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File: flex.info, Node: Top, Next: Copyright, Prev: (dir), Up: (dir)
flex
****
This manual describes `flex', a tool for generating programs that
perform pattern-matching on text. The manual includes both tutorial and
reference sections.
This edition of `The flex Manual' documents `flex' version 2.5.31.
It was last updated on 27 March 2003.
* Menu:
* Copyright::
* Reporting Bugs::
* Introduction::
* Simple Examples::
* Format::
* Patterns::
* Matching::
* Actions::
* Generated Scanner::
* Start Conditions::
* Multiple Input Buffers::
* EOF::
* Misc Macros::
* User Values::
* Yacc::
* Scanner Options::
* Performance::
* Cxx::
* Reentrant::
* Lex and Posix::
* Memory Management::
* Serialized Tables::
* Diagnostics::
* Limitations::
* Bibliography::
* FAQ::
* Appendices::
* Indices::
--- The Detailed Node Listing ---
Format of the Input File
* Definitions Section::
* Rules Section::
* User Code Section::
* Comments in the Input::
Scanner Options
* Options for Specifing Filenames::
* Options Affecting Scanner Behavior::
* Code-Level And API Options::
* Options for Scanner Speed and Size::
* Debugging Options::
* Miscellaneous Options::
Reentrant C Scanners
* Reentrant Uses::
* Reentrant Overview::
* Reentrant Example::
* Reentrant Detail::
* Reentrant Functions::
The Reentrant API in Detail
* Specify Reentrant::
* Extra Reentrant Argument::
* Global Replacement::
* Init and Destroy Functions::
* Accessor Methods::
* Extra Data::
* About yyscan_t::
Memory Management
* The Default Memory Management::
* Overriding The Default Memory Management::
* A Note About yytext And Memory::
Serialized Tables
* Creating Serialized Tables::
* Loading and Unloading Serialized Tables::
* Tables File Format::
FAQ
* When was flex born?::
* How do I expand \ escape sequences in C-style quoted strings?::
* Why do flex scanners call fileno if it is not ANSI compatible?::
* Does flex support recursive pattern definitions?::
* How do I skip huge chunks of input (tens of megabytes) while using flex?::
* Flex is not matching my patterns in the same order that I defined them.::
* My actions are executing out of order or sometimes not at all.::
* How can I have multiple input sources feed into the same scanner at the same time?::
* Can I build nested parsers that work with the same input file?::
* How can I match text only at the end of a file?::
* How can I make REJECT cascade across start condition boundaries?::
* Why cant I use fast or full tables with interactive mode?::
* How much faster is -F or -f than -C?::
* If I have a simple grammar cant I just parse it with flex?::
* Why doesnt yyrestart() set the start state back to INITIAL?::
* How can I match C-style comments?::
* The period isnt working the way I expected.::
* Can I get the flex manual in another format?::
* Does there exist a "faster" NDFA->DFA algorithm?::
* How does flex compile the DFA so quickly?::
* How can I use more than 8192 rules?::
* How do I abandon a file in the middle of a scan and switch to a new file?::
* How do I execute code only during initialization (only before the first scan)?::
* How do I execute code at termination?::
* Where else can I find help?::
* Can I include comments in the "rules" section of the file?::
* I get an error about undefined yywrap().::
* How can I change the matching pattern at run time?::
* How can I expand macros in the input?::
* How can I build a two-pass scanner?::
* How do I match any string not matched in the preceding rules?::
* I am trying to port code from AT&T lex that uses yysptr and yysbuf.::
* Is there a way to make flex treat NULL like a regular character?::
* Whenever flex can not match the input it says "flex scanner jammed".::
* Why doesnt flex have non-greedy operators like perl does?::
* Memory leak - 16386 bytes allocated by malloc.::
* How do I track the byte offset for lseek()?::
* How do I use my own I/O classes in a C++ scanner?::
* How do I skip as many chars as possible?::
* deleteme00::
* Are certain equivalent patterns faster than others?::
* Is backing up a big deal?::
* Can I fake multi-byte character support?::
* deleteme01::
* Can you discuss some flex internals?::
* unput() messes up yy_at_bol::
* The | operator is not doing what I want::
* Why can't flex understand this variable trailing context pattern?::
* The ^ operator isn't working::
* Trailing context is getting confused with trailing optional patterns::
* Is flex GNU or not?::
* ERASEME53::
* I need to scan if-then-else blocks and while loops::
* ERASEME55::
* ERASEME56::
* ERASEME57::
* Is there a repository for flex scanners?::
* How can I conditionally compile or preprocess my flex input file?::
* Where can I find grammars for lex and yacc?::
* I get an end-of-buffer message for each character scanned.::
* unnamed-faq-62::
* unnamed-faq-63::
* unnamed-faq-64::
* unnamed-faq-65::
* unnamed-faq-66::
* unnamed-faq-67::
* unnamed-faq-68::
* unnamed-faq-69::
* unnamed-faq-70::
* unnamed-faq-71::
* unnamed-faq-72::
* unnamed-faq-73::
* unnamed-faq-74::
* unnamed-faq-75::
* unnamed-faq-76::
* unnamed-faq-77::
* unnamed-faq-78::
* unnamed-faq-79::
* unnamed-faq-80::
* unnamed-faq-81::
* unnamed-faq-82::
* unnamed-faq-83::
* unnamed-faq-84::
* unnamed-faq-85::
* unnamed-faq-86::
* unnamed-faq-87::
* unnamed-faq-88::
* unnamed-faq-90::
* unnamed-faq-91::
* unnamed-faq-92::
* unnamed-faq-93::
* unnamed-faq-94::
* unnamed-faq-95::
* unnamed-faq-96::
* unnamed-faq-97::
* unnamed-faq-98::
* unnamed-faq-99::
* unnamed-faq-100::
* unnamed-faq-101::
Appendices
* Makefiles and Flex::
* Bison Bridge::
* M4 Dependency::
Indices
* Concept Index::
* Index of Functions and Macros::
* Index of Variables::
* Index of Data Types::
* Index of Hooks::
* Index of Scanner Options::
File: flex.info, Node: Copyright, Next: Reporting Bugs, Prev: Top, Up: Top
1 Copyright
***********
The flex manual is placed under the same licensing conditions as the
rest of flex:
Copyright (C) 1990, 1997 The Regents of the University of California.
All rights reserved.
This code is derived from software contributed to Berkeley by Vern
Paxson.
The United States Government has rights in this work pursuant to
contract no. DE-AC03-76SF00098 between the United States Department of
Energy and the University of California.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the
distribution.
Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
File: flex.info, Node: Reporting Bugs, Next: Introduction, Prev: Copyright, Up: Top
2 Reporting Bugs
****************
If you have problems with `flex' or think you have found a bug, please
send mail detailing your problem to <lex-help AT lists.net>.
Patches are always welcome.
File: flex.info, Node: Introduction, Next: Simple Examples, Prev: Reporting Bugs, Up: Top
3 Introduction
**************
`flex' is a tool for generating "scanners". A scanner is a program
which recognizes lexical patterns in text. The `flex' program reads
the given input files, or its standard input if no file names are
given, for a description of a scanner to generate. The description is
in the form of pairs of regular expressions and C code, called "rules".
`flex' generates as output a C source file, `lex.yy.c' by default,
which defines a routine `yylex()'. This file can be compiled and
linked with the flex runtime library to produce an executable. When
the executable is run, it analyzes its input for occurrences of the
regular expressions. Whenever it finds one, it executes the
corresponding C code.
File: flex.info, Node: Simple Examples, Next: Format, Prev: Introduction, Up: Top
4 Some Simple Examples
**********************
First some simple examples to get the flavor of how one uses `flex'.
The following `flex' input specifies a scanner which, when it
encounters the string `username' will replace it with the user's login
name:
%%
username printf( "%s", getlogin() );
By default, any text not matched by a `flex' scanner is copied to
the output, so the net effect of this scanner is to copy its input file
to its output with each occurrence of `username' expanded. In this
input, there is just one rule. `username' is the "pattern" and the
`printf' is the "action". The `%%' symbol marks the beginning of the
rules.
Here's another simple example:
int num_lines = 0, num_chars = 0;
%%
\n ++num_lines; ++num_chars;
. ++num_chars;
%%
main()
{
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );
}
This scanner counts the number of characters and the number of lines
in its input. It produces no output other than the final report on the
character and line counts. The first line declares two globals,
`num_lines' and `num_chars', which are accessible both inside `yylex()'
and in the `main()' routine declared after the second `%%'. There are
two rules, one which matches a newline (`\n') and increments both the
line count and the character count, and one which matches any character
other than a newline (indicated by the `.' regular expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include math.h>
%}
DIGIT [0-9]
ID [a-z][a-z0-9]*
%%
{DIGIT}+ {
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );
}
{DIGIT}+"."{DIGIT}* {
printf( "A float: %s (%g)\n", yytext,
atof( yytext ) );
}
if|then|begin|end|procedure|function {
printf( "A keyword: %s\n", yytext );
}
{ID} printf( "An identifier: %s\n", yytext );
"+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext );
"{"[\^{}}\n]*"}" /* eat up one-line comments */
[ \t\n]+ /* eat up whitespace */
. printf( "Unrecognized character: %s\n", yytext );
%%
main( argc, argv )
int argc;
char **argv;
{
++argv, --argc; /* skip over program name */
if ( argc > 0 )
yyin = fopen( argv[0], "r" );
else
yyin = stdin;
yylex();
}
This is the beginnings of a simple scanner for a language like
Pascal. It identifies different types of "tokens" and reports on what
it has seen.
The details of this example will be explained in the following
sections.
File: flex.info, Node: Format, Next: Patterns, Prev: Simple Examples, Up: Top
5 Format of the Input File
**************************
The `flex' input file consists of three sections, separated by a line
containing only `%%'.
definitions
%%
rules
%%
user code
* Menu:
* Definitions Section::
* Rules Section::
* User Code Section::
* Comments in the Input::
File: flex.info, Node: Definitions Section, Next: Rules Section, Prev: Format, Up: Format
5.1 Format of the Definitions Section
=====================================
The "definitions section" contains declarations of simple "name"
definitions to simplify the scanner specification, and declarations of
"start conditions", which are explained in a later section.
Name definitions have the form:
name definition
The `name' is a word beginning with a letter or an underscore (`_')
followed by zero or more letters, digits, `_', or `-' (dash). The
definition is taken to begin at the first non-whitespace character
following the name and continuing to the end of the line. The
definition can subsequently be referred to using `{name}', which will
expand to `(definition)'. For example,
DIGIT [0-9]
ID [a-z][a-z0-9]*
Defines `DIGIT' to be a regular expression which matches a single
digit, and `ID' to be a regular expression which matches a letter
followed by zero-or-more letters-or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a `.' followed by
zero-or-more digits.
An unindented comment (i.e., a line beginning with `/*') is copied
verbatim to the output up to the next `*/'.
Any _indented_ text or text enclosed in `%{' and `%}' is also copied
verbatim to the output (with the %{ and %} symbols removed). The %{
and %} symbols must appear unindented on lines by themselves.
A `%top' block is similar to a `%{' ... `%}' block, except that the
code in a `%top' block is relocated to the _top_ of the generated file,
before any flex definitions (1). The `%top' block is useful when you
want certain preprocessor macros to be defined or certain files to be
included before the generated code. The single characters, `{' and
`}' are used to delimit the `%top' block, as show in the example below:
%top{
/* This code goes at the "top" of the generated file. */
#include <stdint.h>
#include <inttypes.h>
}
Multiple `%top' blocks are allowed, and their order is preserved.
---------- Footnotes ----------
(1) Actually, `yyIN_HEADER' is defined before the `%top' block.
File: flex.info, Node: Rules Section, Next: User Code Section, Prev: Definitions Section, Up: Format
5.2 Format of the Rules Section
===============================
The "rules" section of the `flex' input contains a series of rules of
the form:
pattern action
where the pattern must be unindented and the action must begin on
the same line. *Note Patterns::, for a further description of patterns
and actions.
