1<html> 2<head> 3<title>The Lemon Parser Generator</title> 4</head> 5<body bgcolor=white> 6<h1 align=center>The Lemon Parser Generator</h1> 7 8<p>Lemon is an LALR(1) parser generator for C or C++. 9It does the same job as ``bison'' and ``yacc''. 10But lemon is not another bison or yacc clone. It 11uses a different grammar syntax which is designed to 12reduce the number of coding errors. Lemon also uses a more 13sophisticated parsing engine that is faster than yacc and 14bison and which is both reentrant and thread-safe. 15Furthermore, Lemon implements features that can be used 16to eliminate resource leaks, making is suitable for use 17in long-running programs such as graphical user interfaces 18or embedded controllers.</p> 19 20<p>This document is an introduction to the Lemon 21parser generator.</p> 22 23<h2>Theory of Operation</h2> 24 25<p>The main goal of Lemon is to translate a context free grammar (CFG) 26for a particular language into C code that implements a parser for 27that language. 28The program has two inputs: 29<ul> 30<li>The grammar specification. 31<li>A parser template file. 32</ul> 33Typically, only the grammar specification is supplied by the programmer. 34Lemon comes with a default parser template which works fine for most 35applications. But the user is free to substitute a different parser 36template if desired.</p> 37 38<p>Depending on command-line options, Lemon will generate between 39one and three files of outputs. 40<ul> 41<li>C code to implement the parser. 42<li>A header file defining an integer ID for each terminal symbol. 43<li>An information file that describes the states of the generated parser 44 automaton. 45</ul> 46By default, all three of these output files are generated. 47The header file is suppressed if the ``-m'' command-line option is 48used and the report file is omitted when ``-q'' is selected.</p> 49 50<p>The grammar specification file uses a ``.y'' suffix, by convention. 51In the examples used in this document, we'll assume the name of the 52grammar file is ``gram.y''. A typical use of Lemon would be the 53following command: 54<pre> 55 lemon gram.y 56</pre> 57This command will generate three output files named ``gram.c'', 58``gram.h'' and ``gram.out''. 59The first is C code to implement the parser. The second 60is the header file that defines numerical values for all 61terminal symbols, and the last is the report that explains 62the states used by the parser automaton.</p> 63 64<h3>Command Line Options</h3> 65 66<p>The behavior of Lemon can be modified using command-line options. 67You can obtain a list of the available command-line options together 68with a brief explanation of what each does by typing 69<pre> 70 lemon -? 71</pre> 72As of this writing, the following command-line options are supported: 73<ul> 74<li><tt>-b</tt> 75<li><tt>-c</tt> 76<li><tt>-g</tt> 77<li><tt>-m</tt> 78<li><tt>-q</tt> 79<li><tt>-s</tt> 80<li><tt>-x</tt> 81</ul> 82The ``-b'' option reduces the amount of text in the report file by 83printing only the basis of each parser state, rather than the full 84configuration. 85The ``-c'' option suppresses action table compression. Using -c 86will make the parser a little larger and slower but it will detect 87syntax errors sooner. 88The ``-g'' option causes no output files to be generated at all. 89Instead, the input grammar file is printed on standard output but 90with all comments, actions and other extraneous text deleted. This 91is a useful way to get a quick summary of a grammar. 92The ``-m'' option causes the output C source file to be compatible 93with the ``makeheaders'' program. 94Makeheaders is a program that automatically generates header files 95from C source code. When the ``-m'' option is used, the header 96file is not output since the makeheaders program will take care 97of generated all header files automatically. 98The ``-q'' option suppresses the report file. 99Using ``-s'' causes a brief summary of parser statistics to be 100printed. Like this: 101<pre> 102 Parser statistics: 74 terminals, 70 nonterminals, 179 rules 103 340 states, 2026 parser table entries, 0 conflicts 104</pre> 105Finally, the ``-x'' option causes Lemon to print its version number 106and then stops without attempting to read the grammar or generate a parser.</p> 107 108<h3>The Parser Interface</h3> 109 110<p>Lemon doesn't generate a complete, working program. It only generates 111a few subroutines that implement a parser. This section describes 112the interface to those subroutines. It is up to the programmer to 113call these subroutines in an appropriate way in order to produce a 114complete system.