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From: andrew@frip.UUCP
Newsgroups: comp.unix.wizards,comp.lang.c,comp.unix.questions
Subject: Re: Lex Macros
Message-ID: <9399@tekecs.TEK.COM>
Date: Fri, 20-Nov-87 12:00:30 EST
Article-I.D.: tekecs.9399
Posted: Fri Nov 20 12:00:30 1987
Date-Received: Sun, 22-Nov-87 13:23:30 EST
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Organization: Tektronix, Wilsonville, Oregon
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Xref: utgpu comp.unix.wizards:5136 comp.lang.c:5216 comp.unix.questions:4460

[]

	"I would like some help in interpreting some of the macros used
	in Lex.  Below is the relevant portion of the C code produced
	by Lex.  The macros are quite "slick", but also somewhat
	difficult to understand.  Could someone please describe - in
	detail - how the macros "input()"  and  "unput()"
	operate?????"

Yysbuf is the unget buffer.  It's organized as a stack, growing and
shrinking at the right (high address).  To unget a character, append
it; to get a character, first look to see if yysbuf is nonempty, and if
so take (and remove) the last character.

Yysptr points to the position beyond the last character in the stack.
When yysptr==yysbuf, then stack is empty.

So:

# define input() (((yytchar=yysptr>yysbuf?U(*--yysptr):getc(yyin))==10?
    	    	    (yylineno++,yytchar):yytchar)==EOF?0:yytchar)

yysptr>yysbuf means the unget stack is nonempty, so set yytchar to the
last character in that stack and remove it by decrementing yysptr.
Otherwise the unget stack is empty, so invoke getc to read the next
character from the input file, and, if that character is '\n' (decimal
10 in ASCII), increment the line number in yylineno; if it's EOF,
return 0, else return the character.

# define unput(c) {yytchar= (c);if(yytchar=='\n')yylineno--;*yysptr++=yytchar;}

Just append the character to the unget stack, and decrement the line
number if ungetting a newline.

  -=- Andrew Klossner   (decvax!tektronix!tekecs!andrew)       [UUCP]
                        (andrew%tekecs.tek.com@relay.cs.net)   [ARPA]