In the rules section, any indented or %{ %} enclosed text appearing
before the first rule may be used to declare variables which are local
to the scanning routine and (after the declarations) code which is to be
executed whenever the scanning routine is entered. Other indented or
%{ %} text in the rule section is still copied to the output, but its
meaning is not well-defined and it may well cause compile-time errors
(this feature is present for POSIX compliance. *Note Lex and Posix::,
for other such features).
Any _indented_ text or text enclosed in `%{' and `%}' is copied
verbatim to the output (with the %{ and %} symbols removed). The %{
and %} symbols must appear unindented on lines by themselves.
File: flex.info, Node: User Code Section, Next: Comments in the Input, Prev: Rules Section, Up: Format
5.3 Format of the User Code Section
===================================
The user code section is simply copied to `lex.yy.c' verbatim. It is
used for companion routines which call or are called by the scanner.
The presence of this section is optional; if it is missing, the second
`%%' in the input file may be skipped, too.
File: flex.info, Node: Comments in the Input, Prev: User Code Section, Up: Format
5.4 Comments in the Input
=========================
Flex supports C-style comments, that is, anything between /* and */ is
considered a comment. Whenever flex encounters a comment, it copies the
entire comment verbatim to the generated source code. Comments may
appear just about anywhere, but with the following exceptions:
* Comments may not appear in the Rules Section wherever flex is
expecting a regular expression. This means comments may not appear
at the beginning of a line, or immediately following a list of
scanner states.
* Comments may not appear on an `%option' line in the Definitions
Section.
If you want to follow a simple rule, then always begin a comment on a
new line, with one or more whitespace characters before the initial
`/*'). This rule will work anywhere in the input file.
All the comments in the following example are valid:
%{
/* code block */
%}
/* Definitions Section */
%x STATE_X
%%
/* Rules Section */
ruleA /* after regex */ { /* code block */ } /* after code block */
/* Rules Section (indented) */
<STATE_X>{
ruleC ECHO;
ruleD ECHO;
%{
/* code block */
%}
}
%%
/* User Code Section */
File: flex.info, Node: Patterns, Next: Matching, Prev: Format, Up: Top
6 Patterns
**********
The patterns in the input (see *Note Rules Section::) are written using
an extended set of regular expressions. These are:
`x'
match the character 'x'
`.'
any character (byte) except newline
`[xyz]'
a "character class"; in this case, the pattern matches either an
'x', a 'y', or a 'z'
`[abj-oZ]'
a "character class" with a range in it; matches an 'a', a 'b', any
letter from 'j' through 'o', or a 'Z'
`[^A-Z]'
a "negated character class", i.e., any character but those in the
class. In this case, any character EXCEPT an uppercase letter.
`[^A-Z\n]'
any character EXCEPT an uppercase letter or a newline
`r*'
zero or more r's, where r is any regular expression
`r+'
one or more r's
`r?'
zero or one r's (that is, "an optional r")
`r{2,5}'
anywhere from two to five r's
`r{2,}'
two or more r's
`r{4}'
exactly 4 r's
`{name}'
the expansion of the `name' definition (*note Format::).
`"[xyz]\"foo"'
the literal string: `[xyz]"foo'
`\X'
if X is `a', `b', `f', `n', `r', `t', or `v', then the ANSI-C
interpretation of `\x'. Otherwise, a literal `X' (used to escape
operators such as `*')
`\0'
a NUL character (ASCII code 0)
`\123'
the character with octal value 123
`\x2a'
the character with hexadecimal value 2a
`(r)'
match an `r'; parentheses are used to override precedence (see
below)
`rs'
the regular expression `r' followed by the regular expression `s';
called "concatenation"
`r|s'
either an `r' or an `s'
`r/s'
an `r' but only if it is followed by an `s'. The text matched by
`s' is included when determining whether this rule is the longest
match, but is then returned to the input before the action is
executed. So the action only sees the text matched by `r'. This
type of pattern is called "trailing context". (There are some
combinations of `r/s' that flex cannot match correctly. *Note
Limitations::, regarding dangerous trailing context.)
`^r'
an `r', but only at the beginning of a line (i.e., when just
starting to scan, or right after a newline has been scanned).
`r$'
an `r', but only at the end of a line (i.e., just before a
newline). Equivalent to `r/\n'.
Note that `flex''s notion of "newline" is exactly whatever the C
compiler used to compile `flex' interprets `\n' as; in particular,
on some DOS systems you must either filter out `\r's in the input
yourself, or explicitly use `r/\r\n' for `r$'.
`<s>r'
an `r', but only in start condition `s' (see *Note Start
Conditions:: for discussion of start conditions).
`<s1,s2,s3>r'
same, but in any of start conditions `s1', `s2', or `s3'.
`<*>r'
an `r' in any start condition, even an exclusive one.
`<<EOF>>'
an end-of-file.
`<s1,s2><<EOF>>'
an end-of-file when in start condition `s1' or `s2'
Note that inside of a character class, all regular expression
operators lose their special meaning except escape (`\') and the
character class operators, `-', `]]', and, at the beginning of the
class, `^'.
The regular expressions listed above are grouped according to
precedence, from highest precedence at the top to lowest at the bottom.
Those grouped together have equal precedence (see special note on the
precedence of the repeat operator, `{}', under the documentation for
the `--posix' POSIX compliance option). For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the `*' operator has higher precedence than concatenation, and
concatenation higher than alternation (`|'). This pattern therefore
matches _either_ the string `foo' _or_ the string `ba' followed by
zero-or-more `r''s. To match `foo' or zero-or-more repetitions of the
string `bar', use:
foo|(bar)*
And to match a sequence of zero or more repetitions of `foo' and
`bar':
(foo|bar)*
In addition to characters and ranges of characters, character classes
can also contain "character class expressions". These are expressions
enclosed inside `[': and `:]' delimiters (which themselves must appear
between the `[' and `]' of the character class. Other elements may
occur inside the character class, too). The valid expressions are:
[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]
These expressions all designate a set of characters equivalent to the
corresponding standard C `isXXX' function. For example, `[:alnum:]'
designates those characters for which `isalnum()' returns true - i.e.,
any alphabetic or numeric character. Some systems don't provide
`isblank()', so flex defines `[:blank:]' as a blank or a tab.
For example, the following character classes are all equivalent:
[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:][0-9]]
[a-zA-Z0-9]
Some notes on patterns are in order.
* If your scanner is case-insensitive (the `-i' flag), then
`[:upper:]' and `[:lower:]' are equivalent to `[:alpha:]'.
* Character classes with ranges, such as `[a-Z]', should be used with
caution in a case-insensitive scanner if the range spans upper or
lowercase characters. Flex does not know if you want to fold all
upper and lowercase characters together, or if you want the
literal numeric range specified (with no case folding). When in
doubt, flex will assume that you meant the literal numeric range,
and will issue a warning. The exception to this rule is a
character range such as `[a-z]' or `[S-W]' where it is obvious
that you want case-folding to occur. Here are some examples with
the `-i' flag enabled:
Range Result Literal Range Alternate Range
`[a-t]' ok `[a-tA-T]'
`[A-T]' ok `[a-tA-T]'
`[A-t]' ambiguous `[A-Z\[\\\]_`a-t]' `[a-tA-T]'
`[_-{]' ambiguous `[_`a-z{]' `[_`a-zA-Z{]'
`[@-C]' ambiguous `[@ABC]' `[@A-Z\[\\\]_`abc]'
* A negated character class such as the example `[^A-Z]' above
_will_ match a newline unless `\n' (or an equivalent escape
sequence) is one of the characters explicitly present in the
negated character class (e.g., `[^A-Z\n]'). This is unlike how
many other regular expression tools treat negated character
classes, but unfortunately the inconsistency is historically
entrenched. Matching newlines means that a pattern like `[^"]*'
can match the entire input unless there's another quote in the
input.
* A rule can have at most one instance of trailing context (the `/'
operator or the `$' operator). The start condition, `^', and
`<<EOF>>' patterns can only occur at the beginning of a pattern,
and, as well as with `/' and `$', cannot be grouped inside
parentheses. A `^' which does not occur at the beginning of a
rule or a `$' which does not occur at the end of a rule loses its
special properties and is treated as a normal character.
* The following are invalid:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these can be written `foo/bar\n'.
* The following will result in `$' or `^' being treated as a normal
character:
foo|(bar$)
foo|^bar
If the desired meaning is a `foo' or a
`bar'-followed-by-a-newline, the following could be used (the
special `|' action is explained below, *note Actions::):
foo |
bar$ /* action goes here */
A similar trick will work for matching a `foo' or a
`bar'-at-the-beginning-of-a-line.
File: flex.info, Node: Matching, Next: Actions, Prev: Patterns, Up: Top
7 How the Input Is Matched
**************************
When the generated scanner is run, it analyzes its input looking for
strings which match any of its patterns. If it finds more than one
match, it takes the one matching the most text (for trailing context
rules, this includes the length of the trailing part, even though it
will then be returned to the input). If it finds two or more matches of
the same length, the rule listed first in the `flex' input file is
chosen.
Once the match is determined, the text corresponding to the match
(called the "token") is made available in the global character pointer
`yytext', and its length in the global integer `yyleng'. The "action"
corresponding to the matched pattern is then executed (*note
Actions::), and then the remaining input is scanned for another match.
If no match is found, then the "default rule" is executed: the next
character in the input is considered matched and copied to the standard
output. Thus, the simplest valid `flex' input is:
%%
which generates a scanner that simply copies its input (one
character at a time) to its output.
Note that `yytext' can be defined in two different ways: either as a
character _pointer_ or as a character _array_. You can control which
definition `flex' uses by including one of the special directives
`%pointer' or `%array' in the first (definitions) section of your flex
input. The default is `%pointer', unless you use the `-l' lex
compatibility option, in which case `yytext' will be an array. The
advantage of using `%pointer' is substantially faster scanning and no
buffer overflow when matching very large tokens (unless you run out of
dynamic memory). The disadvantage is that you are restricted in how
your actions can modify `yytext' (*note Actions::), and calls to the
`unput()' function destroys the present contents of `yytext', which can
be a considerable porting headache when moving between different `lex'
versions.
The advantage of `%array' is that you can then modify `yytext' to
your heart's content, and calls to `unput()' do not destroy `yytext'
(*note Actions::). Furthermore, existing `lex' programs sometimes
access `yytext' externally using declarations of the form:
extern char yytext[];
This definition is erroneous when used with `%pointer', but correct
for `%array'.
The `%array' declaration defines `yytext' to be an array of `YYLMAX'
characters, which defaults to a fairly large value. You can change the
size by simply #define'ing `YYLMAX' to a different value in the first
section of your `flex' input. As mentioned above, with `%pointer'
yytext grows dynamically to accommodate large tokens. While this means
your `%pointer' scanner can accommodate very large tokens (such as
matching entire blocks of comments), bear in mind that each time the
scanner must resize `yytext' it also must rescan the entire token from
the beginning, so matching such tokens can prove slow. `yytext'
presently does _not_ dynamically grow if a call to `unput()' results in
too much text being pushed back; instead, a run-time error results.
Also note that you cannot use `%array' with C++ scanner classes
(*note Cxx::).
File: flex.info, Node: Actions, Next: Generated Scanner, Prev: Matching, Up: Top
8 Actions
*********
Each pattern in a rule has a corresponding "action", which can be any
arbitrary C statement. The pattern ends at the first non-escaped
whitespace character; the remainder of the line is its action. If the
action is empty, then when the pattern is matched the input token is
simply discarded. For example, here is the specification for a program
which deletes all occurrences of `zap me' from its input:
%%
"zap me"
This example will copy all other characters in the input to the
output since they will be matched by the default rule.
Here is a program which compresses multiple blanks and tabs down to a
single blank, and throws away whitespace found at the end of a line:
%%
[ \t]+ putchar( ' ' );
[ \t]+$ /* ignore this token */
If the action contains a `}', then the action spans till the
balancing `}' is found, and the action may cross multiple lines.
`flex' knows about C strings and comments and won't be fooled by braces
found within them, but also allows actions to begin with `%{' and will
consider the action to be all the text up to the next `%}' (regardless
of ordinary braces inside the action).
An action consisting solely of a vertical bar (`|') means "same as
the action for the next rule". See below for an illustration.