</p> 115 116<p>Before a program begins using a Lemon-generated parser, the program 117must first create the parser. 118A new parser is created as follows: 119<pre> 120 void *pParser = ParseAlloc( malloc ); 121</pre> 122The ParseAlloc() routine allocates and initializes a new parser and 123returns a pointer to it. 124The actual data structure used to represent a parser is opaque -- 125its internal structure is not visible or usable by the calling routine. 126For this reason, the ParseAlloc() routine returns a pointer to void 127rather than a pointer to some particular structure. 128The sole argument to the ParseAlloc() routine is a pointer to the 129subroutine used to allocate memory. Typically this means ``malloc()''.</p> 130 131<p>After a program is finished using a parser, it can reclaim all 132memory allocated by that parser by calling 133<pre> 134 ParseFree(pParser, free); 135</pre> 136The first argument is the same pointer returned by ParseAlloc(). The 137second argument is a pointer to the function used to release bulk 138memory back to the system.</p> 139 140<p>After a parser has been allocated using ParseAlloc(), the programmer 141must supply the parser with a sequence of tokens (terminal symbols) to 142be parsed. This is accomplished by calling the following function 143once for each token: 144<pre> 145 Parse(pParser, hTokenID, sTokenData, pArg); 146</pre> 147The first argument to the Parse() routine is the pointer returned by 148ParseAlloc(). 149The second argument is a small positive integer that tells the parse the 150type of the next token in the data stream. 151There is one token type for each terminal symbol in the grammar. 152The gram.h file generated by Lemon contains #define statements that 153map symbolic terminal symbol names into appropriate integer values. 154(A value of 0 for the second argument is a special flag to the 155parser to indicate that the end of input has been reached.) 156The third argument is the value of the given token. By default, 157the type of the third argument is integer, but the grammar will 158usually redefine this type to be some kind of structure. 159Typically the second argument will be a broad category of tokens 160such as ``identifier'' or ``number'' and the third argument will 161be the name of the identifier or the value of the number.</p> 162 163<p>The Parse() function may have either three or four arguments, 164depending on the grammar. If the grammar specification file request 165it, the Parse() function will have a fourth parameter that can be 166of any type chosen by the programmer. The parser doesn't do anything 167with this argument except to pass it through to action routines. 168This is a convenient mechanism for passing state information down 169to the action routines without having to use global variables.</p> 170 171<p>A typical use of a Lemon parser might look something like the 172following: 173<pre> 174 01 ParseTree *ParseFile(const char *zFilename){ 175 02 Tokenizer *pTokenizer; 176 03 void *pParser; 177 04 Token sToken; 178 05 int hTokenId; 179 06 ParserState sState; 180 07 181 08 pTokenizer = TokenizerCreate(zFilename); 182 09 pParser = ParseAlloc( malloc ); 183 10 InitParserState(&sState); 184 11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){ 185 12 Parse(pParser, hTokenId, sToken, &sState); 186 13 } 187 14 Parse(pParser, 0, sToken, &sState); 188 15 ParseFree(pParser, free ); 189 16 TokenizerFree(pTokenizer); 190 17 return sState.treeRoot; 191 18 } 192</pre> 193This example shows a user-written routine that parses a file of 194text and returns a pointer to the parse tree. 195(We've omitted all error-handling from this example to keep it 196simple.) 197We assume the existence of some kind of tokenizer which is created 198using TokenizerCreate() on line 8 and deleted by TokenizerFree() 199on line 16. The GetNextToken() function on line 11 retrieves the 200next token from the input file and puts its type in the 201integer variable hTokenId. The sToken variable is assumed to be 202some kind of structure that contains details about each token, 203such as its complete text, what line it occurs on, etc. </p> 204 205<p>This example also assumes the existence of structure of type 206ParserState that holds state information about a particular parse. 207An instance of such a structure is created on line 6 and initialized 208on line 10. A pointer to this structure is passed into the Parse() 209routine as the optional 4th argument. 