The C Users' Group Library 1994 August

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FLEX(1) UNIX Programmer's Manual FLEX(1) NAME flex - fast lexical analyzer generator SYNOPSIS flex [ -dfirstvFILT -c[efmF] -Sskeleton_file ] [ filename ] DESCRIPTION flex is a rewrite of lex intended to right some of that tool's deficiencies: in particular, flex generates lexical analyzers much faster, and the analyzers use smaller tables and run faster. OPTIONS In addition to lex's -t flag, flex has the following options: -d makes the generated scanner run in debug mode. When- ever a pattern is recognized the scanner will write to stderr a line of the form: --accepting rule #n Rules are numbered sequentially with the first one being 1. -f has the same effect as lex's -f flag (do not compress the scanner tables); the mnemonic changes from fast compilation to (take your pick) full table or fast scanner. The actual compilation takes longer, since flex is I/O bound writing out the big table. This option is equivalent to -cf (see below). -i instructs flex to generate a case-insensitive scanner. The case of letters given in the flex input patterns will be ignored, and the rules will be matched regard- less of case. The matched text given in yytext will have the preserved case (i.e., it will not be folded). -r specifies that the scanner uses the REJECT action. -s 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. -v has the same meaning as for lex (print to stderr a sum- mary of statistics of the generated scanner). Many more statistics are printed, though, and the summary spans several lines. Most of the statistics are mean- ingless to the casual flex user. Printed 12/28/88 13 May 1987 1 FLEX(1) UNIX Programmer's Manual FLEX(1) -F specifies that the fast scanner table representation should be used. This representation is about as fast as the full table representation (-f), and for some sets of patterns will be considerably smaller (and for others, larger). In general, if the pattern set con- tains 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 representa- tion. If only the "identifier" rule is present and you then use a hash table or some such to detect the key- words, you're better off using -F. This option is equivalent to -cF (see below). -I instructs flex to generate an interactive scanner. Normally, scanners generated by flex always look ahead one character before deciding that a rule has been matched. At the possible cost of some scanning over- head (it's not clear that more overhead is involved), flex will generate a scanner which only looks ahead when needed. Such scanners are called interactive because if you want to write a scanner for an interac- tive system such as a command shell, you will probably want the user's input to be terminated with a newline, and without -I the user will have to type a character in addition to the newline in order to have the newline recognized. This leads to dreadful interactive perfor- mance. If all this seems to confusing, here's the general rule: if a human will be typing in input to your scanner, use -I, otherwise don't; if you don't care about how fast your scanners run and don't want to make any assumptions about the input to your scanner, always use -I. Note, -I cannot be used in conjunction with full or fast tables, i.e., the -f, -F, -cf, or -cF flags. -L instructs flex to not generate #line directives (see below). -T makes flex run in trace mode. It will generate a lot of messages to standard out concerning the form of the input and the resultant non-deterministic and Printed 12/28/88 13 May 1987 2 FLEX(1) UNIX Programmer's Manual FLEX(1) deterministic finite automatons. This option is mostly for use in maintaining flex. -c[efmF] controls the degree of table compression. -ce 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). -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 speci- fies that the alternate fast scanner representation (described above under the -F flag) should be used. - cm 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. A lone -c specifies that the scanner tables should be compressed but neither equivalence classes nor meta-equivalence classes should be used. 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. Other- wise the options may be freely mixed. 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 -ce -cm -c -c{f,F}e -c{f,F} fastest largest -Sskeleton_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. INCOMPATIBILITIES WITH LEX Printed 12/28/88 13 May 1987 3 FLEX(1) UNIX Programmer's Manual FLEX(1) flex is fully compatible with lex with the following excep- tions: - There is no run-time library to link with. You needn't specify -ll when linking, and you must supply a main program. (Hacker's note: since the lex library con- tains a main() which simply calls yylex(), you actually can be lazy and not supply your own main program and link with -ll.) - lex's %r (Ratfor scanners) and %t (translation table) options are not supported. - The do-nothing -n flag is not supported. - When definitions are expanded, flex encloses them in parentheses. With lex, the following NAME [A-Z][A-Z0-9]* %% foo{NAME}? printf( "Found it\n" ); %% 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 asso- ciated 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. - yymore() is not supported. - The undocumented lex-scanner internal variable yylineno is not supported. - If your input uses REJECT, you must run flex with the -r flag. If you leave out the flag, the scanner will abort at run-time with a message that the scanner was compiled without the flag being specified. - The input() routine is not redefinable, though may be called to read characters following whatever has been matched by a rule. If input() encounters and end-of- file the normal yywrap() processing is done. A ``real'' end-of-file is returned as EOF. Input can be controlled by redefining 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 buffer "buf" and return in the integer variable "result" either the number of characters read or the constant Printed 12/28/88 13 May 1987 4 FLEX(1) UNIX Programmer's Manual FLEX(1) YY_NULL (0 on Unix systems) systems) to indicate EOF. The default YY_INPUT reads from the file-pointer "yyin" (which is by default stdin), so if you just want to change the input file, you needn't redefine YY_INPUT - just point yyin at the input file. A sample redefinition of YY_INPUT (in the first section of the input file): %{ #undef YY_INPUT #define YY_INPUT(buf,result,max_size) \ result = (buf[0] = getchar()) == EOF ? YY_NULL : 1; %} You also can add in things like