Actions can include arbitrary C code, including `return' statements
to return a value to whatever routine called `yylex()'. Each time
`yylex()' is called it continues processing tokens from where it last
left off until it either reaches the end of the file or executes a
return.
Actions are free to modify `yytext' except for lengthening it
(adding characters to its end-these will overwrite later characters in
the input stream). This however does not apply when using `%array'
(*note Matching::). In that case, `yytext' may be freely modified in
any way.
Actions are free to modify `yyleng' except they should not do so if
the action also includes use of `yymore()' (see below).
There are a number of special directives which can be included
within an action:
`ECHO'
copies yytext to the scanner's output.
`BEGIN'
followed by the name of a start condition places the scanner in the
corresponding start condition (see below).
`REJECT'
directs the scanner to proceed on to the "second best" rule which
matched the input (or a prefix of the input). The rule is chosen
as described above in *Note Matching::, and `yytext' and `yyleng'
set up appropriately. It may either be one which matched as much
text as the originally chosen rule but came later in the `flex'
input file, or one which matched less text. For example, the
following will both count the words in the input and call the
routine `special()' whenever `frob' is seen:
int word_count = 0;
%%
frob special(); REJECT;
[^ \t\n]+ ++word_count;
Without the `REJECT', any occurences of `frob' in the input would
not be counted as words, since the scanner normally executes only
one action per token. Multiple uses of `REJECT' are allowed, each
one finding the next best choice to the currently active rule. For
example, when the following scanner scans the token `abcd', it will
write `abcdabcaba' to the output:
%%
a |
ab |
abc |
abcd ECHO; REJECT;
.|\n /* eat up any unmatched character */
The first three rules share the fourth's action since they use the
special `|' action.
`REJECT' is a particularly expensive feature in terms of scanner
performance; if it is used in _any_ of the scanner's actions it
will slow down _all_ of the scanner's matching. Furthermore,
`REJECT' cannot be used with the `-Cf' or `-CF' options (*note
Scanner Options::).
Note also that unlike the other special actions, `REJECT' is a
_branch_. code immediately following it in the action will _not_
be executed.
`yymore()'
tells the scanner that the next time it matches a rule, the
corresponding token should be _appended_ onto the current value of
`yytext' rather than replacing it. For example, given the input
`mega-kludge' the following will write `mega-mega-kludge' to the
output:
%%
mega- ECHO; yymore();
kludge ECHO;
First `mega-' is matched and echoed to the output. Then `kludge'
is matched, but the previous `mega-' is still hanging around at the
beginning of `yytext' so the `ECHO' for the `kludge' rule will
actually write `mega-kludge'.
Two notes regarding use of `yymore()'. First, `yymore()' depends on
the value of `yyleng' correctly reflecting the size of the current
token, so you must not modify `yyleng' if you are using `yymore()'.
Second, the presence of `yymore()' in the scanner's action entails a
minor performance penalty in the scanner's matching speed.
`yyless(n)' returns all but the first `n' characters of the current
token back to the input stream, where they will be rescanned when the
scanner looks for the next match. `yytext' and `yyleng' are adjusted
appropriately (e.g., `yyleng' will now be equal to `n'). For example,
on the input `foobar' the following will write out `foobarbar':
%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;
An argument of 0 to `yyless()' will cause the entire current input
string to be scanned again. Unless you've changed how the scanner will
subsequently process its input (using `BEGIN', for example), this will
result in an endless loop.
Note that `yyless()' is a macro and can only be used in the flex
input file, not from other source files.
`unput(c)' puts the character `c' back onto the input stream. It
will be the next character scanned. The following action will take the
current token and cause it to be rescanned enclosed in parentheses.
{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( '(' );
free( yycopy );
}
Note that since each `unput()' puts the given character back at the
_beginning_ of the input stream, pushing back strings must be done
back-to-front.
An important potential problem when using `unput()' is that if you
are using `%pointer' (the default), a call to `unput()' _destroys_ the
contents of `yytext', starting with its rightmost character and
devouring one character to the left with each call. If you need the
value of `yytext' preserved after a call to `unput()' (as in the above
example), you must either first copy it elsewhere, or build your
scanner using `%array' instead (*note Matching::).
Finally, note that you cannot put back `EOF' to attempt to mark the
input stream with an end-of-file.
`input()' reads the next character from the input stream. For
example, the following is one way to eat up C comments:
%%
"/*" {
register int c;
for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
; /* eat up text of comment */
if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break; /* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using `C++', then `input()' is
instead referred to as yyinput(), in order to avoid a name clash with
the `C++' stream by the name of `input'.)
`YY_FLUSH_BUFFER()' flushes the scanner's internal buffer so that
the next time the scanner attempts to match a token, it will first
refill the buffer using `YY_INPUT()' (*note Generated Scanner::). This
action is a special case of the more general `yy_flush_buffer()'
function, described below (*note Multiple Input Buffers::)
`yyterminate()' can be used in lieu of a return statement in an
action. It terminates the scanner and returns a 0 to the scanner's
caller, indicating "all done". By default, `yyterminate()' is also
called when an end-of-file is encountered. It is a macro and may be
redefined.
File: flex.info, Node: Generated Scanner, Next: Start Conditions, Prev: Actions, Up: Top
9 The Generated Scanner
***********************
The output of `flex' is the file `lex.yy.c', which contains the
scanning routine `yylex()', a number of tables used by it for matching
tokens, and a number of auxiliary routines and macros. By default,
`yylex()' is declared as follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it will be
`int yylex( void )'.) This definition may be changed by defining the
`YY_DECL' macro. For example, you could use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name `lexscan', returning a float,
and taking two floats as arguments. Note that if you give arguments to
the scanning routine using a K&R-style/non-prototyped function
declaration, you must terminate the definition with a semi-colon (;).
`flex' generates `C99' function definitions by default. However flex
does have the ability to generate obsolete, er, `traditional', function
definitions. This is to support bootstrapping gcc on old systems.
Unfortunately, traditional definitions prevent us from using any
standard data types smaller than int (such as short, char, or bool) as
function arguments. For this reason, future versions of `flex' may
generate standard C99 code only, leaving K&R-style functions to the
historians. Currently, if you do *not* want `C99' definitions, then
you must use `%option noansi-definitions'.
Whenever `yylex()' is called, it scans tokens from the global input
file `yyin' (which defaults to stdin). It continues until it either
reaches an end-of-file (at which point it returns the value 0) or one
of its actions executes a `return' statement.
If the scanner reaches an end-of-file, subsequent calls are undefined
unless either `yyin' is pointed at a new input file (in which case
scanning continues from that file), or `yyrestart()' is called.
`yyrestart()' takes one argument, a `FILE *' pointer (which can be
NULL, if you've set up `YY_INPUT' to scan from a source other than
`yyin'), and initializes `yyin' for scanning from that file.
Essentially there is no difference between just assigning `yyin' to a
new input file or using `yyrestart()' to do so; the latter is available
for compatibility with previous versions of `flex', and because it can
be used to switch input files in the middle of scanning. It can also
be used to throw away the current input buffer, by calling it with an
argument of `yyin'; but it would be better to use `YY_FLUSH_BUFFER'
(*note Actions::). Note that `yyrestart()' does _not_ reset the start
condition to `INITIAL' (*note Start Conditions::).
If `yylex()' stops scanning due to executing a `return' statement in
one of the actions, the scanner may then be called again and it will
resume scanning where it left off.
By default (and for purposes of efficiency), the scanner uses
block-reads rather than simple `getc()' calls to read characters from
`yyin'. The nature of how it gets its input can be controlled by
defining the `YY_INPUT' macro. The calling sequence for `YY_INPUT()'
is `YY_INPUT(buf,result,max_size)'. Its action is to place up to
`max_size' characters in the character array `buf' and return in the
integer variable `result' either the number of characters read or the
constant `YY_NULL' (0 on Unix systems) to indicate `EOF'. The default
`YY_INPUT' reads from the global file-pointer `yyin'.
Here is a sample definition of `YY_INPUT' (in the definitions
section of the input file):
%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
This definition will change the input processing to occur one
character at a time.
When the scanner receives an end-of-file indication from YY_INPUT, it
then checks the `yywrap()' function. If `yywrap()' returns false
(zero), then it is assumed that the function has gone ahead and set up
`yyin' to point to another input file, and scanning continues. If it
returns true (non-zero), then the scanner terminates, returning 0 to
its caller. Note that in either case, the start condition remains
unchanged; it does _not_ revert to `INITIAL'.
If you do not supply your own version of `yywrap()', then you must
either use `%option noyywrap' (in which case the scanner behaves as
though `yywrap()' returned 1), or you must link with `-lfl' to obtain
the default version of the routine, which always returns 1.
For scanning from in-memory buffers (e.g., scanning strings), see
*Note Scanning Strings::. *Note Multiple Input Buffers::.
The scanner writes its `ECHO' output to the `yyout' global (default,
`stdout'), which may be redefined by the user simply by assigning it to
some other `FILE' pointer.
File: flex.info, Node: Start Conditions, Next: Multiple Input Buffers, Prev: Generated Scanner, Up: Top
10 Start Conditions
*******************
`flex' provides a mechanism for conditionally activating rules. Any
rule whose pattern is prefixed with `<sc>' will only be active when the
scanner is in the "start condition" named `sc'. For example,
<STRING>[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the `STRING' start
condition, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is either
`INITIAL', `STRING', or `QUOTE'.
Start conditions are declared in the definitions (first) section of
the input using unindented lines beginning with either `%s' or `%x'
followed by a list of names. The former declares "inclusive" start
conditions, the latter "exclusive" start conditions. A start condition
is activated using the `BEGIN' action. Until the next `BEGIN' action
is executed, rules with the given start condition will be active and
rules with other start conditions will be inactive. If the start
condition is inclusive, then rules with no start conditions at all will
also be active. If it is exclusive, then _only_ rules qualified with
the start condition will be active. A set of rules contingent on the
same exclusive start condition describe a scanner which is independent
of any of the other rules in the `flex' input. Because of this,
exclusive start conditions make it easy to specify "mini-scanners"
which scan portions of the input that are syntactically different from
the rest (e.g., comments).
If the distinction between inclusive and exclusive start conditions
is still a little vague, here's a simple example illustrating the
connection between the two. The set of rules:
%s example
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the `<INITIAL,example>' qualifier, the `bar' pattern in the
second example wouldn't be active (i.e., couldn't match) when in start
condition `example'. If we just used `example>' to qualify `bar',
though, then it would only be active in `example' and not in `INITIAL',
while in the first example it's active in both, because in the first
example the `example' start condition is an inclusive `(%s)' start
condition.
Also note that the special start-condition specifier `<*>' matches
every start condition. Thus, the above example could also have been
written:
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to `ECHO' any unmatched character) remains active
in start conditions. It is equivalent to:
<*>.|\n ECHO;
`BEGIN(0)' returns to the original state where only the rules with
no start conditions are active. This state can also be referred to as
the start-condition `INITIAL', so `BEGIN(INITIAL)' is equivalent to
`BEGIN(0)'. (The parentheses around the start condition name are not
required but are considered good style.)
`BEGIN' actions can also be given as indented code at the beginning
of the rules section. For example, the following will cause the scanner
to enter the `SPECIAL' start condition whenever `yylex()' is called and
the global variable `enter_special' is true:
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a scanner which
provides two different interpretations of a string like `123.456'. By
default it will treat it as three tokens, the integer `123', a dot
(`.'), and the integer `456'. But if the string is preceded earlier in
the line by the string `expect-floats' it will treat it as a single
token, the floating-point number `123.456':
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+@samp{.}[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments while
maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text as
possible with each rule. In general, when attempting to write a
high-speed scanner try to match as much possible in each rule, as it's
a big win.
Note that start-conditions names are really integer values and can
be stored as such. Thus, the above could be extended in the following
fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, you can access the current start condition using the
integer-valued `YY_START' macro. For example, the above assignments to
`comment_caller' could instead be written
comment_caller = YY_START;
Flex provides `YYSTATE' as an alias for `YY_START' (since that is
what's used by AT&T `lex').