210The action routine specified by the grammar for the parser can use 211the ParserState structure to hold whatever information is useful and 212appropriate. In the example, we note that the treeRoot field of 213the ParserState structure is left pointing to the root of the parse 214tree.</p> 215 216<p>The core of this example as it relates to Lemon is as follows: 217<pre> 218 ParseFile(){ 219 pParser = ParseAlloc( malloc ); 220 while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){ 221 Parse(pParser, hTokenId, sToken); 222 } 223 Parse(pParser, 0, sToken); 224 ParseFree(pParser, free ); 225 } 226</pre> 227Basically, what a program has to do to use a Lemon-generated parser 228is first create the parser, then send it lots of tokens obtained by 229tokenizing an input source. When the end of input is reached, the 230Parse() routine should be called one last time with a token type 231of 0. This step is necessary to inform the parser that the end of 232input has been reached. Finally, we reclaim memory used by the 233parser by calling ParseFree().</p> 234 235<p>There is one other interface routine that should be mentioned 236before we move on. 237The ParseTrace() function can be used to generate debugging output 238from the parser. A prototype for this routine is as follows: 239<pre> 240 ParseTrace(FILE *stream, char *zPrefix); 241</pre> 242After this routine is called, a short (one-line) message is written 243to the designated output stream every time the parser changes states 244or calls an action routine. Each such message is prefaced using 245the text given by zPrefix. This debugging output can be turned off 246by calling ParseTrace() again with a first argument of NULL (0).</p> 247 248<h3>Differences With YACC and BISON</h3> 249 250<p>Programmers who have previously used the yacc or bison parser 251generator will notice several important differences between yacc and/or 252bison and Lemon. 253<ul> 254<li>In yacc and bison, the parser calls the tokenizer. In Lemon, 255 the tokenizer calls the parser. 256<li>Lemon uses no global variables. Yacc and bison use global variables 257 to pass information between the tokenizer and parser. 258<li>Lemon allows multiple parsers to be running simultaneously. Yacc 259 and bison do not. 260</ul> 261These differences may cause some initial confusion for programmers 262with prior yacc and bison experience. 263But after years of experience using Lemon, I firmly 264believe that the Lemon way of doing things is better.</p> 265 266<h2>Input File Syntax</h2> 267 268<p>The main purpose of the grammar specification file for Lemon is 269to define the grammar for the parser. But the input file also 270specifies additional information Lemon requires to do its job. 271Most of the work in using Lemon is in writing an appropriate 272grammar file.</p> 273 274<p>The grammar file for lemon is, for the most part, free format. 275It does not have sections or divisions like yacc or bison. Any 276declaration can occur at any point in the file. 277Lemon ignores whitespace (except where it is needed to separate 278tokens) and it honors the same commenting conventions as C and C++.</p> 279 280<h3>Terminals and Nonterminals</h3> 281 282<p>A terminal symbol (token) is any string of alphanumeric 283and underscore characters 284that begins with an upper case letter. 285A terminal can contain lower class letters after the first character, 286but the usual convention is to make terminals all upper case. 287A nonterminal, on the other hand, is any string of alphanumeric 288and underscore characters than begins with a lower case letter. 289Again, the usual convention is to make nonterminals use all lower 290case letters.</p> 291 292<p>In Lemon, terminal and nonterminal symbols do not need to 293be declared or identified in a separate section of the grammar file. 294Lemon is able to generate a list of all terminals and nonterminals 295by examining the grammar rules, and it can always distinguish a 296terminal from a nonterminal by checking the case of the first 297character of the name.</p> 298 299<p>Yacc and bison allow terminal symbols to have either alphanumeric 300names or to be individual characters included in single quotes, like 301this: ')' or '$'. Lemon does not allow this alternative form for 302terminal symbols. With Lemon, all symbols, terminals and nonterminals, 303must have alphanumeric names.</p> 304 305<h3>Grammar Rules</h3> 306 307<p>The main component of a Lemon grammar file is a sequence of grammar 308rules. 309Each grammar rule consists of a nonterminal symbol followed by 310the special symbol ``::='' and then a list of terminals and/or nonterminals. 