Manual Pages  — FLEX

    Description
        a brief overview of the tool

    Some Simple Examples

    Format Of The Input File

    Patterns
        the extended regular expressions used by flex

    How The Input Is Matched
        the rules for determining what has been matched

    Actions
        how to specify what to do when a pattern is matched

    The Generated Scanner
        details regarding the scanner that flex produces;
        how to control the input source

    Start Conditions
        introducing context into your scanners, and
        managing "mini-scanners"

    Multiple Input Buffers
        how to manipulate multiple input sources; how to
        scan from strings instead of files

    End-of-file Rules
        special rules for matching the end of the input

    Miscellaneous Macros
        a summary of macros available to the actions

    Values Available To The User
        a summary of values available to the actions

    Interfacing With Yacc
        connecting flex scanners together with yacc parsers

    Options
        flex command-line options, and the "%option"
        directive

    Performance Considerations
        how to make your scanner go as fast as possible

    Generating C++ Scanners
        the (experimental) facility for generating C++
        scanner classes

    Incompatibilities With Lex And POSIX
        how flex differs from AT&T lex and the POSIX lex
        standard

    Diagnostics
        those error messages produced by flex (or scanners
        it generates) whose meanings might not be apparent

    Files
        files used by flex

    Deficiencies / Bugs
        known problems with flex

    See Also
        other documentation, related tools

    Author
        includes contact information

    %%
    username    printf( "%s", getlogin() );

Here's another simple example:

    %{
            int num_lines = 0, num_chars = 0;
    %}

    %%
          ++num_lines; ++num_chars;
    .       ++num_chars;

    %%
    main()
            {
            yylex();
            printf( "# of lines = %d, # of chars = %d ,
                    num_lines, num_chars );
            }

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) , yytext,
                        atoi( yytext ) );
                }

    {DIGIT}+"."{DIGIT}*        {
                printf( "A float: %s (%g) , yytext,
                        atof( yytext ) );
                }

    if|then|begin|end|procedure|function        {
                printf( "A keyword: %s , yytext );
                }

    {ID}        printf( "An identifier: %s , yytext );

    "+"|"-"|"*"|"/"   printf( "An operator: %s , yytext );

    "{"[^} *"}"     /* eat up one-line comments */

    [ \t +          /* eat up whitespace */

    .           printf( "Unrecognized character: %s , 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();
        }

The details of this example will be explained in the following sections.

    definitions
    %%
    rules
    %%
    user code

Name definitions have the form:

    name definition

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*

    {DIGIT}+"."{DIGIT}*

    ([0-9])+"."([0-9])*

The rules section of the flex input contains a series of rules of the form:

    pattern   action

See below for a further description of patterns and actions.

Finally, 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.

In the definitions and rules sections, any indented text or text enclosed in %{ and %} is copied verbatim to the output (with the %{}'s removed). The %{}'s must appear unindented on lines by themselves.

In the rules section, any indented or %{} 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; see below for other such features).

In the definitions section (but not in the rules section), an unindented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/".

    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    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
               (see above)
    "[xyz]\"foo"
               the literal string: [xyz]"foo
    \X         if X is an '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; see notes in the
                 Deficiencies / Bugs section below 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/ .

               Note that flex's notion of "newline" is exactly
               whatever the C compiler used to compile flex
               interprets '  as; in particular, on some DOS
               systems you must either filter out \r's in the
               input yourself, or explicitly use r/\r for "r$".


    <s>r       an r, but only in start condition s (see
                 below 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

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. For example,

    foo|bar*

    (foo)|(ba(r*))

    foo|(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:]

For example, the following character classes are all equivalent:

    [[:alnum:]]
    [[:alpha:][:digit:]]
    [[:alpha:]0-9]
    [a-zA-Z0-9]

Some notes on patterns:

-	A negated character class such as the example "[^A-Z]" above will match a newline unless "  (or an equivalent escape sequence) is one of the characters explicitly present in the negated character class (e.g., "[^A-Z "). 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 '$' which does not occur at the end of a rule loses its special properties and is treated as a normal character.
	The following are illegal: foo/bar$ <sc1>foo<sc2>bar Note that the first of these, can be written "foo/bar .
	The following will result in '$' or '^' being treated as a normal character: foo|(bar$) foo|^bar If what's wanted is a "foo" or a bar-followed-by-a-newline, the following could be used (the special '|' action is explained below): foo | bar$ /* action goes here */ A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line.