For historical reasons, start conditions do not have their own
name-space within the generated scanner. The start condition names are
unmodified in the generated scanner and generated header. *Note
option-header::. *Note option-prefix::.
Finally, here's an example of how to match C-style quoted strings
using exclusive start conditions, including expanded escape sequences
(but not including checking for a string that's too long):
%x str
%%
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\" string_buf_ptr = string_buf; BEGIN(str);
<str>\" { /* saw closing quote - all done */
BEGIN(INITIAL);
*string_buf_ptr = '\0';
/* return string constant token type and
* value to parser
*/
}
<str>\n {
/* error - unterminated string constant */
/* generate error message */
}
<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;
(void) sscanf( yytext + 1, "%o", &result );
if ( result > 0xff )
/* error, constant is out-of-bounds */
*string_buf_ptr++ = result;
}
<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like '\48' or '\0777777'
*/
}
<str>\\n *string_buf_ptr++ = '\n';
<str>\\t *string_buf_ptr++ = '\t';
<str>\\r *string_buf_ptr++ = '\r';
<str>\\b *string_buf_ptr++ = '\b';
<str>\\f *string_buf_ptr++ = '\f';
<str>\\(.|\n) *string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+ {
char *yptr = yytext;
while ( *yptr )
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, you wind up writing a
whole bunch of rules all preceded by the same start condition(s). Flex
makes this a little easier and cleaner by introducing a notion of start
condition "scope". A start condition scope is begun with:
<SCs>{
where `SCs' is a list of one or more start conditions. Inside the
start condition scope, every rule automatically has the prefix `SCs>'
applied to it, until a `}' which matches the initial `{'. So, for
example,
<ESC>{
"\\n" return '\n';
"\\r" return '\r';
"\\f" return '\f';
"\\0" return '\0';
}
is equivalent to:
<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';
Start condition scopes may be nested.
The following routines are available for manipulating stacks of
start conditions:
-- Function: void yy_push_state ( int `new_state' )
pushes the current start condition onto the top of the start
condition stack and switches to `new_state' as though you had used
`BEGIN new_state' (recall that start condition names are also
integers).
-- Function: void yy_pop_state ()
pops the top of the stack and switches to it via `BEGIN'.
-- Function: int yy_top_state ()
returns the top of the stack without altering the stack's contents.
The start condition stack grows dynamically and so has no built-in
size limitation. If memory is exhausted, program execution aborts.
To use start condition stacks, your scanner must include a `%option
stack' directive (*note Scanner Options::).
File: flex.info, Node: Multiple Input Buffers, Next: EOF, Prev: Start Conditions, Up: Top
11 Multiple Input Buffers
*************************
Some scanners (such as those which support "include" files) require
reading from several input streams. As `flex' scanners do a large
amount of buffering, one cannot control where the next input will be
read from by simply writing a `YY_INPUT()' which is sensitive to the
scanning context. `YY_INPUT()' is only called when the scanner reaches
the end of its buffer, which may be a long time after scanning a
statement such as an `include' statement which requires switching the
input source.
To negotiate these sorts of problems, `flex' provides a mechanism
for creating and switching between multiple input buffers. An input
buffer is created by using:
-- Function: YY_BUFFER_STATE yy_create_buffer ( FILE *file, int size )
which takes a `FILE' pointer and a size and creates a buffer
associated with the given file and large enough to hold `size'
characters (when in doubt, use `YY_BUF_SIZE' for the size). It returns
a `YY_BUFFER_STATE' handle, which may then be passed to other routines
(see below). The `YY_BUFFER_STATE' type is a pointer to an opaque
`struct yy_buffer_state' structure, so you may safely initialize
`YY_BUFFER_STATE' variables to `((YY_BUFFER_STATE) 0)' if you wish, and
also refer to the opaque structure in order to correctly declare input
buffers in source files other than that of your scanner. Note that the
`FILE' pointer in the call to `yy_create_buffer' is only used as the
value of `yyin' seen by `YY_INPUT'. If you redefine `YY_INPUT()' so it
no longer uses `yyin', then you can safely pass a NULL `FILE' pointer to
`yy_create_buffer'. You select a particular buffer to scan from using:
-- Function: void yy_switch_to_buffer ( YY_BUFFER_STATE new_buffer )
The above function switches the scanner's input buffer so subsequent
tokens will come from `new_buffer'. Note that `yy_switch_to_buffer()'
may be used by `yywrap()' to set things up for continued scanning,
instead of opening a new file and pointing `yyin' at it. If you are
looking for a stack of input buffers, then you want to use
`yypush_buffer_state()' instead of this function. Note also that
switching input sources via either `yy_switch_to_buffer()' or
`yywrap()' does _not_ change the start condition.
-- Function: void yy_delete_buffer ( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer. (`buffer'
can be NULL, in which case the routine does nothing.) You can also
clear the current contents of a buffer using:
-- Function: void yypush_buffer_state ( YY_BUFFER_STATE buffer )
This function pushes the new buffer state onto an internal stack.
The pushed state becomes the new current state. The stack is maintained
by flex and will grow as required. This function is intended to be used
instead of `yy_switch_to_buffer', when you want to change states, but
preserve the current state for later use.
-- Function: void yypop_buffer_state ( )
This function removes the current state from the top of the stack,
and deletes it by calling `yy_delete_buffer'. The next state on the
stack, if any, becomes the new current state.
-- Function: void yy_flush_buffer ( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next time the
scanner attempts to match a token from the buffer, it will first fill
the buffer anew using `YY_INPUT()'.
-- Function: YY_BUFFER_STATE yy_new_buffer ( FILE *file, int size )
is an alias for `yy_create_buffer()', provided for compatibility
with the C++ use of `new' and `delete' for creating and destroying
dynamic objects.
`YY_CURRENT_BUFFER' macro returns a `YY_BUFFER_STATE' handle to the
current buffer. It should not be used as an lvalue.
Here are two examples of using these features for writing a scanner
which expands include files (the `<<EOF>>' feature is discussed below).
This first example uses yypush_buffer_state and yypop_buffer_state.
Flex maintains the stack internally.
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%%
include BEGIN(incl);
[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;
<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yypush_buffer_state(yy_create_buffer( yyin, YY_BUF_SIZE ));
BEGIN(INITIAL);
}
<<EOF>> {
yypop_buffer_state();
if ( !YY_CURRENT_BUFFER )
{
yyterminate();
}
}
The second example, below, does the same thing as the previous
example did, but manages its own input buffer stack manually (instead
of letting flex do it).
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}
%%
include BEGIN(incl);
[a-z]+ ECHO;
[^a-z\n]*\n? ECHO;
<incl>[ \t]* /* eat the whitespace */
<incl>[^ \t\n]+ { /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL);
}
<<EOF>> {
if ( --include_stack_ptr 0 )
{
yyterminate();
}
else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
}
The following routines are available for setting up input buffers for
scanning in-memory strings instead of files. All of them create a new
input buffer for scanning the string, and return a corresponding
`YY_BUFFER_STATE' handle (which you should delete with
`yy_delete_buffer()' when done with it). They also switch to the new
buffer using `yy_switch_to_buffer()', so the next call to `yylex()'
will start scanning the string.
-- Function: YY_BUFFER_STATE yy_scan_string ( const char *str )
scans a NUL-terminated string.
-- Function: YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, int
len )
scans `len' bytes (including possibly `NUL's) starting at location
`bytes'.
Note that both of these functions create and scan a _copy_ of the
string or bytes. (This may be desirable, since `yylex()' modifies the
contents of the buffer it is scanning.) You can avoid the copy by
using:
-- Function: YY_BUFFER_STATE yy_scan_buffer (char *base, yy_size_t
size)
which scans in place the buffer starting at `base', consisting of
`size' bytes, the last two bytes of which _must_ be
`YY_END_OF_BUFFER_CHAR' (ASCII NUL). These last two bytes are not
scanned; thus, scanning consists of `base[0]' through
`base[size-2]', inclusive.
If you fail to set up `base' in this manner (i.e., forget the final
two `YY_END_OF_BUFFER_CHAR' bytes), then `yy_scan_buffer()' returns a
NULL pointer instead of creating a new input buffer.
-- Data type: yy_size_t
is an integral type to which you can cast an integer expression
reflecting the size of the buffer.
File: flex.info, Node: EOF, Next: Misc Macros, Prev: Multiple Input Buffers, Up: Top
12 End-of-File Rules
********************
The special rule `<<EOF>>' indicates actions which are to be taken when
an end-of-file is encountered and `yywrap()' returns non-zero (i.e.,
indicates no further files to process). The action must finish by
doing one of the following things:
* assigning `yyin' to a new input file (in previous versions of
`flex', after doing the assignment you had to call the special
action `YY_NEW_FILE'. This is no longer necessary.)
* executing a `return' statement;
* executing the special `yyterminate()' action.
* or, switching to a new buffer using `yy_switch_to_buffer()' as
shown in the example above.
<<EOF>> rules may not be used with other patterns; they may only be
qualified with a list of start conditions. If an unqualified <<EOF>>
rule is given, it applies to _all_ start conditions which do not
already have <<EOF>> actions. To specify an <<EOF>> rule for only the
initial start condition, use:
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed comments.
An example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>> {
error( "unterminated quote" );
yyterminate();
}
<<EOF>> {
if ( *++filelist )
yyin = fopen( *filelist, "r" );
else
yyterminate();
}
File: flex.info, Node: Misc Macros, Next: User Values, Prev: EOF, Up: Top
13 Miscellaneous Macros
***********************
The macro `YY_USER_ACTION' can be defined to provide an action which is
always executed prior to the matched rule's action. For example, it
could be #define'd to call a routine to convert yytext to lower-case.
When `YY_USER_ACTION' is invoked, the variable `yy_act' gives the
number of the matched rule (rules are numbered starting with 1).
Suppose you want to profile how often each of your rules is matched.
The following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where `ctr' is an array to hold the counts for the different rules.
Note that the macro `YY_NUM_RULES' gives the total number of rules
(including the default rule), even if you use `-s)', so a correct
declaration for `ctr' is:
int ctr[YY_NUM_RULES];
The macro `YY_USER_INIT' may be defined to provide an action which
is always executed before the first scan (and before the scanner's
internal initializations are done). For example, it could be used to
call a routine to read in a data table or open a logging file.
The macro `yy_set_interactive(is_interactive)' can be used to
control whether the current buffer is considered "interactive". An
interactive buffer is processed more slowly, but must be used when the
scanner's input source is indeed interactive to avoid problems due to
waiting to fill buffers (see the discussion of the `-I' flag in *Note
Scanner Options::). A non-zero value in the macro invocation marks the
buffer as interactive, a zero value as non-interactive. Note that use
of this macro overrides `%option always-interactive' or `%option
never-interactive' (*note Scanner Options::). `yy_set_interactive()'
must be invoked prior to beginning to scan the buffer that is (or is
not) to be considered interactive.
The macro `yy_set_bol(at_bol)' can be used to control whether the
current buffer's scanning context for the next token match is done as
though at the beginning of a line. A non-zero macro argument makes
rules anchored with `^' active, while a zero argument makes `^' rules
inactive.
The macro `YY_AT_BOL()' returns true if the next token scanned from
the current buffer will have `^' rules active, false otherwise.
In the generated scanner, the actions are all gathered in one large
switch statement and separated using `YY_BREAK', which may be
redefined. By default, it is simply a `break', to separate each rule's
action from the following rule's. Redefining `YY_BREAK' allows, for
example, C++ users to #define YY_BREAK to do nothing (while being very
careful that every rule ends with a `break'" or a `return'!) to avoid
suffering from unreachable statement warnings where because a rule's
action ends with `return', the `YY_BREAK' is inaccessible.
File: flex.info, Node: User Values, Next: Yacc, Prev: Misc Macros, Up: Top
14 Values Available To the User
*******************************
This chapter summarizes the various values available to the user in the
rule actions.
`char *yytext'
holds the text of the current token. It may be modified but not
lengthened (you cannot append characters to the end).