311The rule is terminated by a period. 312The list of terminals and nonterminals on the right-hand side of the 313rule can be empty. 314Rules can occur in any order, except that the left-hand side of the 315first rule is assumed to be the start symbol for the grammar (unless 316specified otherwise using the <tt>%start</tt> directive described below.) 317A typical sequence of grammar rules might look something like this: 318<pre> 319 expr ::= expr PLUS expr. 320 expr ::= expr TIMES expr. 321 expr ::= LPAREN expr RPAREN. 322 expr ::= VALUE. 323</pre> 324</p> 325 326<p>There is one non-terminal in this example, ``expr'', and five 327terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'', 328``RPAREN'' and ``VALUE''.</p> 329 330<p>Like yacc and bison, Lemon allows the grammar to specify a block 331of C code that will be executed whenever a grammar rule is reduced 332by the parser. 333In Lemon, this action is specified by putting the C code (contained 334within curly braces <tt>{...}</tt>) immediately after the 335period that closes the rule. 336For example: 337<pre> 338 expr ::= expr PLUS expr. { printf("Doing an addition...\n"); } 339</pre> 340</p> 341 342<p>In order to be useful, grammar actions must normally be linked to 343their associated grammar rules. 344In yacc and bison, this is accomplished by embedding a ``$$'' in the 345action to stand for the value of the left-hand side of the rule and 346symbols ``$1'', ``$2'', and so forth to stand for the value of 347the terminal or nonterminal at position 1, 2 and so forth on the 348right-hand side of the rule. 349This idea is very powerful, but it is also very error-prone. The 350single most common source of errors in a yacc or bison grammar is 351to miscount the number of symbols on the right-hand side of a grammar 352rule and say ``$7'' when you really mean ``$8''.</p> 353 354<p>Lemon avoids the need to count grammar symbols by assigning symbolic 355names to each symbol in a grammar rule and then using those symbolic 356names in the action. 357In yacc or bison, one would write this: 358<pre> 359 expr -> expr PLUS expr { $$ = $1 + $3; }; 360</pre> 361But in Lemon, the same rule becomes the following: 362<pre> 363 expr(A) ::= expr(B) PLUS expr(C). { A = B+C; } 364</pre> 365In the Lemon rule, any symbol in parentheses after a grammar rule 366symbol becomes a place holder for that symbol in the grammar rule. 367This place holder can then be used in the associated C action to 368stand for the value of that symbol.<p> 369 370<p>The Lemon notation for linking a grammar rule with its reduce 371action is superior to yacc/bison on several counts. 372First, as mentioned above, the Lemon method avoids the need to 373count grammar symbols. 374Secondly, if a terminal or nonterminal in a Lemon grammar rule 375includes a linking symbol in parentheses but that linking symbol 376is not actually used in the reduce action, then an error message 377is generated. 378For example, the rule 379<pre> 380 expr(A) ::= expr(B) PLUS expr(C). { A = B; } 381</pre> 382will generate an error because the linking symbol ``C'' is used 383in the grammar rule but not in the reduce action.</p> 384 385<p>The Lemon notation for linking grammar rules to reduce actions 386also facilitates the use of destructors for reclaiming memory 387allocated by the values of terminals and nonterminals on the 388right-hand side of a rule.</p> 389 390<h3>Precedence Rules</h3> 391 392<p>Lemon resolves parsing ambiguities in exactly the same way as 393yacc and bison. A shift-reduce conflict is resolved in favor 394of the shift, and a reduce-reduce conflict is resolved by reducing 395whichever rule comes first in the grammar file.</p> 396 397<p>Just like in 398yacc and bison, Lemon allows a measure of control 399over the resolution of paring conflicts using precedence rules. 400A precedence value can be assigned to any terminal symbol 401using the %left, %right or %nonassoc directives. Terminal symbols 402mentioned in earlier directives have a lower precedence that 403terminal symbols mentioned in later directives. For example:</p> 404 405<p><pre> 406 %left AND. 407 %left OR. 408 %nonassoc EQ NE GT GE LT LE. 409 %left PLUS MINUS. 410 %left TIMES DIVIDE MOD. 411 %right EXP NOT. 412</pre></p> 413 414<p>In the preceding sequence of directives, the AND operator is 415defined to have the lowest precedence. The OR operator is one 416precedence level higher. And so forth. Hence, the grammar would 417attempt to group the ambiguous expression 418<pre> 419 a AND b OR c 420</pre> 421like this 422<pre> 423 a AND (b OR c). 