Modern compiler design [PDF]

Default Actions
What happens when no regular expression matches the next characters in the input? Lex will execute the
default action, which merely prints the unmatched characters to standard out. The easiest way to understand
how default actions work is to assume that the lines:
. { printf("%s",yytext); }
\n { printf("%s", yytext); }
appear at the end of our lex file, to be used if no other rules match the input. These default rules allow us
to easily write lex code to make slight modifications to an input file. We only need to specify what changes
we want to make, and the rest of the file is left alone. Examples of using lex to transform files can be found
in Figures 2.6 and 2.7.
What if we do not want to use the default rules? All we need to do is make sure that our regular
expressions match all possible strings. If we have our own rules for the regular expressions “,” and “\n”,
then the default rules will never be used.
yywrap
When lex reaches the end of an input file, it calls the function int yywrap(void). If yywrap returns 1, then
yylex exits, returning a 0. If yywrap returns 0, lex will continue to try to match regular expressions to the
input file. We can use yywrap to allow lex to analyze several files one after the other, by changing the file
pointer yyin and returning 0. An example of using yywrap in this way can be found in the changedates.lex
file, in Figure 2.7. If we no not wish to use multiple input files, and we don’t need lex to do any cleanup
work when the end of the file is reached, then we can use the default version of yywrap:
int yyrwap(void) {
return 1;
}
If we do not include a definition of yywrap, lex is supposed to use this default definition. However, on
some implementations of lex, if we do not include the above definition in the C code segment of lex, the
lexical analyzer will not compile correctly.
Including a Main Program in Lex
Any C functions that we place after the final %% in the lex file will be copied directly into the C code that
lex generates. We can use this feature to create a stand alone program in lex, by including a main function
after the final %%. Consider the lex program in Figure 2.6. When we run lex on this file, and then compile
lex.yy.c, we get a complete, stand-alone program that removes the comments from a C source file. How
does this program work? After setting yyin (so that lex reads from a file instead of from standard input),
the main program calls yylex. Since none of the actions include a return statement, yylex will continue
matching expressions and executing their actions until the end of file is reached. Then the function yywrap
will be called, and yylex will end. The main program will then print out some statistics. Note that we could
have put the line
printf("/* Comments removed = %d */", commentsRemoved);
in the body of yywrap instead of in the main program, without changing the output of the program. Also,
since the regular expressions:
. { printf("%s",yytext); }
\n { printf("%s", yytext); }
just implement the default action, we could remove them without affecting the output of our program.
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