If the special directive `%array' appears in the first section of
the scanner description, then `yytext' is instead declared `char
yytext[YYLMAX]', where `YYLMAX' is a macro definition that you can
redefine in the first section if you don't like the default value
(generally 8KB). Using `%array' results in somewhat slower
scanners, but the value of `yytext' becomes immune to calls to
`unput()', which potentially destroy its value when `yytext' is a
character pointer. The opposite of `%array' is `%pointer', which
is the default.
You cannot use `%array' when generating C++ scanner classes (the
`-+' flag).
`int yyleng'
holds the length of the current token.
`FILE *yyin'
is the file which by default `flex' reads from. It may be
redefined but doing so only makes sense before scanning begins or
after an EOF has been encountered. Changing it in the midst of
scanning will have unexpected results since `flex' buffers its
input; use `yyrestart()' instead. Once scanning terminates
because an end-of-file has been seen, you can assign `yyin' at the
new input file and then call the scanner again to continue
scanning.
`void yyrestart( FILE *new_file )'
may be called to point `yyin' at the new input file. The
switch-over to the new file is immediate (any previously
buffered-up input is lost). Note that calling `yyrestart()' with
`yyin' as an argument thus throws away the current input buffer
and continues scanning the same input file.
`FILE *yyout'
is the file to which `ECHO' actions are done. It can be reassigned
by the user.
`YY_CURRENT_BUFFER'
returns a `YY_BUFFER_STATE' handle to the current buffer.
`YY_START'
returns an integer value corresponding to the current start
condition. You can subsequently use this value with `BEGIN' to
return to that start condition.
File: flex.info, Node: Yacc, Next: Scanner Options, Prev: User Values, Up: Top
15 Interfacing with Yacc
************************
One of the main uses of `flex' is as a companion to the `yacc'
parser-generator. `yacc' parsers expect to call a routine named
`yylex()' to find the next input token. The routine is supposed to
return the type of the next token as well as putting any associated
value in the global `yylval'. To use `flex' with `yacc', one specifies
the `-d' option to `yacc' to instruct it to generate the file `y.tab.h'
containing definitions of all the `%tokens' appearing in the `yacc'
input. This file is then included in the `flex' scanner. For example,
if one of the tokens is `TOK_NUMBER', part of the scanner might look
like:
%{
#include "y.tab.h"
%}
%%
[0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
File: flex.info, Node: Scanner Options, Next: Performance, Prev: Yacc, Up: Top
16 Scanner Options
******************
The various `flex' options are categorized by function in the following
menu. If you want to lookup a particular option by name, *Note Index of
Scanner Options::.
* Menu:
* Options for Specifing Filenames::
* Options Affecting Scanner Behavior::
* Code-Level And API Options::
* Options for Scanner Speed and Size::
* Debugging Options::
* Miscellaneous Options::
Even though there are many scanner options, a typical scanner might
only specify the following options:
%option 8bit reentrant bison-bridge
%option warn nodefault
%option yylineno
%option outfile="scanner.c" header-file="scanner.h"
The first line specifies the general type of scanner we want. The
second line specifies that we are being careful. The third line asks
flex to track line numbers. The last line tells flex what to name the
files. (The options can be specified in any order. We just dividied
them.)
`flex' also provides a mechanism for controlling options within the
scanner specification itself, rather than from the flex command-line.
This is done by including `%option' directives in the first section of
the scanner specification. You can specify multiple options with a
single `%option' directive, and multiple directives in the first
section of your flex input file.
Most options are given simply as names, optionally preceded by the
word `no' (with no intervening whitespace) to negate their meaning.
The names are the same as their long-option equivalents (but without the
leading `--' ).
`flex' scans your rule actions to determine whether you use the
`REJECT' or `yymore()' features. The `REJECT' and `yymore' options are
available to override its decision as to whether you use the options,
either by setting them (e.g., `%option reject)' to indicate the feature
is indeed used, or unsetting them to indicate it actually is not used
(e.g., `%option noyymore)'.
A number of options are available for lint purists who want to
suppress the appearance of unneeded routines in the generated scanner.
Each of the following, if unset (e.g., `%option nounput'), results in
the corresponding routine not appearing in the generated scanner:
input, unput
yy_push_state, yy_pop_state, yy_top_state
yy_scan_buffer, yy_scan_bytes, yy_scan_string
yyget_extra, yyset_extra, yyget_leng, yyget_text,
yyget_lineno, yyset_lineno, yyget_in, yyset_in,
yyget_out, yyset_out, yyget_lval, yyset_lval,
yyget_lloc, yyset_lloc, yyget_debug, yyset_debug
(though `yy_push_state()' and friends won't appear anyway unless you
use `%option stack)'.
File: flex.info, Node: Options for Specifing Filenames, Next: Options Affecting Scanner Behavior, Prev: Scanner Options, Up: Scanner Options
16.1 Options for Specifing Filenames
====================================
`--header-file=FILE, `%option header-file="FILE"''
instructs flex to write a C header to `FILE'. This file contains
function prototypes, extern variables, and types used by the
scanner. Only the external API is exported by the header file.
Many macros that are usable from within scanner actions are not
exported to the header file. This is due to namespace problems and
the goal of a clean external API.
While in the header, the macro `yyIN_HEADER' is defined, where `yy'
is substituted with the appropriate prefix.
The `--header-file' option is not compatible with the `--c++'
option, since the C++ scanner provides its own header in
`yyFlexLexer.h'.
`-oFILE, --outfile=FILE, `%option outfile="FILE"''
directs flex to write the scanner to the file `FILE' instead of
`lex.yy.c'. If you combine `--outfile' with the `--stdout' option,
then the scanner is written to `stdout' but its `#line' directives
(see the `-l' option above) refer to the file `FILE'.
`-t, --stdout, `%option stdout''
instructs `flex' to write the scanner it generates to standard
output instead of `lex.yy.c'.
`-SFILE, --skel=FILE'
overrides the default skeleton file from which `flex' constructs
its scanners. You'll never need this option unless you are doing
`flex' maintenance or development.
`--tables-file=FILE'
Write serialized scanner dfa tables to FILE. The generated scanner
will not contain the tables, and requires them to be loaded at
runtime. *Note serialization::.
`--tables-verify'
This option is for flex development. We document it here in case
you stumble upon it by accident or in case you suspect some
inconsistency in the serialized tables. Flex will serialize the
scanner dfa tables but will also generate the in-code tables as it
normally does. At runtime, the scanner will verify that the
serialized tables match the in-code tables, instead of loading
them.
File: flex.info, Node: Options Affecting Scanner Behavior, Next: Code-Level And API Options, Prev: Options for Specifing Filenames, Up: Scanner Options
16.2 Options Affecting Scanner Behavior
=======================================
`-i, --case-insensitive, `%option case-insensitive''
instructs `flex' to generate a "case-insensitive" scanner. The
case of letters given in the `flex' input patterns will be ignored,
and tokens in the input will be matched regardless of case. The
matched text given in `yytext' will have the preserved case (i.e.,
it will not be folded). For tricky behavior, see *Note case and
character ranges::.
`-l, --lex-compat, `%option lex-compat''
turns on maximum compatibility with the original AT&T `lex'
implementation. Note that this does not mean _full_ compatibility.
Use of this option costs a considerable amount of performance, and
it cannot be used with the `--c++', `--full', `--fast', `-Cf', or
`-CF' options. For details on the compatibilities it provides, see
*Note Lex and Posix::. This option also results in the name
`YY_FLEX_LEX_COMPAT' being `#define''d in the generated scanner.
`-B, --batch, `%option batch''
instructs `flex' to generate a "batch" scanner, the opposite of
_interactive_ scanners generated by `--interactive' (see below).
In general, you use `-B' when you are _certain_ that your scanner
will never be used interactively, and you want to squeeze a
_little_ more performance out of it. If your goal is instead to
squeeze out a _lot_ more performance, you should be using the
`-Cf' or `-CF' options, which turn on `--batch' automatically
anyway.
`-I, --interactive, `%option interactive''
instructs `flex' to generate an interactive scanner. An
interactive scanner is one that only looks ahead to decide what
token has been matched if it absolutely must. It turns out that
always looking one extra character ahead, even if the scanner has
already seen enough text to disambiguate the current token, is a
bit faster than only looking ahead when necessary. But scanners
that always look ahead give dreadful interactive performance; for
example, when a user types a newline, it is not recognized as a
newline token until they enter _another_ token, which often means
typing in another whole line.
`flex' scanners default to `interactive' unless you use the `-Cf'
or `-CF' table-compression options (*note Performance::). That's
because if you're looking for high-performance you should be using
one of these options, so if you didn't, `flex' assumes you'd
rather trade off a bit of run-time performance for intuitive
interactive behavior. Note also that you _cannot_ use
`--interactive' in conjunction with `-Cf' or `-CF'. Thus, this
option is not really needed; it is on by default for all those
cases in which it is allowed.
You can force a scanner to _not_ be interactive by using `--batch'
`-7, --7bit, `%option 7bit''
instructs `flex' to generate a 7-bit scanner, i.e., one which can
only recognize 7-bit characters in its input. The advantage of
using `--7bit' is that the scanner's tables can be up to half the
size of those generated using the `--8bit'. The disadvantage is
that such scanners often hang or crash if their input contains an
8-bit character.
Note, however, that unless you generate your scanner using the
`-Cf' or `-CF' table compression options, use of `--7bit' will
save only a small amount of table space, and make your scanner
considerably less portable. `Flex''s default behavior is to
generate an 8-bit scanner unless you use the `-Cf' or `-CF', in
which case `flex' defaults to generating 7-bit scanners unless
your site was always configured to generate 8-bit scanners (as will
often be the case with non-USA sites). You can tell whether flex
generated a 7-bit or an 8-bit scanner by inspecting the flag
summary in the `--verbose' output as described above.
Note that if you use `-Cfe' or `-CFe' `flex' still defaults to
generating an 8-bit scanner, since usually with these compression
options full 8-bit tables are not much more expensive than 7-bit
tables.
`-8, --8bit, `%option 8bit''
instructs `flex' to generate an 8-bit scanner, i.e., one which can
recognize 8-bit characters. This flag is only needed for scanners
generated using `-Cf' or `-CF', as otherwise flex defaults to
generating an 8-bit scanner anyway.
See the discussion of `--7bit' above for `flex''s default behavior
and the tradeoffs between 7-bit and 8-bit scanners.
`--default, `%option default''
generate the default rule.
`--always-interactive, `%option always-interactive''
instructs flex to generate a scanner which always considers its
input _interactive_. Normally, on each new input file the scanner
calls `isatty()' in an attempt to determine whether the scanner's
input source is interactive and thus should be read a character at
a time. When this option is used, however, then no such call is
made.
`--never-interactive, `--never-interactive''
instructs flex to generate a scanner which never considers its
input interactive. This is the opposite of `always-interactive'.
`-X, --posix, `%option posix''
turns on maximum compatibility with the POSIX 1003.2-1992
definition of `lex'. Since `flex' was originally designed to
implement the POSIX definition of `lex' this generally involves
very few changes in behavior. At the current writing the known
differences between `flex' and the POSIX standard are:
* In POSIX and AT&T `lex', the repeat operator, `{}', has lower
precedence than concatenation (thus `ab{3}' yields `ababab').
Most POSIX utilities use an Extended Regular Expression (ERE)
precedence that has the precedence of the repeat operator
higher than concatenation (which causes `ab{3}' to yield
`abbb'). By default, `flex' places the precedence of the
repeat operator higher than concatenation which matches the
ERE processing of other POSIX utilities. When either
`--posix' or `-l' are specified, `flex' will use the
traditional AT&T and POSIX-compliant precedence for the
repeat operator where concatenation has higher precedence
than the repeat operator.
`--stack, `%option stack''
enables the use of start condition stacks (*note Start
Conditions::).
`--stdinit, `%option stdinit''
if set (i.e., %option stdinit) initializes `yyin' and `yyout' to
`stdin' and `stdout', instead of the default of `NULL'. Some
existing `lex' programs depend on this behavior, even though it is
not compliant with ANSI C, which does not require `stdin' and
`stdout' to be compile-time constant. In a reentrant scanner,
however, this is not a problem since initialization is performed
in `yylex_init' at runtime.