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 (a more detailed description of actions follows), 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 legal flex input is:

    %%

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 (see the next section), 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 (see below). Furthermore, existing lex programs sometimes access yytext externally using declarations of the form:

    extern char yytext[];

%array 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 (the c++ option; see below).

    %%
    "zap me"

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 (see above); 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 "How the Input is Matched", 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 + ++word_count; Without the REJECT, any "frob"'s in the input would not be counted as words, since the scanner normally executes only one action per token. Multiple REJECT's 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; .|  /* 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 (see below).
	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 (see How The Input Is Matched).
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: %% "/*" { 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 (see The Generated Scanner, below). This action is a special case of the more general yy_flush_buffer() function, described below in the section 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.

    int yylex()
        {
        ... various definitions and the actions in here ...
        }

    #define YY_DECL float lexscan( a, b ) float a, b;

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 nil, 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 better is to use YY_FLUSH_BUFFER (see above). Note that yyrestart() does not reset the start condition to INITIAL (see Start Conditions, below).

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. YY_INPUT's calling sequence 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".

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); \
        }
    %}

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 -ll to obtain the default version of the routine, which always returns 1.

Three routines are available for scanning from in-memory buffers rather than files: yy_scan_string(), yy_scan_bytes(), and yy_scan_buffer(). See the discussion of them below in the section 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.

    <STRING>[^"]*        { /* eat up the string body ... */
                ...
                }

    <INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */
                ...
                }

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();

    %x example
    %%

    <example>foo   do_something();

    <INITIAL,example>bar    something_else();

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:

    <*>.|     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]+"."[0-9]+      {
                printf( "found a float, = %f ,
                        atof( yytext ) );
                }
    <expect>           {
                /* 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 ,
                        atoi( yytext ) );
                }

    "."         printf( "found a dot  );

    %x comment
    %%
            int line_num = 1;

    "/*"         BEGIN(comment);

    <comment>[^* *        /* eat anything that's not a '*' */
    <comment>"*"+[^*/ *   /* eat up '*'s not followed by '/'s */
    <comment>             ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

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>[^* *        /* eat anything that's not a '*' */
    <comment>"*"+[^*/ *   /* eat up '*'s not followed by '/'s */
    <comment>             ++line_num;
    <comment>"*"+"/"        BEGIN(comment_caller);

    comment_caller = YY_START;

Note that start conditions do not have their own name-space; %s's and %x's declare names in the same fashion as #define's.

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>        {
            /* 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>nbsp; *string_buf_ptr++ = ' ;
    <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>\\(.|   *string_buf_ptr++ = yytext[1];

    <str>[^\nbsp;
            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>{

    <ESC>{
        "nbsp;   return ' ;
        "\\r"   return '\r';
        "\\f"   return '\f';
        "\\0"   return '\0';
    }

    <ESC>"nbsp;  return ' ;
    <ESC>"\\r"  return '\r';
    <ESC>"\\f"  return '\f';
    <ESC>"\\0"  return '\0';

Three routines are available for manipulating stacks of start conditions:

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).
void yy_pop_state()
	pops the top of the stack and switches to it via BEGIN.
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 (see Options below).

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:

    YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

    void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

    void yy_delete_buffer( YY_BUFFER_STATE buffer )

    void yy_flush_buffer( YY_BUFFER_STATE buffer )

yy_new_buffer() is an alias for yy_create_buffer(), provided for compatibility with the C++ use of new and delete for creating and destroying dynamic objects.

Finally, the YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer.

Here is an example of using these features for writing a scanner which expands include files (the <<EOF>> feature is discussed below):

    /* 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 *         ECHO;

    <incl>[ \t]*      /* eat the whitespace */
    <incl>[^ \t +   { /* 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] );
                }
            }

    <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();
             }

    #define YY_USER_ACTION ++ctr[yy_act]

    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 below). 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 interactive, %option always-interactive or %option never-interactive (see Options below). 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.