424</pre> 425The associativity (left, right or nonassoc) is used to determine 426the grouping when the precedence is the same. AND is left-associative 427in our example, so 428<pre> 429 a AND b AND c 430</pre> 431is parsed like this 432<pre> 433 (a AND b) AND c. 434</pre> 435The EXP operator is right-associative, though, so 436<pre> 437 a EXP b EXP c 438</pre> 439is parsed like this 440<pre> 441 a EXP (b EXP c). 442</pre> 443The nonassoc precedence is used for non-associative operators. 444So 445<pre> 446 a EQ b EQ c 447</pre> 448is an error.</p> 449 450<p>The precedence of non-terminals is transferred to rules as follows: 451The precedence of a grammar rule is equal to the precedence of the 452left-most terminal symbol in the rule for which a precedence is 453defined. This is normally what you want, but in those cases where 454you want to precedence of a grammar rule to be something different, 455you can specify an alternative precedence symbol by putting the 456symbol in square braces after the period at the end of the rule and 457before any C-code. For example:</p> 458 459<p><pre> 460 expr = MINUS expr. [NOT] 461</pre></p> 462 463<p>This rule has a precedence equal to that of the NOT symbol, not the 464MINUS symbol as would have been the case by default.</p> 465 466<p>With the knowledge of how precedence is assigned to terminal 467symbols and individual 468grammar rules, we can now explain precisely how parsing conflicts 469are resolved in Lemon. Shift-reduce conflicts are resolved 470as follows: 471<ul> 472<li> If either the token to be shifted or the rule to be reduced 473 lacks precedence information, then resolve in favor of the 474 shift, but report a parsing conflict. 475<li> If the precedence of the token to be shifted is greater than 476 the precedence of the rule to reduce, then resolve in favor 477 of the shift. No parsing conflict is reported. 478<li> If the precedence of the token it be shifted is less than the 479 precedence of the rule to reduce, then resolve in favor of the 480 reduce action. No parsing conflict is reported. 481<li> If the precedences are the same and the shift token is 482 right-associative, then resolve in favor of the shift. 483 No parsing conflict is reported. 484<li> If the precedences are the same the the shift token is 485 left-associative, then resolve in favor of the reduce. 486 No parsing conflict is reported. 487<li> Otherwise, resolve the conflict by doing the shift and 488 report the parsing conflict. 489</ul> 490Reduce-reduce conflicts are resolved this way: 491<ul> 492<li> If either reduce rule 493 lacks precedence information, then resolve in favor of the 494 rule that appears first in the grammar and report a parsing 495 conflict. 496<li> If both rules have precedence and the precedence is different 497 then resolve the dispute in favor of the rule with the highest 498 precedence and do not report a conflict. 499<li> Otherwise, resolve the conflict by reducing by the rule that 500 appears first in the grammar and report a parsing conflict. 501</ul> 502 503<h3>Special Directives</h3> 504 505<p>The input grammar to Lemon consists of grammar rules and special 506directives. We've described all the grammar rules, so now we'll 507talk about the special directives.</p> 508 509<p>Directives in lemon can occur in any order. You can put them before 510the grammar rules, or after the grammar rules, or in the mist of the 511grammar rules. It doesn't matter. The relative order of 512directives used to assign precedence to terminals is important, but 513other than that, the order of directives in Lemon is arbitrary.</p> 514 515<p>Lemon supports the following special directives: 516<ul> 517<li><tt>%code</tt> 518<li><tt>%default_destructor</tt> 519<li><tt>%default_type</tt> 520<li><tt>%destructor</tt> 521<li><tt>%extra_argument</tt> 522<li><tt>%include</tt> 523<li><tt>%left</tt> 524<li><tt>%name</tt> 525<li><tt>%nonassoc</tt> 526<li><tt>%parse_accept</tt> 527<li><tt>%parse_failure </tt> 528<li><tt>%right</tt> 529<li><tt>%stack_overflow</tt> 530<li><tt>%stack_size</tt> 531<li><tt>%start_symbol</tt> 532<li><tt>%syntax_error</tt> 533<li><tt>%token_destructor</tt> 534<li><tt>%token_prefix</tt> 535<li><tt>%token_type</tt> 536<li><tt>%type</tt> 537</ul> 538Each of these directives will be described separately in the 539following sections:</p> 540 541<h4>The <tt>%code</tt> directive</h4> 542 543<p>The %code directive is used to specify addition C/C++ code that 544is added to the end of the main output file. This is similar to 545the %include directive except that %include is inserted at the 546beginning of the main output file.