`--yylineno, `%option yylineno''
directs `flex' to generate a scanner that maintains the number of
the current line read from its input in the global variable
`yylineno'. This option is implied by `%option lex-compat'. In a
reentrant C scanner, the macro `yylineno' is accessible regardless
of the value of `%option yylineno', however, its value is not
modified by `flex' unless `%option yylineno' is enabled.
`--yywrap, `%option yywrap''
if unset (i.e., `--noyywrap)', makes the scanner not call
`yywrap()' upon an end-of-file, but simply assume that there are no
more files to scan (until the user points `yyin' at a new file and
calls `yylex()' again).
File: flex.info, Node: Code-Level And API Options, Next: Options for Scanner Speed and Size, Prev: Options Affecting Scanner Behavior, Up: Scanner Options
16.3 Code-Level And API Options
===============================
`--ansi-definitions, `%option ansi-definitions''
instruct flex to generate ANSI C99 definitions for functions.
This option is enabled by default. If `%option
noansi-definitions' is specified, then the obsolete style is
generated.
`--ansi-prototypes, `%option ansi-prototypes''
instructs flex to generate ANSI C99 prototypes for functions.
This option is enabled by default. If `noansi-prototypes' is
specified, then prototypes will have empty parameter lists.
`--bison-bridge, `%option bison-bridge''
instructs flex to generate a C scanner that is meant to be called
by a `GNU bison' parser. The scanner has minor API changes for
`bison' compatibility. In particular, the declaration of `yylex'
is modified to take an additional parameter, `yylval'. *Note
Bison Bridge::.
`--bison-locations, `%option bison-locations''
instruct flex that `GNU bison' `%locations' are being used. This
means `yylex' will be passed an additional parameter, `yylloc'.
This option implies `%option bison-bridge'. *Note Bison Bridge::.
`-L, --noline, `%option noline''
instructs `flex' not to generate `#line' directives. Without this
option, `flex' peppers the generated scanner with `#line'
directives so error messages in the actions will be correctly
located with respect to either the original `flex' input file (if
the errors are due to code in the input file), or `lex.yy.c' (if
the errors are `flex''s fault - you should report these sorts of
errors to the email address given in *Note Reporting Bugs::).
`-R, --reentrant, `%option reentrant''
instructs flex to generate a reentrant C scanner. The generated
scanner may safely be used in a multi-threaded environment. The
API for a reentrant scanner is different than for a non-reentrant
scanner *note Reentrant::). Because of the API difference between
reentrant and non-reentrant `flex' scanners, non-reentrant flex
code must be modified before it is suitable for use with this
option. This option is not compatible with the `--c++' option.
The option `--reentrant' does not affect the performance of the
scanner.
`-+, --c++, `%option c++''
specifies that you want flex to generate a C++ scanner class.
*Note Cxx::, for details.
`--array, `%option array''
specifies that you want yytext to be an array instead of a char*
`--pointer, `%option pointer''
specify that `yytext' should be a `char *', not an array. This
default is `char *'.
`-PPREFIX, --prefix=PREFIX, `%option prefix="PREFIX"''
changes the default `yy' prefix used by `flex' for all
globally-visible variable and function names to instead be
`PREFIX'. For example, `--prefix=foo' changes the name of
`yytext' to `footext'. It also changes the name of the default
output file from `lex.yy.c' to `lex.foo.c'. Here is a partial
list of the names affected:
yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
yyalloc
yyrealloc
yyfree
(If you are using a C++ scanner, then only `yywrap' and
`yyFlexLexer' are affected.) Within your scanner itself, you can
still refer to the global variables and functions using either
version of their name; but externally, they have the modified name.
This option lets you easily link together multiple `flex' programs
into the same executable. Note, though, that using this option
also renames `yywrap()', so you now _must_ either provide your own
(appropriately-named) version of the routine for your scanner, or
use `%option noyywrap', as linking with `-lfl' no longer provides
one for you by default.
`--main, `%option main''
directs flex to provide a default `main()' program for the
scanner, which simply calls `yylex()'. This option implies
`noyywrap' (see below).
`--nounistd, `%option nounistd''
suppresses inclusion of the non-ANSI header file `unistd.h'. This
option is meant to target environments in which `unistd.h' does
not exist. Be aware that certain options may cause flex to
generate code that relies on functions normally found in
`unistd.h', (e.g. `isatty()', `read()'.) If you wish to use these
functions, you will have to inform your compiler where to find
them. *Note option-always-interactive::. *Note option-read::.
`--yyclass, `%option yyclass="NAME"''
only applies when generating a C++ scanner (the `--c++' option).
It informs `flex' that you have derived `foo' as a subclass of
`yyFlexLexer', so `flex' will place your actions in the member
function `foo::yylex()' instead of `yyFlexLexer::yylex()'. It
also generates a `yyFlexLexer::yylex()' member function that emits
a run-time error (by invoking `yyFlexLexer::LexerError())' if
called. *Note Cxx::.
File: flex.info, Node: Options for Scanner Speed and Size, Next: Debugging Options, Prev: Code-Level And API Options, Up: Scanner Options
16.4 Options for Scanner Speed and Size
=======================================
`-C[aefFmr]'
controls the degree of table compression and, more generally,
trade-offs between small scanners and fast scanners.
`-C'
A lone `-C' specifies that the scanner tables should be
compressed but neither equivalence classes nor
meta-equivalence classes should be used.
`-Ca, --align, `%option align''
("align") instructs flex to trade off larger tables in the
generated scanner for faster performance because the elements
of the tables are better aligned for memory access and
computation. On some RISC architectures, fetching and
manipulating longwords is more efficient than with
smaller-sized units such as shortwords. This option can
quadruple the size of the tables used by your scanner.
`-Ce, --ecs, `%option ecs''
directs `flex' to construct "equivalence classes", i.e., sets
of characters which have identical lexical properties (for
example, if the only appearance of digits in the `flex' input
is in the character class "[0-9]" then the digits '0', '1',
..., '9' will all be put in the same equivalence class).
Equivalence classes usually give dramatic reductions in the
final table/object file sizes (typically a factor of 2-5) and
are pretty cheap performance-wise (one array look-up per
character scanned).
`-Cf'
specifies that the "full" scanner tables should be generated -
`flex' should not compress the tables by taking advantages of
similar transition functions for different states.
`-CF'
specifies that the alternate fast scanner representation
(described above under the `--fast' flag) should be used.
This option cannot be used with `--c++'.
`-Cm, --meta-ecs, `%option meta-ecs''
directs `flex' to construct "meta-equivalence classes", which
are sets of equivalence classes (or characters, if equivalence
classes are not being used) that are commonly used together.
Meta-equivalence classes are often a big win when using
compressed tables, but they have a moderate performance
impact (one or two `if' tests and one array look-up per
character scanned).
`-Cr, --read, `%option read''
causes the generated scanner to _bypass_ use of the standard
I/O library (`stdio') for input. Instead of calling
`fread()' or `getc()', the scanner will use the `read()'
system call, resulting in a performance gain which varies
from system to system, but in general is probably negligible
unless you are also using `-Cf' or `-CF'. Using `-Cr' can
cause strange behavior if, for example, you read from `yyin'
using `stdio' prior to calling the scanner (because the
scanner will miss whatever text your previous reads left in
the `stdio' input buffer). `-Cr' has no effect if you define
`YY_INPUT()' (*note Generated Scanner::).
The options `-Cf' or `-CF' and `-Cm' do not make sense together -
there is no opportunity for meta-equivalence classes if the table
is not being compressed. Otherwise the options may be freely
mixed, and are cumulative.
The default setting is `-Cem', which specifies that `flex' should
generate equivalence classes and meta-equivalence classes. This
setting provides the highest degree of table compression. You can
trade off faster-executing scanners at the cost of larger tables
with the following generally being true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
-C{f,F}a
fastest & largest
Note that scanners with the smallest tables are usually generated
and compiled the quickest, so during development you will usually
want to use the default, maximal compression.
`-Cfe' is often a good compromise between speed and size for
production scanners.
`-f, --full, `%option full''
specifies "fast scanner". No table compression is done and
`stdio' is bypassed. The result is large but fast. This option
is equivalent to `--Cfr'
`-F, --fast, `%option fast''
specifies that the _fast_ scanner table representation should be
used (and `stdio' bypassed). This representation is about as fast
as the full table representation `--full', and for some sets of
patterns will be considerably smaller (and for others, larger). In
general, if the pattern set contains both _keywords_ and a
catch-all, _identifier_ rule, such as in the set:
"case" return TOK_CASE;
"switch" return TOK_SWITCH;
...
"default" return TOK_DEFAULT;
[a-z]+ return TOK_ID;
then you're better off using the full table representation. If
only the _identifier_ rule is present and you then use a hash
table or some such to detect the keywords, you're better off using
`--fast'.
This option is equivalent to `-CFr' (see below). It cannot be used
with `--c++'.
File: flex.info, Node: Debugging Options, Next: Miscellaneous Options, Prev: Options for Scanner Speed and Size, Up: Scanner Options
16.5 Debugging Options
======================
`-b, --backup, `%option backup''
Generate backing-up information to `lex.backup'. This is a list of
scanner states which require backing up and the input characters on
which they do so. By adding rules one can remove backing-up
states. If _all_ backing-up states are eliminated and `-Cf' or
`-CF' is used, the generated scanner will run faster (see the
`--perf-report' flag). Only users who wish to squeeze every last
cycle out of their scanners need worry about this option. (*note
Performance::).
`-d, --debug, `%option debug''
makes the generated scanner run in "debug" mode. Whenever a
pattern is recognized and the global variable `yy_flex_debug' is
non-zero (which is the default), the scanner will write to
`stderr' a line of the form:
-accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file
defining the scanner (i.e., the file that was fed to flex).
Messages are also generated when the scanner backs up, accepts the
default rule, reaches the end of its input buffer (or encounters a
NUL; at this point, the two look the same as far as the scanner's
concerned), or reaches an end-of-file.
`-p, --perf-report, `%option perf-report''
generates a performance report to `stderr'. The report consists of
comments regarding features of the `flex' input file which will
cause a serious loss of performance in the resulting scanner. If
you give the flag twice, you will also get comments regarding
features that lead to minor performance losses.
Note that the use of `REJECT', and variable trailing context
(*note Limitations::) entails a substantial performance penalty;
use of `yymore()', the `^' operator, and the `--interactive' flag
entail minor performance penalties.
`-s, --nodefault, `%option nodefault''
causes the _default rule_ (that unmatched scanner input is echoed
to `stdout)' to be suppressed. If the scanner encounters input
that does not match any of its rules, it aborts with an error.
This option is useful for finding holes in a scanner's rule set.
`-T, --trace, `%option trace''
makes `flex' run in "trace" mode. It will generate a lot of
messages to `stderr' concerning the form of the input and the
resultant non-deterministic and deterministic finite automata.
This option is mostly for use in maintaining `flex'.
`-w, --nowarn, `%option nowarn''
suppresses warning messages.
`-v, --verbose, `%option verbose''
specifies that `flex' should write to `stderr' a summary of
statistics regarding the scanner it generates. Most of the
statistics are meaningless to the casual `flex' user, but the
first line identifies the version of `flex' (same as reported by
`--version'), and the next line the flags used when generating the
scanner, including those that are on by default.
`--warn, `%option warn''
warn about certain things. In particular, if the default rule can
be matched but no defualt rule has been given, the flex will warn
you. We recommend using this option always.
File: flex.info, Node: Miscellaneous Options, Prev: Debugging Options, Up: Scanner Options
16.6 Miscellaneous Options
==========================
`-c'
is a do-nothing option included for POSIX compliance.
generates
`-h, -?, --help'
generates a "help" summary of `flex''s options to `stdout' and
then exits.
`-n'
is another do-nothing option included only for POSIX compliance.
`-V, --version'
prints the version number to `stdout' and exits.