    %{
    #include "y.tab.h"
    %}

    %%

    [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;

    --accepting rule at line 53 ("the matched text")

    "case"    return TOK_CASE;
    "switch"  return TOK_SWITCH;
    ...
    "default" return TOK_DEFAULT;
    [a-z]+    return TOK_ID;

    slowest & smallest
          -Cem
          -Cm
          -Ce
          -C
          -C{f,F}e
          -C{f,F}
          -C{f,F}a
    fastest & largest

    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

    7bit            -7 option
    8bit            -8 option
    align           -Ca option
    backup          -b option
    batch           -B option
    c++             -+ option

    caseful or
    case-sensitive  opposite of -i (default)

    case-insensitive or
    caseless        -i option

    debug           -d option
    default         opposite of -s option
    ecs             -Ce option
    fast            -F option
    full            -f option
    interactive     -I option
    lex-compat      -l option
    meta-ecs        -Cm option
    perf-report     -p option
    read            -Cr option
    stdout          -t option
    verbose         -v option
    warn            opposite of -w option
                    (use "%option nowarn" for -w)

    array           equivalent to "%array"
    pointer         equivalent to "%pointer" (default)

Three options take string-delimited values, offset with '=':

    %option outfile="ABC"

    %option prefix="XYZ"

    %option yyclass="foo"

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

    REJECT
    %option yylineno
    arbitrary trailing context

    pattern sets that require backing up
    %array
    %option interactive
    %option always-interactive

    '^' beginning-of-line operator
    yymore()

REJECT should be avoided at all costs when performance is important. It is a particularly expensive option.

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;

    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 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;

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();

    %%
    mouse/cat|dog         run();
    rat/cat|dog           run();

    %%
    mouse|rat/cat         run();
    mouse|rat/dog         run();

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>[^* *
    <comment>"*"+[^*/ *
    <comment>             ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

    %x comment
    %%
            int line_num = 1;

    "/*"         BEGIN(comment);

    <comment>[^* *
    <comment>[^* *      ++line_num;
    <comment>"*"+[^*/ *
    <comment>"*"+[^*/ * ++line_num;
    <comment>"*"+"/"        BEGIN(INITIAL);

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 */

    .|     /* it's not a keyword */

    %%
    asm      |
    auto     |
    break    |
    ... etc ...
    volatile |
    while    /* it's a keyword */

    [a-z]+   |
    .|     /* it's not a keyword */

    %%
    asm    |
    auto   |
    break  |
    ... etc ...
    volatile |
    while  /* it's a keyword */

    [a-z]+ |
    .|     /* it's not a keyword */

    %%
    asm    |
    auto   |
    break  |
    ... etc ...
    volatile |
    while  /* it's a keyword */

    [a-z]+ |
    [a-z]+   |
    .|     /* it's not a keyword */

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 above in the section How the Input is Matched, 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/token.

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 (see the Options section above). Note that you must build the scanner using %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 std::istream* object pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a std::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( std::istream* arg_yyin = 0, std::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(std::istream* new_in = 0, std::ostream* new_out = 0) reassigns yyin to new_in (if non-nil) and yyout to new_out (ditto), deleting the previous input buffer if yyin is reassigned.
	int yylex( std::istream* new_in, std::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) 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.
	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. 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.

    %{
    int mylineno = 0;
    %}

    string  \"[^ ]+\"

    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 == ' )
                    ++mylineno;

                else if(c == '*')
                    {
                    if((c = yyinput()) == '/')
                        break;
                    else
                        unput(c);
                    }
                }
            }

    {number}  cout << "number " << YYText() << ' ;

            mylineno++;

    {name}    cout << "name " << YYText() << ' ;

    {string}  cout << "string " << YYText() << ' ;

    %%

    int main( int /* argc */, char** /* argv */ )
        {
        FlexLexer* lexer = new yyFlexLexer;
        while(lexer->yylex() != 0)
            ;
        return 0;
        }

    #undef yyFlexLexer
    #define yyFlexLexer xxFlexLexer
    #include <FlexLexer.h>

    #undef yyFlexLexer
    #define yyFlexLexer zzFlexLexer
    #include <FlexLexer.h>

IMPORTANT: the present form of the scanning class is experimental and may change considerably between major releases.

In this section we discuss all of the known areas of incompatibility between flex, AT&T lex, and the POSIX specification.

flex's -l option turns on maximum compatibility with the original AT&T lex implementation, at the cost of a major loss in the generated scanner's performance. We note below which incompatibilities can be overcome using the -l option.

flex is fully compatible with lex with the following exceptions:

