1 # LR(1) Parser Generator #
3 This parser generator takes a grammar description combined with code
4 fragments, analyses it, and can report details about the analysis and
5 write out C code files which can be compiled to make a parser.
7 There are several distinct sections.
9 1. `grammar_read` will read a grammar from a literate-code file and
10 store the productions, symbols, and code fragments.
11 2. `grammar_analyse` will take that grammar and build LR parsing
12 states and look-ahead tables.
13 3. `grammar_report` will present the details of the analysis
14 in a readable format and will report any conflicts.
15 4. `parser_generate` will write out C code files with various
16 tables and with the code fragments arranged in useful places.
17 5. `parser_run` is a library function intended to be linked together
18 with the generated parser tables to complete the implementation of
20 6. `calc.cgm` is a test grammar for a simple calculator.
22 ###### File: parsergen.c
27 ## forward declarations
38 ###### File: libparser.c
47 ###### File: parsergen.mk
50 parsergen.c parsergen.mk calc.cgm libparser.c parser.h : parsergen.mdc
53 ## Reading the grammar
55 The grammar must be stored in a markdown literate code file as parsed
56 by "[mdcode][]". It must have three top level (i.e. unreferenced)
57 sections: `header`, `code`, and `grammar`. The first two will be
58 literally copied into the generated `.c` and `.h`. files. The last
59 contains the grammar. This is tokenised with "[scanner][]".
62 [scanner]: scanner.html
68 ###### parser includes
72 The grammar contains production sets, precedence/associativity
73 declarations, and data type declarations. These are all parsed with
74 _ad hoc_ parsing as we don't have a parser generator yet.
76 The precedence and associativity can be set for each production, but
77 can be inherited from symbols. The data type is potentially defined
78 for each non-terminal and describes what C structure is used to store
79 information about each symbol.
82 enum assoc {Left, Right, Non};
83 char *assoc_names[] = {"Left","Right","Non"};
86 struct text struct_name;
88 unsigned short precedence;
92 unsigned short precedence;
100 The strings reported by `mdcode` and `scanner` are `struct text` which have
101 length rather than being null terminated. To help with printing and
102 comparing we define `text_is` and `prtxt`, which should possibly go in
103 `mdcode`. `scanner` does provide `text_dump` which is useful for strings
104 which might contain control characters.
107 static int text_is(struct text t, char *s)
109 return (strlen(s) == t.len &&
110 strncmp(s, t.txt, t.len) == 0);
112 static void prtxt(struct text t)
114 printf("%.*s", t.len, t.txt);
119 Productions are comprised primarily of symbols - terminal and
120 non-terminal. We do not make a syntactic distinction between the two,
121 though non-terminals must be identifiers. Non-terminal symbols are
122 those which appear in the head of a production, terminal symbols are
123 those which don't. There are also "virtual" symbols used for precedence
124 marking discussed later, and sometimes we won't know what type a symbol
127 ###### forward declarations
128 enum symtype { Unknown, Virtual, Terminal, Nonterminal };
129 char *symtypes = "UVTN";
133 Symbols can be either `TK_ident` or `TK_mark`. They are saved in a
134 table of known symbols and the resulting parser will report them as
135 `TK_reserved + N`. A small set of identifiers are reserved for the
136 different token types that `scanner` can report.
139 static char *reserved_words[] = {
140 [TK_error] = "ERROR",
141 [TK_number] = "NUMBER",
142 [TK_ident] = "IDENTIFIER",
144 [TK_string] = "STRING",
145 [TK_multi_string] = "MULTI_STRING",
148 [TK_newline] = "NEWLINE",
154 Note that `TK_eof` and the two `TK_*_comment` tokens cannot be
155 recognised. The former is automatically expected at the end of the text
156 being parsed. The latter are always ignored by the parser.
158 All of these symbols are stored in a simple symbol table. We use the
159 `struct text` from `mdcode` to store the name and link them together
160 into a sorted list using an insertion sort.
162 We don't have separate `find` and `insert` functions as any symbol we
163 find needs to be remembered. We simply expect `find` to always return a
164 symbol, but its type might be `Unknown`.
173 ###### grammar fields
178 static int text_cmp(struct text a, struct text b)
183 int cmp = strncmp(a.txt, b.txt, len);
187 return a.len - b.len;
190 static struct symbol *sym_find(struct grammar *g, struct text s)
192 struct symbol **l = &g->syms;
197 (cmp = text_cmp((*l)->name, s)) < 0)
201 n = calloc(1, sizeof(*n));
210 static void symbols_init(struct grammar *g)
212 int entries = sizeof(reserved_words)/sizeof(reserved_words[0]);
214 for (i = 0; i < entries; i++) {
217 t.txt = reserved_words[i];
220 t.len = strlen(t.txt);
227 ### Data types and precedence.