</p> 547 548<p>%code is typically used to include some action routines or perhaps 549a tokenizer as part of the output file.</p> 550 551<h4>The <tt>%default_destructor</tt> directive</h4> 552 553<p>The %default_destructor directive specifies a destructor to 554use for non-terminals that do not have their own destructor 555specified by a separate %destructor directive. See the documentation 556on the %destructor directive below for additional information.</p> 557 558<p>In some grammers, many different non-terminal symbols have the 559same datatype and hence the same destructor. This directive is 560a convenience way to specify the same destructor for all those 561non-terminals using a single statement.</p> 562 563<h4>The <tt>%default_type</tt> directive</h4> 564 565<p>The %default_type directive specifies the datatype of non-terminal 566symbols that do no have their own datatype defined using a separate 567%type directive. See the documentation on %type below for addition 568information.</p> 569 570<h4>The <tt>%destructor</tt> directive</h4> 571 572<p>The %destructor directive is used to specify a destructor for 573a non-terminal symbol. 574(See also the %token_destructor directive which is used to 575specify a destructor for terminal symbols.)</p> 576 577<p>A non-terminal's destructor is called to dispose of the 578non-terminal's value whenever the non-terminal is popped from 579the stack. This includes all of the following circumstances: 580<ul> 581<li> When a rule reduces and the value of a non-terminal on 582 the right-hand side is not linked to C code. 583<li> When the stack is popped during error processing. 584<li> When the ParseFree() function runs. 585</ul> 586The destructor can do whatever it wants with the value of 587the non-terminal, but its design is to deallocate memory 588or other resources held by that non-terminal.</p> 589 590<p>Consider an example: 591<pre> 592 %type nt {void*} 593 %destructor nt { free($$); } 594 nt(A) ::= ID NUM. { A = malloc( 100 ); } 595</pre> 596This example is a bit contrived but it serves to illustrate how 597destructors work. The example shows a non-terminal named 598``nt'' that holds values of type ``void*''. When the rule for 599an ``nt'' reduces, it sets the value of the non-terminal to 600space obtained from malloc(). Later, when the nt non-terminal 601is popped from the stack, the destructor will fire and call 602free() on this malloced space, thus avoiding a memory leak. 603(Note that the symbol ``$$'' in the destructor code is replaced 604by the value of the non-terminal.)</p> 605 606<p>It is important to note that the value of a non-terminal is passed 607to the destructor whenever the non-terminal is removed from the 608stack, unless the non-terminal is used in a C-code action. If 609the non-terminal is used by C-code, then it is assumed that the 610C-code will take care of destroying it if it should really 611be destroyed. More commonly, the value is used to build some 612larger structure and we don't want to destroy it, which is why 613the destructor is not called in this circumstance.</p> 614 615<p>By appropriate use of destructors, it is possible to 616build a parser using Lemon that can be used within a long-running 617program, such as a GUI, that will not leak memory or other resources. 618To do the same using yacc or bison is much more difficult.</p> 619 620<h4>The <tt>%extra_argument</tt> directive</h4> 621 622The %extra_argument directive instructs Lemon to add a 4th parameter 623to the parameter list of the Parse() function it generates. Lemon 624doesn't do anything itself with this extra argument, but it does 625make the argument available to C-code action routines, destructors, 626and so forth. For example, if the grammar file contains:</p> 627 628<p><pre> 629 %extra_argument { MyStruct *pAbc } 630</pre></p> 631 632<p>Then the Parse() function generated will have an 4th parameter 633of type ``MyStruct*'' and all action routines will have access to 634a variable named ``pAbc'' that is the value of the 4th parameter 635in the most recent call to Parse().</p> 636 637<h4>The <tt>%include</tt> directive</h4> 638 639<p>The %include directive specifies C code that is included at the 640top of the generated parser. You can include any text you want -- 641the Lemon parser generator copies it blindly. If you have multiple 642%include directives in your grammar file the value of the last 643%include directive overwrites all the others.