File: flex.info, Node: Performance, Next: Cxx, Prev: Scanner Options, Up: Top
17 Performance Considerations
*****************************
The main design goal of `flex' is that it generate high-performance
scanners. It has been optimized for dealing well with large sets of
rules. Aside from the effects on scanner speed of the table compression
`-C' options outlined above, there are a number of options/actions
which degrade performance. These are, from most expensive to least:
REJECT
arbitrary trailing context
pattern sets that require backing up
%option yylineno
%array
%option interactive
%option always-interactive
@samp{^} beginning-of-line operator
yymore()
with the first two all being quite expensive and the last two being
quite cheap. Note also that `unput()' is implemented as a routine call
that potentially does quite a bit of work, while `yyless()' is a
quite-cheap macro. So if you are just putting back some excess text you
scanned, use `ss()'.
`REJECT' should be avoided at all costs when performance is
important. It is a particularly expensive option.
There is one case when `%option yylineno' can be expensive. That is
when your patterns match long tokens that could _possibly_ contain a
newline character. There is no performance penalty for rules that can
not possibly match newlines, since flex does not need to check them for
newlines. In general, you should avoid rules such as `[^f]+', which
match very long tokens, including newlines, and may possibly match your
entire file! A better approach is to separate `[^f]+' into two rules:
%option yylineno
%%
[^f\n]+
\n+
The above scanner does not incur a performance penalty.
Getting rid of backing up is messy and often may be an enormous
amount of work for a complicated scanner. In principal, one begins by
using the `-b' flag to generate a `lex.backup' file. For example, on
the input:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting -
associated rule line numbers:
2 3
out-transitions: [ o ]
jam-transitions: EOF [ \001-n p-\177 ]
State #8 is non-accepting -
associated rule line numbers:
3
out-transitions: [ a ]
jam-transitions: EOF [ \001-` b-\177 ]
State #9 is non-accepting -
associated rule line numbers:
3
out-transitions: [ r ]
jam-transitions: EOF [ \001-q s-\177 ]
Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it
can make a transition on an 'o' but not on any other character, and
that in that state the currently scanned text does not match any rule.
The state occurs when trying to match the rules found at lines 2 and 3
in the input file. If the scanner is in that state and then reads
something other than an 'o', it will have to back up to find a rule
which is matched. With a bit of headscratching one can see that this
must be the state it's in when it has seen `fo'. When this has
happened, if anything other than another `o' is seen, the scanner will
have to back up to simply match the `f' (by the default rule).
The comment regarding State #8 indicates there's a problem when
`foob' has been scanned. Indeed, on any character other than an `a',
the scanner will have to back up to accept "foo". Similarly, the
comment for State #9 concerns when `fooba' has been scanned and an `r'
does not follow.
The final comment reminds us that there's no point going to all the
trouble of removing backing up from the rules unless we're using `-Cf'
or `-CF', since there's no performance gain doing so with compressed
scanners.
The way to remove the backing up is to add "error" rules:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
fooba |
foob |
fo {
/* false alarm, not really a keyword */
return TOK_ID;
}
Eliminating backing up among a list of keywords can also be done
using a "catch-all" rule:
%%
foo return TOK_KEYWORD;
foobar return TOK_KEYWORD;
[a-z]+ return TOK_ID;
This is usually the best solution when appropriate.
Backing up messages tend to cascade. With a complicated set of rules
it's not uncommon to get hundreds of messages. If one can decipher
them, though, it often only takes a dozen or so rules to eliminate the
backing up (though it's easy to make a mistake and have an error rule
accidentally match a valid token. A possible future `flex' feature
will be to automatically add rules to eliminate backing up).
It's important to keep in mind that you gain the benefits of
eliminating backing up only if you eliminate _every_ instance of
backing up. Leaving just one means you gain nothing.
_Variable_ trailing context (where both the leading and trailing
parts do not have a fixed length) entails almost the same performance
loss as `REJECT' (i.e., substantial). So when possible a rule like:
%%
mouse|rat/(cat|dog) run();
is better written:
%%
mouse/cat|dog run();
rat/cat|dog run();
or as
%%
mouse|rat/cat run();
mouse|rat/dog run();
Note that here the special '|' action does _not_ provide any
savings, and can even make things worse (*note Limitations::).
Another area where the user can increase a scanner's performance (and
one that's easier to implement) arises from the fact that the longer the
tokens matched, the faster the scanner will run. This is because with
long tokens the processing of most input characters takes place in the
(short) inner scanning loop, and does not often have to go through the
additional work of setting up the scanning environment (e.g., `yytext')
for the action. Recall the scanner for C comments:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This could be sped up by writing it as:
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]*
<comment>[^*\n]*\n ++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
Now instead of each newline requiring the processing of another
action, recognizing the newlines is distributed over the other rules to
keep the matched text as long as possible. Note that _adding_ rules
does _not_ slow down the scanner! The speed of the scanner is
independent of the number of rules or (modulo the considerations given
at the beginning of this section) how complicated the rules are with
regard to operators such as `*' and `|'.
A final example in speeding up a scanner: suppose you want to scan
through a file containing identifiers and keywords, one per line and
with no other extraneous characters, and recognize all the keywords. A
natural first approach is:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
.|\n /* it's not a keyword */
To eliminate the back-tracking, introduce a catch-all rule:
%%
asm |
auto |
break |
... etc ...
volatile |
while /* it's a keyword */
[a-z]+ |
.|\n /* it's not a keyword */
Now, if it's guaranteed that there's exactly one word per line, then
we can reduce the total number of matches by a half by merging in the
recognition of newlines with that of the other tokens:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
.|\n /* it's not a keyword */
One has to be careful here, as we have now reintroduced backing up
into the scanner. In particular, while _we_ know that there will never
be any characters in the input stream other than letters or newlines,
`flex' can't figure this out, and it will plan for possibly needing to
back up when it has scanned a token like `auto' and then the next
character is something other than a newline or a letter. Previously it
would then just match the `auto' rule and be done, but now it has no
`auto' rule, only a `auto\n' rule. To eliminate the possibility of
backing up, we could either duplicate all rules but without final
newlines, or, since we never expect to encounter such an input and
therefore don't how it's classified, we can introduce one more
catch-all rule, this one which doesn't include a newline:
%%
asm\n |
auto\n |
break\n |
... etc ...
volatile\n |
while\n /* it's a keyword */
[a-z]+\n |
[a-z]+ |
.|\n /* it's not a keyword */
Compiled with `-Cf', this is about as fast as one can get a `flex'
scanner to go for this particular problem.
A final note: `flex' is slow when matching `NUL's, particularly when
a token contains multiple `NUL's. It's best to write rules which match
_short_ amounts of text if it's anticipated that the text will often
include `NUL's.
Another final note regarding performance: as mentioned in *Note
Matching::, dynamically resizing `yytext' to accommodate huge tokens is
a slow process because it presently requires that the (huge) token be
rescanned from the beginning. Thus if performance is vital, you should
attempt to match "large" quantities of text but not "huge" quantities,
where the cutoff between the two is at about 8K characters per token.
File: flex.info, Node: Cxx, Next: Reentrant, Prev: Performance, Up: Top
18 Generating C++ Scanners
**************************
*IMPORTANT*: the present form of the scanning class is _experimental_
and may change considerably between major releases.
`flex' provides two different ways to generate scanners for use with
C++. The first way is to simply compile a scanner generated by `flex'
using a C++ compiler instead of a C compiler. You should not encounter
any compilation errors (*note Reporting Bugs::). You can then use C++
code in your rule actions instead of C code. Note that the default
input source for your scanner remains `yyin', and default echoing is
still done to `yyout'. Both of these remain `FILE *' variables and not
C++ _streams_.
You can also use `flex' to generate a C++ scanner class, using the
`-+' option (or, equivalently, `%option c++)', which is automatically
specified if the name of the `flex' executable ends in a '+', such as
`flex++'. When using this option, `flex' defaults to generating the
scanner to the file `lex.yy.cc' instead of `lex.yy.c'. The generated
scanner includes the header file `FlexLexer.h', which defines the
interface to two C++ classes.
The first class, `FlexLexer', provides an abstract base class
defining the general scanner class interface. It provides the
following member functions:
`const char* YYText()'
returns the text of the most recently matched token, the
equivalent of `yytext'.
`int YYLeng()'
returns the length of the most recently matched token, the
equivalent of `yyleng'.
`int lineno() const'
returns the current input line number (see `%option yylineno)', or
`1' if `%option yylineno' was not used.
`void set_debug( int flag )'
sets the debugging flag for the scanner, equivalent to assigning to
`yy_flex_debug' (*note Scanner Options::). Note that you must
build the scannerusing `%option debug' to include debugging
information in it.
`int debug() const'
returns the current setting of the debugging flag.
Also provided are member functions equivalent to
`yy_switch_to_buffer()', `yy_create_buffer()' (though the first
argument is an `istream*' object pointer and not a `FILE*)',
`yy_flush_buffer()', `yy_delete_buffer()', and `yyrestart()' (again,
the first argument is a `istream*' object pointer).
The second class defined in `FlexLexer.h' is `yyFlexLexer', which is
derived from `FlexLexer'. It defines the following additional member
functions:
`yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )'
constructs a `yyFlexLexer' object using the given streams for input
and output. If not specified, the streams default to `cin' and
`cout', respectively.
`virtual int yylex()'
performs the same role is `yylex()' does for ordinary `flex'
scanners: it scans the input stream, consuming tokens, until a
rule's action returns a value. If you derive a subclass `S' from
`yyFlexLexer' and want to access the member functions and variables
of `S' inside `yylex()', then you need to use `%option
yyclass="S"' to inform `flex' that you will be using that subclass
instead of `yyFlexLexer'. In this case, rather than generating
`yyFlexLexer::yylex()', `flex' generates `S::yylex()' (and also
generates a dummy `yyFlexLexer::yylex()' that calls
`yyFlexLexer::LexerError()' if called).
`virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)'
reassigns `yyin' to `new_in' (if non-null) and `yyout' to
`new_out' (if non-null), deleting the previous input buffer if
`yyin' is reassigned.
`int yylex( istream* new_in, ostream* new_out = 0 )'
first switches the input streams via `switch_streams( new_in,
new_out )' and then returns the value of `yylex()'.
In addition, `yyFlexLexer' defines the following protected virtual
functions which you can redefine in derived classes to tailor the
scanner:
`virtual int LexerInput( char* buf, int max_size )'
reads up to `max_size' characters into `buf' and returns the
number of characters read. To indicate end-of-input, return 0
characters. Note that `interactive' scanners (see the `-B' and
`-I' flags in *Note Scanner Options::) define the macro
`YY_INTERACTIVE'. If you redefine `LexerInput()' and need to take
different actions depending on whether or not the scanner might be
scanning an interactive input source, you can test for the
presence of this name via `#ifdef' statements.
`virtual void LexerOutput( const char* buf, int size )'
writes out `size' characters from the buffer `buf', which, while
`NUL'-terminated, may also contain internal `NUL's if the
scanner's rules can match text with `NUL's in them.
`virtual void LexerError( const char* msg )'
reports a fatal error message. The default version of this
function writes the message to the stream `cerr' and exits.
Note that a `yyFlexLexer' object contains its _entire_ scanning
state. Thus you can use such objects to create reentrant scanners, but
see also *Note Reentrant::. You can instantiate multiple instances of
the same `yyFlexLexer' class, and you can also combine multiple C++
scanner classes together in the same program using the `-P' option
discussed above.
Finally, note that the `%array' feature is not available to C++
scanner classes; you must use `%pointer' (the default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class.
%option noyywrap
%{
int mylineno = 0;
%}
string \"[^\n"]+\"
ws [ \t]+
alpha [A-Za-z]
dig [0-9]
name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)?
num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
number {num1}|{num2}
%%
{ws} /* skip blanks and tabs */
"/*" {
int c;
while((c = yyinput()) != 0)
{
if(c == '\n')
++mylineno;
else if(c == '*')
{
if((c = yyinput()) == '/')
break;
else
unput(c);
}
}
}
{number} std::cout << "number " << YYText() << '\n';
\n mylineno++;
{name} std::cout << "name " << YYText() << '\n';
{string} std::cout << "string " << YYText() << '\n';
%%
int main( int /* argc */, char** /* argv */ )
{
FlexLexer* lexer = new yyFlexLexer;
while(lexer->yylex() != 0)
;
return 0;
}
If you want to create multiple (different) lexer classes, you use the
`-P' flag (or the `prefix=' option) to rename each `yyFlexLexer' to
some other `xxFlexLexer'. You then can include `FlexLexer>' in your
other sources once per lexer class, first renaming `yyFlexLexer' as
follows:
#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <FflexLexer>
#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include FlexLexer>
if, for example, you used `%option prefix="xx"' for one of your
scanners and `%option prefix="zz"' for the other.