229 Data type specification and precedence specification are both
230 introduced by a dollar sign at the start of the line. If the next
231 word is `LEFT`, `RIGHT` or `NON`, then the line specifies a
232 precedence, otherwise it specifies a data type.
234 The data type name is simply stored and applied to the head of all
235 subsequent productions. It must be the name of a structure, so `$expr`
236 maps to `struct expr`.
238 Any productions given before the first data type will have no data type
239 and can carry no information. In order to allow other non-terminals to
240 have no type, the data type `$void` can be given. This does *not* mean
241 that `struct void` will be used, but rather than no type will be
242 associated with future non-terminals.
244 The precedence line must contain a list of symbols - typically
245 terminal symbols, but not necessarily. It can only contain symbols
246 that have not been seen yet, so precedence declaration must precede
247 the productions that they affect.
249 A precedence line may also contain a "virtual" symbol which is an
250 identifier preceded by `$$`. Virtual terminals carry precedence
251 information but are not included in the grammar. A production can
252 declare that it inherits the precedence of a given virtual symbol.
254 This use for `$$` precludes it from being used as a symbol in the
255 described language. Two other symbols: `${` and `}$` are also
258 Each new precedence line introduces a new precedence level and
259 declares how it associates. This level is stored in each symbol
260 listed and may be inherited by any production which uses the symbol. A
261 production inherits from the last symbol which has a precedence.
263 ###### grammar fields
264 struct text current_type;
268 enum symbols { TK_virtual = TK_reserved, TK_open, TK_close };
269 static const char *known[] = { "$$", "${", "}$" };
272 static char *dollar_line(struct token_state *ts, struct grammar *g)
274 struct token t = token_next(ts);
279 if (t.num != TK_ident) {
280 err = "type or assoc expected after '$'";
283 if (text_is(t.txt, "LEFT"))
285 else if (text_is(t.txt, "RIGHT"))
287 else if (text_is(t.txt, "NON"))
290 g->current_type = t.txt;
291 if (text_is(t.txt, "void"))
292 g->current_type.txt = NULL;
294 if (t.num != TK_newline) {
295 err = "Extra tokens after type name";
301 // This is a precedence line, need some symbols.
305 while (t.num != TK_newline) {
306 enum symtype type = Terminal;
308 if (t.num == TK_virtual) {
311 if (t.num != TK_ident) {
312 err = "$$ must be followed by a word";
315 } else if (t.num != TK_ident &&
317 err = "Illegal token in precedence line";
320 s = sym_find(g, t.txt);
321 if (s->type != Unknown) {
322 err = "Symbols in precedence line must not already be known.";
326 s->precedence = g->prec_levels;
331 err = "No symbols given on precedence line";
335 while (t.num != TK_newline && t.num != TK_eof)
342 A production either starts with an identifier which is the head
343 non-terminal, or a vertical bar (`|`) in which case this production
344 uses the same head as the previous one. The identifier must be
345 followed by a `->` mark. All productions for a given non-terminal must
346 be grouped together with the `nonterminal ->` given only once.
348 After this a (possibly empty) sequence of identifiers and marks form
349 the body of the production. A virtual symbol may be given after the
350 body by preceding it with `$$`. If a virtual symbol is given, the
351 precedence of the production is that for the virtual symbol. If none
352 is given, the precedence is inherited from the last symbol in the
353 production which has a precedence specified.
355 After the optional precedence may come the `${` mark. This indicates
356 the start of a code fragment. If present, this must be on the same
357 line as the start of the production.
359 All of the text from the `${` through to the matching `}$` is
360 collected and forms the code-fragment for the production. It must all
361 be in one `code_node` of the literate code. The `}$` must be
362 at the end of a line.
364 Text in the code fragment will undergo substitutions where `$N` for
365 some numeric `N` will be replaced with a variable holding the parse
366 information for the particular symbol in the production. `$0` is the
367 head of the production, `$1` is the first symbol of the body, etc.
368 The type of `$N` for a terminal symbol is `struct token`. For
369 a non-terminal, it is whatever has been declared for that symbol.
