.l and mapparser.y files?
 
I'm just in need of a thorough explanation, please please? 
 
Thanks very much in advance
 
--
Andrew Arace
aarace at geonetics.com 
Software Engineer
Geonetics, Inc
(617) 896 - 4409

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