counting keeping track of the input line number this way; but don't expect your scanner to go very fast. - output() is not supported. Output from the ECHO macro is done to the file-pointer "yyout" (default stdout). - Trailing context is restricted to patterns which have either a fixed-sized leading part or a fixed-sized trailing part. For example, "a*/b" and "a/b*" are okay, but not "a*/b*". This restriction is due to a bug in the trailing context algorithm given in Princi- ples of Compiler Design (and Compilers - Principles, Techniques, and Tools) which can result in mismatches. Try the following lex program %% x+/xy printf( "I found \"%s\"\n", yytext ); on the input "xxy". (If anyone knows of a fast algo- rithm for finding the beginning of trailing context for an arbitrary pair of regular expressions, please let me know!) If you must have arbitrary trailing context, you can use yyless() to effect it. - flex reads only one input file, while lex's input is made up of the concatenation of its input files. ENHANCEMENTS - Exclusive start-conditions can be declared by using %x instead of %s. These start-conditions have the property that when they are active, no other rules are active. Thus a set of rules governed by the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input. This feature makes it easy to specify "mini-scanners" which scan portions of the input that are syntactically Printed 12/28/88 13 May 1987 5 FLEX(1) UNIX Programmer's Manual FLEX(1) different from the rest (e.g., comments). - flex dynamically resizes its internal tables, so direc- tives like "%a 3000" are not needed when specifying large scanners. - The scanning routine generated by flex is declared using the macro YY_DECL. By redefining this macro you can change the routine's name and its calling sequence. For example, you could use: #undef YY_DECL #define YY_DECL float lexscan( a, b ) float a, b; to give it the name lexscan, returning a float, and taking two floats as arguments. - flex generates #line directives mapping lines in the output to their origin in the input file. - You can put multiple actions on the same line, separated with semi-colons. With lex, the following foo handle_foo(); return 1; is truncated to foo handle_foo(); flex does not truncate the action. Actions that are not enclosed in braces are terminated at the end of the line. - Actions can be begun with %{ and terminated with %}. In this case, flex does not count braces to figure out where the action ends - actions are terminated by the closing %}. This feature is useful when the enclosed action has extraneous braces in it (usually in comments or inside inactive #ifdef's) that throw off the brace- count. - All of the scanner actions (e.g., ECHO, yywrap ...) except the unput() and input() routines, are written as macros, so they can be redefined if necessary without requiring a separate library to link to. FILES flex.skel skeleton scanner flex.fastskel skeleton scanner for -f and -F Printed 12/28/88 13 May 1987 6 FLEX(1) UNIX Programmer's Manual FLEX(1) flexskelcom.h common definitions for skeleton files flexskeldef.h definitions for compressed skeleton file fastskeldef.h definitions for -f, -F skeleton file SEE ALSO lex(1) M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator AUTHOR Vern Paxson, with the help of many ideas and much inspira- tion from Van Jacobson. Original version by Jef Poskanzer. Fast table representation is a partial implementation of a design done by Van Jacobson. The implementation was done by Kevin Gong and Vern Paxson. Thanks to the many flex beta-testers, especially Casey Lee- dom, Nick Christopher, Chris Faylor, Eric Goldman, Craig Leres, Mohamed el Lozy, Esmond Pitt, Jef Poskanzer, and Dave Tallman. Thanks to John Gilmore, Bob Mulcahy, Rich Salz, and Richard Stallman for help with various distribution headaches. Send comments to: Vern Paxson Real Time Systems Bldg. 46A Lawrence Berkeley Laboratory 1 Cyclotron Rd. Berkeley, CA 94720 (415) 486-6411 vern@lbl-{csam,rtsg}.arpa ucbvax!lbl-csam.arpa!vern DIAGNOSTICS flex scanner jammed - a scanner compiled with -s has encoun- tered an input string which wasn't matched by any of its rules. flex input buffer overflowed - a scanner rule matched a string long enough to overflow the scanner's internal input buffer (as large as BUFSIZ in "/usr/include/stdio.h"). You can edit flexskelcom.h and increase YY_BUF_SIZE and Printed 12/28/88 13 May 1987 7 FLEX(1) UNIX Programmer's Manual FLEX(1) YY_MAX_LINE to increase this limit. REJECT used and scanner was not generated using -r - jus like it sounds. Your scanner uses REJECT. You must run flex on the scanner description using the -r flag. old-style lex command ignored - the flex input contains a lex command (e.g., "%n 1000") which is being ignored. BUGS Use of unput() or input() trashes the current yytext and yyleng. Use of unput() to push back more text than was matched can result in the pushed-back text matching a beginning-of-line ('^') rule even though it didn't come at the beginning of the line. Nulls are not allowed in flex inputs or in the inputs to scanners generated by flex. Their presence generates fatal errors. Do not mix trailing context with the '|' operator used to specify that multiple rules use the same action. That is, avoid constructs like: foo/bar | bletch | bugprone { ... } They can result in subtle mismatches. This is actually not a problem if there is only one rule using trailing context and it is the first in the list (so the above example will actually work okay). The problem is due to fall-through in the action switch statement, causing non-trailing-context rules to execute the trailing-context code of their fellow rules. This should be fixed, as it's a nasty bug and not obvious. The proper fix is for flex to spit out a FLEX_TRAILING_CONTEXT_USED #define and then have the backup logic in a separate table which is consulted for each rule- match, rather than as part of the rule action. The place to do the tweaking is in add_accept() - any kind soul want to be a hero? The pattern: x{3} is considered to be variable-length for the purposes of trailing context, even though it has a clear fixed length. Due to both buffering of input and read-ahead, you cannot Printed 12/28/88 13 May 1987 8 FLEX(1) UNIX Programmer's Manual FLEX(1) intermix calls to, 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 was not compiled with -r, and greater than the number of states if it was. The scanner run-time speeds have not been optimized as much as they deserve. Van Jacobson's work shows that the can go quite a bit faster still. Printed 12/28/88 13 May 1987 9