</p. 644 645<p>The %include directive is very handy for getting some extra #include 646preprocessor statements at the beginning of the generated parser. 647For example:</p> 648 649<p><pre> 650 %include {#include <unistd.h>} 651</pre></p> 652 653<p>This might be needed, for example, if some of the C actions in the 654grammar call functions that are prototyed in unistd.h.</p> 655 656<h4>The <tt>%left</tt> directive</h4> 657 658The %left directive is used (along with the %right and 659%nonassoc directives) to declare precedences of terminal 660symbols. Every terminal symbol whose name appears after 661a %left directive but before the next period (``.'') is 662given the same left-associative precedence value. Subsequent 663%left directives have higher precedence. For example:</p> 664 665<p><pre> 666 %left AND. 667 %left OR. 668 %nonassoc EQ NE GT GE LT LE. 669 %left PLUS MINUS. 670 %left TIMES DIVIDE MOD. 671 %right EXP NOT. 672</pre></p> 673 674<p>Note the period that terminates each %left, %right or %nonassoc 675directive.</p> 676 677<p>LALR(1) grammars can get into a situation where they require 678a large amount of stack space if you make heavy use or right-associative 679operators. For this reason, it is recommended that you use %left 680rather than %right whenever possible.</p> 681 682<h4>The <tt>%name</tt> directive</h4> 683 684<p>By default, the functions generated by Lemon all begin with the 685five-character string ``Parse''. You can change this string to something 686different using the %name directive. For instance:</p> 687 688<p><pre> 689 %name Abcde 690</pre></p> 691 692<p>Putting this directive in the grammar file will cause Lemon to generate 693functions named 694<ul> 695<li> AbcdeAlloc(), 696<li> AbcdeFree(), 697<li> AbcdeTrace(), and 698<li> Abcde(). 699</ul> 700The %name directive allows you to generator two or more different 701parsers and link them all into the same executable. 702</p> 703 704<h4>The <tt>%nonassoc</tt> directive</h4> 705 706<p>This directive is used to assign non-associative precedence to 707one or more terminal symbols. See the section on precedence rules 708or on the %left directive for additional information.</p> 709 710<h4>The <tt>%parse_accept</tt> directive</h4> 711 712<p>The %parse_accept directive specifies a block of C code that is 713executed whenever the parser accepts its input string. To ``accept'' 714an input string means that the parser was able to process all tokens 715without error.</p> 716 717<p>For example:</p> 718 719<p><pre> 720 %parse_accept { 721 printf("parsing complete!\n"); 722 } 723</pre></p> 724 725 726<h4>The <tt>%parse_failure</tt> directive</h4> 727 728<p>The %parse_failure directive specifies a block of C code that 729is executed whenever the parser fails complete. This code is not 730executed until the parser has tried and failed to resolve an input 731error using is usual error recovery strategy. The routine is 732only invoked when parsing is unable to continue.</p> 733 734<p><pre> 735 %parse_failure { 736 fprintf(stderr,"Giving up. Parser is hopelessly lost...\n"); 737 } 738</pre></p> 739 740<h4>The <tt>%right</tt> directive</h4> 741 742<p>This directive is used to assign right-associative precedence to 743one or more terminal symbols. See the section on precedence rules 744or on the %left directive for additional information.</p> 745 746<h4>The <tt>%stack_overflow</tt> directive</h4> 747 748<p>The %stack_overflow directive specifies a block of C code that 749is executed if the parser's internal stack ever overflows. Typically 750this just prints an error message. After a stack overflow, the parser 751will be unable to continue and must be reset.</p> 752 753<p><pre> 754 %stack_overflow { 755 fprintf(stderr,"Giving up. Parser stack overflow\n"); 756 } 757</pre></p> 758 759<p>You can help prevent parser stack overflows by avoiding the use 760of right recursion and right-precedence operators in your grammar. 761Use left recursion and and left-precedence operators instead, to 762encourage rules to reduce sooner and keep the stack size down. 763For example, do rules like this: 764<pre> 765 list ::= list element. // left-recursion. Good! 766 list ::= . 767</pre> 768Not like this: 769<pre> 770 list ::= element list. // right-recursion. Bad! 771 list ::= . 772</pre> 773 774<h4>The <tt>%stack_size</tt> directive</h4> 775 776<p>If stack overflow is a problem and you can't resolve the trouble 777by using left-recursion, then you might want to increase the size 778of the parser's stack using this directive. Put an positive integer 779after the %stack_size directive and Lemon will generate a parse 780with a stack of the requested size. The default value is 100.