File: flex.info, Node: Reentrant, Next: Lex and Posix, Prev: Cxx, Up: Top
19 Reentrant C Scanners
***********************
`flex' has the ability to generate a reentrant C scanner. This is
accomplished by specifying `%option reentrant' (`-R') The generated
scanner is both portable, and safe to use in one or more separate
threads of control. The most common use for reentrant scanners is from
within multi-threaded applications. Any thread may create and execute
a reentrant `flex' scanner without the need for synchronization with
other threads.
* Menu:
* Reentrant Uses::
* Reentrant Overview::
* Reentrant Example::
* Reentrant Detail::
* Reentrant Functions::
File: flex.info, Node: Reentrant Uses, Next: Reentrant Overview, Prev: Reentrant, Up: Reentrant
19.1 Uses for Reentrant Scanners
================================
However, there are other uses for a reentrant scanner. For example, you
could scan two or more files simultaneously to implement a `diff' at
the token level (i.e., instead of at the character level):
/* Example of maintaining more than one active scanner. */
do {
int tok1, tok2;
tok1 = yylex( scanner_1 );
tok2 = yylex( scanner_2 );
if( tok1 != tok2 )
printf("Files are different.");
} while ( tok1 && tok2 );
Another use for a reentrant scanner is recursion. (Note that a
recursive scanner can also be created using a non-reentrant scanner and
buffer states. *Note Multiple Input Buffers::.)
The following crude scanner supports the `eval' command by invoking
another instance of itself.
/* Example of recursive invocation. */
%option reentrant
%%
"eval(".+")" {
yyscan_t scanner;
YY_BUFFER_STATE buf;
yylex_init( &scanner );
yytext[yyleng-1] = ' ';
buf = yy_scan_string( yytext + 5, scanner );
yylex( scanner );
yy_delete_buffer(buf,scanner);
yylex_destroy( scanner );
}
...
%%
File: flex.info, Node: Reentrant Overview, Next: Reentrant Example, Prev: Reentrant Uses, Up: Reentrant
19.2 An Overview of the Reentrant API
=====================================
The API for reentrant scanners is different than for non-reentrant
scanners. Here is a quick overview of the API:
`%option reentrant' must be specified.
* All functions take one additional argument: `yyscanner'
* All global variables are replaced by their macro equivalents. (We
tell you this because it may be important to you during debugging.)
* `yylex_init' and `yylex_destroy' must be called before and after
`yylex', respectively.
* Accessor methods (get/set functions) provide access to common
`flex' variables.
* User-specific data can be stored in `yyextra'.
File: flex.info, Node: Reentrant Example, Next: Reentrant Detail, Prev: Reentrant Overview, Up: Reentrant
19.3 Reentrant Example
======================
First, an example of a reentrant scanner:
/* This scanner prints "//" comments. */
%option reentrant stack
%x COMMENT
%%
"//" yy_push_state( COMMENT, yyscanner);
.|\n
<COMMENT>\n yy_pop_state( yyscanner );
<COMMENT>[^\n]+ fprintf( yyout, "%s\n", yytext);
%%
int main ( int argc, char * argv[] )
{
yyscan_t scanner;
yylex_init ( &scanner );
yylex ( scanner );
yylex_destroy ( scanner );
return 0;
}
File: flex.info, Node: Reentrant Detail, Next: Reentrant Functions, Prev: Reentrant Example, Up: Reentrant
19.4 The Reentrant API in Detail
================================
Here are the things you need to do or know to use the reentrant C API of
`flex'.
* Menu:
* Specify Reentrant::
* Extra Reentrant Argument::
* Global Replacement::
* Init and Destroy Functions::
* Accessor Methods::
* Extra Data::
* About yyscan_t::
File: flex.info, Node: Specify Reentrant, Next: Extra Reentrant Argument, Prev: Reentrant Detail, Up: Reentrant Detail
19.4.1 Declaring a Scanner As Reentrant
---------------------------------------
%option reentrant (-reentrant) must be specified.
Notice that `%option reentrant' is specified in the above example
(*note Reentrant Example::. Had this option not been specified, `flex'
would have happily generated a non-reentrant scanner without
complaining. You may explicitly specify `%option noreentrant', if you
do _not_ want a reentrant scanner, although it is not necessary. The
default is to generate a non-reentrant scanner.
File: flex.info, Node: Extra Reentrant Argument, Next: Global Replacement, Prev: Specify Reentrant, Up: Reentrant Detail
19.4.2 The Extra Argument
-------------------------
All functions take one additional argument: `yyscanner'.
Notice that the calls to `yy_push_state' and `yy_pop_state' both
have an argument, `yyscanner' , that is not present in a non-reentrant
scanner. Here are the declarations of `yy_push_state' and
`yy_pop_state' in the generated scanner:
static void yy_push_state ( int new_state , yyscan_t yyscanner ) ;
static void yy_pop_state ( yyscan_t yyscanner ) ;
Notice that the argument `yyscanner' appears in the declaration of
both functions. In fact, all `flex' functions in a reentrant scanner
have this additional argument. It is always the last argument in the
argument list, it is always of type `yyscan_t' (which is typedef'd to
`void *') and it is always named `yyscanner'. As you may have guessed,
`yyscanner' is a pointer to an opaque data structure encapsulating the
current state of the scanner. For a list of function declarations, see
*Note Reentrant Functions::. Note that preprocessor macros, such as
`BEGIN', `ECHO', and `REJECT', do not take this additional argument.
File: flex.info, Node: Global Replacement, Next: Init and Destroy Functions, Prev: Extra Reentrant Argument, Up: Reentrant Detail
19.4.3 Global Variables Replaced By Macros
------------------------------------------
All global variables in traditional flex have been replaced by macro
equivalents.
Note that in the above example, `yyout' and `yytext' are not plain
variables. These are macros that will expand to their equivalent lvalue.
All of the familiar `flex' globals have been replaced by their macro
equivalents. In particular, `yytext', `yyleng', `yylineno', `yyin',
`yyout', `yyextra', `yylval', and `yylloc' are macros. You may safely
use these macros in actions as if they were plain variables. We only
tell you this so you don't expect to link to these variables
externally. Currently, each macro expands to a member of an internal
struct, e.g.,
#define yytext (((struct yyguts_t*)yyscanner)->yytext_r)
One important thing to remember about `yytext' and friends is that
`yytext' is not a global variable in a reentrant scanner, you can not
access it directly from outside an action or from other functions. You
must use an accessor method, e.g., `yyget_text', to accomplish this.
(See below).
File: flex.info, Node: Init and Destroy Functions, Next: Accessor Methods, Prev: Global Replacement, Up: Reentrant Detail
19.4.4 Init and Destroy Functions
---------------------------------
`yylex_init' and `yylex_destroy' must be called before and after
`yylex', respectively.
int yylex_init ( yyscan_t * ptr_yy_globals ) ;
int yylex ( yyscan_t yyscanner ) ;
int yylex_destroy ( yyscan_t yyscanner ) ;
The function `yylex_init' must be called before calling any other
function. The argument to `yylex_init' is the address of an
uninitialized pointer to be filled in by `flex'. The contents of
`ptr_yy_globals' need not be initialized, since `flex' will overwrite
it anyway. The value stored in `ptr_yy_globals' should thereafter be
passed to `yylex()' and yylex_destroy(). Flex does not save the
argument passed to `yylex_init', so it is safe to pass the address of a
local pointer to `yylex_init'. The function `yylex' should be familiar
to you by now. The reentrant version takes one argument, which is the
value returned (via an argument) by `yylex_init'. Otherwise, it
behaves the same as the non-reentrant version of `yylex'.
`yylex_init' returns 0 (zero) on success, or non-zero on failure, in
which case, errno is set to one of the following values:
* ENOMEM Memory allocation error. *Note memory-management::.
* EINVAL Invalid argument.
The function `yylex_destroy' should be called to free resources used
by the scanner. After `yylex_destroy' is called, the contents of
`yyscanner' should not be used. Of course, there is no need to destroy
a scanner if you plan to reuse it. A `flex' scanner (both reentrant
and non-reentrant) may be restarted by calling `yyrestart'.
Below is an example of a program that creates a scanner, uses it,
then destroys it when done:
int main ()
{
yyscan_t scanner;
int tok;
yylex_init(&scanner);
while ((tok=yylex()) > 0)
printf("tok=%d yytext=%s\n", tok, yyget_text(scanner));
yylex_destroy(scanner);
return 0;
}
File: flex.info, Node: Accessor Methods, Next: Extra Data, Prev: Init and Destroy Functions, Up: Reentrant Detail
19.4.5 Accessing Variables with Reentrant Scanners
--------------------------------------------------
Accessor methods (get/set functions) provide access to common `flex'
variables.
Many scanners that you build will be part of a larger project.
Portions of your project will need access to `flex' values, such as
`yytext'. In a non-reentrant scanner, these values are global, so
there is no problem accessing them. However, in a reentrant scanner,
there are no global `flex' values. You can not access them directly.
Instead, you must access `flex' values using accessor methods (get/set
functions). Each accessor method is named `yyget_NAME' or `yyset_NAME',
where `NAME' is the name of the `flex' variable you want. For example:
/* Set the last character of yytext to NULL. */
void chop ( yyscan_t scanner )
{
int len = yyget_leng( scanner );
yyget_text( scanner )[len - 1] = '\0';
}
The above code may be called from within an action like this:
%%
.+\n { chop( yyscanner );}
You may find that `%option header-file' is particularly useful for
generating prototypes of all the accessor functions. *Note
option-header::.
File: flex.info, Node: Extra Data, Next: About yyscan_t, Prev: Accessor Methods, Up: Reentrant Detail
19.4.6 Extra Data
-----------------
User-specific data can be stored in `yyextra'.
In a reentrant scanner, it is unwise to use global variables to
communicate with or maintain state between different pieces of your
program. However, you may need access to external data or invoke
external functions from within the scanner actions. Likewise, you may
need to pass information to your scanner (e.g., open file descriptors,
or database connections). In a non-reentrant scanner, the only way to
do this would be through the use of global variables. `Flex' allows
you to store arbitrary, "extra" data in a scanner. This data is
accessible through the accessor methods `yyget_extra' and `yyset_extra'
from outside the scanner, and through the shortcut macro `yyextra' from
within the scanner itself. They are defined as follows:
#define YY_EXTRA_TYPE void*
YY_EXTRA_TYPE yyget_extra ( yyscan_t scanner );
void yyset_extra ( YY_EXTRA_TYPE arbitrary_data , yyscan_t scanner);
By default, `YY_EXTRA_TYPE' is defined as type `void *'. You will
have to cast `yyextra' and the return value from `yyget_extra' to the
appropriate value each time you access the extra data. To avoid
casting, you may override the default type by defining `YY_EXTRA_TYPE'
in section 1 of your scanner:
/* An example of overriding YY_EXTRA_TYPE. */
%{
#include <sys/stat.h>
#include <unistd.h>
#define YY_EXTRA_TYPE struct stat*
%}
%option reentrant
%%
__filesize__ printf( "%ld", yyextra->st_size );
__lastmod__ printf( "%ld", yyextra->st_mtime );
%%
void scan_file( char* filename )
{
yyscan_t scanner;
struct stat buf;
yylex_init ( &scanner );
yyset_in( fopen(filename,"r"), scanner );
stat( filename, &buf);
yyset_extra( &buf, scanner );
yylex ( scanner );
yylex_destroy( scanner );
}
File: flex.info, Node: About yyscan_t, Prev: Extra Data, Up: Reentrant Detail
19.4.7 About yyscan_t
---------------------
`yyscan_t' is defined as:
typedef void* yyscan_t;
It is initialized by `yylex_init()' to point to an internal
structure. You should never access this value directly. In particular,
you should never attempt to free it (use `yylex_destroy()' instead.)
File: flex.info, Node: Reentran |