-	The undocumented lex scanner internal variable yylineno is not supported unless -l or %option yylineno is used.
	yylineno should be maintained on a per-buffer basis, rather than a per-scanner (single global variable) basis.
	yylineno is not part of the POSIX specification.
-	The input() routine is not redefinable, though it may be called to read characters following whatever has been matched by a rule. If input() encounters an end-of-file the normal yywrap() processing is done. A ``real'' end-of-file is returned by input() as EOF.
	Input is instead controlled by defining the YY_INPUT macro.
	The flex restriction that input() cannot be redefined is in accordance with the POSIX specification, which simply does not specify any way of controlling the scanner's input other than by making an initial assignment to yyin.
-	The unput() routine is not redefinable. This restriction is in accordance with POSIX.
-	flex scanners are not as reentrant as lex scanners. In particular, if you have an interactive scanner and an interrupt handler which long-jumps out of the scanner, and the scanner is subsequently called again, you may get the following message: fatal flex scanner internal error--end of buffer missed To reenter the scanner, first use yyrestart( yyin ); Note that this call will throw away any buffered input; usually this isn't a problem with an interactive scanner.
	Also note that flex C++ scanner classes are reentrant, so if using C++ is an option for you, you should use them instead. See "Generating C++ Scanners" above for details.
-	output() is not supported. Output from the ECHO macro is done to the file-pointer yyout (default stdout).
	output() is not part of the POSIX specification.
-	lex does not support exclusive start conditions (%x), though they are in the POSIX specification.
-	When definitions are expanded, flex encloses them in parentheses. With lex, the following: NAME [A-Z][A-Z0-9]* %% foo{NAME}? printf( "Found it  ); %% will not match the string "foo" because when the macro is expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is such that the '?' is associated with "[A-Z0-9]*". With flex, the rule will be expanded to "foo([A-Z][A-Z0-9]*)?" and so the string "foo" will match.
	Note that if the definition begins with ^ or ends with $ then it is not expanded with parentheses, to allow these operators to appear in definitions without losing their special meanings. But the <s>, /, and <<EOF>> operators cannot be used in a flex definition.
	Using -l results in the lex behavior of no parentheses around the definition.
	The POSIX specification is that the definition be enclosed in parentheses.
-	Some implementations of lex allow a rule's action to begin on a separate line, if the rule's pattern has trailing whitespace: %% foo|bar<space here> { foobar_action(); } flex does not support this feature.
-	The lex %r (generate a Ratfor scanner) option is not supported. It is not part of the POSIX specification.
-	After a call to unput(), yytext is undefined until the next token is matched, unless the scanner was built using %array. This is not the case with lex or the POSIX specification. The -l option does away with this incompatibility.
-	The precedence of the {} (numeric range) operator is different. lex interprets "abc{1,3}" as "match one, two, or three occurrences of 'abc'", whereas flex interprets it as "match 'ab' followed by one, two, or three occurrences of 'c'". The latter is in agreement with the POSIX specification.
-	The precedence of the ^ operator is different. lex interprets "^foo|bar" as "match either 'foo' at the beginning of a line, or 'bar' anywhere", whereas flex interprets it as "match either 'foo' or 'bar' if they come at the beginning of a line". The latter is in agreement with the POSIX specification.
-	The special table-size declarations such as %a supported by lex are not required by flex scanners; flex ignores them.
-	The name FLEX_SCANNER is #define'd so scanners may be written for use with either flex or lex. Scanners also include YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION indicating which version of flex generated the scanner (for example, for the 2.5 release, these defines would be 2 and 5 respectively).

The following flex features are not included in lex or the POSIX specification:

    C++ scanners
    %option
    start condition scopes
    start condition stacks
    interactive/non-interactive scanners
    yy_scan_string() and friends
    yyterminate()
    yy_set_interactive()
    yy_set_bol()
    YY_AT_BOL()
    <<EOF>>
    <*>
    YY_DECL
    YY_START
    YY_USER_ACTION
    YY_USER_INIT
    #line directives
    %{}'s around actions
    multiple actions on a line

    foo    handle_foo(); ++num_foos_seen;

    foo    handle_foo();

    [a-z]+    got_identifier();
    foo       got_foo();

warning, -s option given but default rule can be matched means that it is possible (perhaps only in a particular start condition) that the default rule (match any single character) is the only one that will match a particular input. Since -s was given, presumably this is not intended.

reject_used_but_not_detected undefined or yymore_used_but_not_detected undefined - These errors can occur at compile time. They indicate that the scanner uses REJECT or yymore() but that flex failed to notice the fact, meaning that flex scanned the first two sections looking for occurrences of these actions and failed to find any, but somehow you snuck some in (via a #include file, for example). Use %option reject or %option yymore to indicate to flex that you really do use these features.

flex scanner jammed - a scanner compiled with -s has encountered an input string which wasn't matched by any of its rules. This error can also occur due to internal problems.