</p> 781 782<p><pre> 783 %stack_size 2000 784</pre></p> 785 786<h4>The <tt>%start_symbol</tt> directive</h4> 787 788<p>By default, the start-symbol for the grammar that Lemon generates 789is the first non-terminal that appears in the grammar file. But you 790can choose a different start-symbol using the %start_symbol directive.</p> 791 792<p><pre> 793 %start_symbol prog 794</pre></p> 795 796<h4>The <tt>%token_destructor</tt> directive</h4> 797 798<p>The %destructor directive assigns a destructor to a non-terminal 799symbol. (See the description of the %destructor directive above.) 800This directive does the same thing for all terminal symbols.</p> 801 802<p>Unlike non-terminal symbols which may each have a different data type 803for their values, terminals all use the same data type (defined by 804the %token_type directive) and so they use a common destructor. Other 805than that, the token destructor works just like the non-terminal 806destructors.</p> 807 808<h4>The <tt>%token_prefix</tt> directive</h4> 809 810<p>Lemon generates #defines that assign small integer constants 811to each terminal symbol in the grammar. If desired, Lemon will 812add a prefix specified by this directive 813to each of the #defines it generates. 814So if the default output of Lemon looked like this: 815<pre> 816 #define AND 1 817 #define MINUS 2 818 #define OR 3 819 #define PLUS 4 820</pre> 821You can insert a statement into the grammar like this: 822<pre> 823 %token_prefix TOKEN_ 824</pre> 825to cause Lemon to produce these symbols instead: 826<pre> 827 #define TOKEN_AND 1 828 #define TOKEN_MINUS 2 829 #define TOKEN_OR 3 830 #define TOKEN_PLUS 4 831</pre> 832 833<h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4> 834 835<p>These directives are used to specify the data types for values 836on the parser's stack associated with terminal and non-terminal 837symbols. The values of all terminal symbols must be of the same 838type. This turns out to be the same data type as the 3rd parameter 839to the Parse() function generated by Lemon. Typically, you will 840make the value of a terminal symbol by a pointer to some kind of 841token structure. Like this:</p> 842 843<p><pre> 844 %token_type {Token*} 845</pre></p> 846 847<p>If the data type of terminals is not specified, the default value 848is ``int''.</p> 849 850<p>Non-terminal symbols can each have their own data types. Typically 851the data type of a non-terminal is a pointer to the root of a parse-tree 852structure that contains all information about that non-terminal. 853For example:</p> 854 855<p><pre> 856 %type expr {Expr*} 857</pre></p> 858 859<p>Each entry on the parser's stack is actually a union containing 860instances of all data types for every non-terminal and terminal symbol. 861Lemon will automatically use the correct element of this union depending 862on what the corresponding non-terminal or terminal symbol is. But 863the grammar designer should keep in mind that the size of the union 864will be the size of its largest element. So if you have a single 865non-terminal whose data type requires 1K of storage, then your 100 866entry parser stack will require 100K of heap space. If you are willing 867and able to pay that price, fine. You just need to know.</p> 868 869<h3>Error Processing</h3> 870 871<p>After extensive experimentation over several years, it has been 872discovered that the error recovery strategy used by yacc is about 873as good as it gets. And so that is what Lemon uses.</p> 874 875<p>When a Lemon-generated parser encounters a syntax error, it 876first invokes the code specified by the %syntax_error directive, if 877any. It then enters its error recovery strategy. The error recovery 878strategy is to begin popping the parsers stack until it enters a 879state where it is permitted to shift a special non-terminal symbol 880named ``error''. It then shifts this non-terminal and continues 881parsing. But the %syntax_error routine will not be called again 882until at least three new tokens have been successfully shifted.</p> 883 884<p>If the parser pops its stack until the stack is empty, and it still 885is unable to shift the error symbol, then the %parse_failed routine 886is invoked and the parser resets itself to its start state, ready 887to begin parsing a new file. This is what will happen at the very 888first syntax error, of course, if there are no instances of the 889``error'' non-terminal in your grammar.</p> 890 891</body> 892</html> 893