token too large, exceeds YYLMAX - your scanner uses %array and one of its rules matched a string longer than the YYLMAX constant (8K bytes by default). You can increase the value by #define'ing YYLMAX in the definitions section of your flex input.

scanner requires -8 flag to use the character 'x' - Your scanner specification includes recognizing the 8-bit character 'x' and you did not specify the -8 flag, and your scanner defaulted to 7-bit because you used the -Cf or -CF table compression options. See the discussion of the -7 flag for details.

flex scanner push-back overflow - you used unput() to push back so much text that the scanner's buffer could not hold both the pushed-back text and the current token in yytext. Ideally the scanner should dynamically resize the buffer in this case, but at present it does not.

input buffer overflow, can't enlarge buffer because scanner uses REJECT - the scanner was working on matching an extremely large token and needed to expand the input buffer. This doesn't work with scanners that use REJECT.

fatal flex scanner internal error--end of buffer missed - This can occur in a scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use:

    yyrestart( yyin );

too many start conditions in <> construct! - you listed more start conditions in a <> construct than exist (so you must have listed at least one of them twice).

For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the above mentioned performance loss. In particular, parts using '|' or {n} (such as "foo{3}") are always considered variable-length.

Combining trailing context with the special '|' action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, in the following:

    %%
    abc      |
    xyz/def

Use of unput() invalidates yytext and yyleng, unless the %array directive or the -l option has been used.

Pattern-matching of NUL's is substantially slower than matching other characters.

Dynamic resizing of the input buffer is slow, as it entails rescanning all the text matched so far by the current (generally huge) token.

Due to both buffering of input and read-ahead, you cannot intermix calls to <stdio.h> routines, such as, for example, getchar(), with flex rules and expect it to work. Call input() instead.

The total table entries listed by the -v flag excludes the number of table entries needed to determine what rule has been matched. The number of entries is equal to the number of DFA states if the scanner does not use REJECT, and somewhat greater than the number of states if it does.

REJECT cannot be used with the -f or -F options.

The flex internal algorithms need documentation.

Thanks to the many flex beta-testers, feedbackers, and contributors, especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Adermann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, Nelson H.F. Beebe, benson@odi.com, Karl Berry, Peter A. Bigot, Simon Blanchard, Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick Christopher, Brian Clapper, J.T. Conklin, Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis, Scott David Daniels, Chris G. Demetriou, Theo de Raadt, Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R. Ghazi, Wolfgang Glunz, Eric Goldman, Christopher M. Gould, Ulrich Grepel, Peer Griebel, Jan Hajic, Charles Hemphill, NORO Hideo, Jarkko Hietaniemi, Scott Hofmann, Jeff Honig, Dana Hudes, Eric Hughes, John Interrante, Ceriel Jacobs, Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry Juengst, Klaus Kaempf, Jonathan I. Kamens, Terrence O Kane, Amir Katz, ken@ken.hilco.com, Kevin B. Kenny, Steve Kirsch, Winfried Koenig, Marq Kole, Ronald Lamprecht, Greg Lee, Rohan Lenard, Craig Leres, John Levine, Steve Liddle, David Loffredo, Mike Long, Mohamed el Lozy, Brian Madsen, Malte, Joe Marshall, Bengt Martensson, Chris Metcalf, Luke Mewburn, Jim Meyering, R. Alexander Milowski, Erik Naggum, G.T. Nicol, Landon Noll, James Nordby, Marc Nozell, Richard Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Walter Pelissero, Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Jarmo Raiha, Frederic Raimbault, Pat Rankin, Rick Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim Roskind, Alberto Santini, Andreas Scherer, Darrell Schiebel, Raf Schietekat, Doug Schmidt, Philippe Schnoebelen, Andreas Schwab, Larry Schwimmer, Alex Siegel, Eckehard Stolz, Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman, Ian Lance Taylor, Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik, Frank Whaley, Gerhard Wilhelms, Kent Williams, Ken Yap, Ron Zellar, Nathan Zelle, David Zuhn, and those whose names have slipped my marginal mail-archiving skills but whose contributions are appreciated all the same.

Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T. Nicol, Francois Pinard, Rich Salz, and Richard Stallman for help with various distribution headaches.

Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to Benson Margulies and Fred Burke for C++ support; to Kent Williams and Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL's; and to Eric Hughes for support of multiple buffers.

This work was primarily done when I was with the Real Time Systems Group at the Lawrence Berkeley Laboratory in Berkeley, CA. Many thanks to all there for the support I received.

Send comments to vern@ee.lbl.gov.