371 While building productions we will need to add to an array which needs to
375 static void array_add(void *varray, int *cnt, void *new)
377 void ***array = varray;
380 current = ((*cnt-1) | (step-1))+1;
381 if (*cnt == current) {
384 *array = realloc(*array, current * sizeof(void*));
386 (*array)[*cnt] = new;
390 Collecting the code fragment simply involves reading tokens until we
391 hit the end token `}$`, and noting the character position of the start and
395 static struct text collect_code(struct token_state *state,
400 code.txt = start.txt.txt + start.txt.len;
402 t = token_next(state);
403 while (t.node == start.node &&
404 t.num != TK_close && t.num != TK_error &&
406 if (t.num == TK_close && t.node == start.node)
407 code.len = t.txt.txt - code.txt;
413 Now we have all the bit we need to parse a full production.
418 ###### grammar fields
419 struct production **productions;
420 int production_count;
422 ###### production fields
424 struct symbol **body;
429 int first_production;
432 static char *parse_production(struct grammar *g,
434 struct token_state *state)
436 /* Head has already been parsed. */
439 struct production p, *pp;
441 memset(&p, 0, sizeof(p));
443 tk = token_next(state);
444 while (tk.num == TK_ident || tk.num == TK_mark) {
445 struct symbol *bs = sym_find(g, tk.txt);
446 if (bs->type == Unknown)
448 if (bs->type == Virtual) {
449 err = "Virtual symbol not permitted in production";
452 if (bs->precedence) {
453 p.precedence = bs->precedence;
456 array_add(&p.body, &p.body_size, bs);
457 tk = token_next(state);
459 if (tk.num == TK_virtual) {
461 tk = token_next(state);
462 if (tk.num != TK_ident) {
463 err = "word required after $$";
466 vs = sym_find(g, tk.txt);
467 if (vs->type != Virtual) {
468 err = "symbol after $$ must be virtual";
471 p.precedence = vs->precedence;
473 tk = token_next(state);
475 if (tk.num == TK_open) {
476 p.code = collect_code(state, tk);
477 if (p.code.txt == NULL) {
478 err = "code fragment not closed properly";
481 tk = token_next(state);
483 if (tk.num != TK_newline && tk.num != TK_eof) {
484 err = "stray tokens at end of line";
487 pp = malloc(sizeof(*pp));
489 array_add(&g->productions, &g->production_count, pp);
492 while (tk.num != TK_newline && tk.num != TK_eof)
493 tk = token_next(state);
497 With the ability to parse production and dollar-lines, we have nearly all
498 that we need to parse a grammar from a `code_node`.
500 The head of the first production will effectively be the `start` symbol of
501 the grammar. However it won't _actually_ be so. Processing the grammar is
502 greatly simplified if the real start symbol only has a single production,
503 and expects `$eof` as the final terminal. So when we find the first
504 explicit production we insert an extra production as production zero which
507 ###### Example: production 0
510 where `START` is the first non-terminal given.
512 ###### create production zero
513 struct production *p = calloc(1,sizeof(*p));
514 struct text start = {"$start",6};
515 struct text eof = {"$eof",4};
516 p->head = sym_find(g, start);
517 p->head->type = Nonterminal;
518 array_add(&p->body, &p->body_size, head);
519 array_add(&p->body, &p->body_size, sym_find(g, eof));
520 g->start = p->head->num;
521 p->head->first_production = g->production_count;
522 array_add(&g->productions, &g->production_count, p);
524 Now we are ready to read in the grammar.
526 ###### grammar fields
527 int start; // the 'start' symbol.
530 static struct grammar *grammar_read(struct code_node *code)
532 struct token_config conf = {
535 .words_marks = known,
536 .known_count = sizeof(known)/sizeof(known[0]),
538 .ignored = (1 << TK_line_comment)
539 | (1 << TK_block_comment)
542 | (1 << TK_multi_string)
547 struct token_state *state = token_open(code, &conf);
548 struct token tk = token_next(state);
549 struct symbol *head = NULL;
553 g = calloc(1, sizeof(*g));
556 for (tk = token_next(state); tk.num != TK_eof;
557 tk = token_next(state)) {
558 if (tk.num == TK_newline)
560 if (tk.num == TK_ident) {
562 head = sym_find(g, tk.txt);
563 if (head->type == Nonterminal)
564 err = "This non-terminal has already be used.";
565 else if (head->type == Virtual)
566 err = "Virtual symbol not permitted in head of production";
568 head->type = Nonterminal;
569 head->struct_name = g->current_type;
571 ## create production zero
573 head->first_production = g->production_count;
574 tk = token_next(state);
575 if (tk.num == TK_mark &&
576 text_is(tk.txt, "->"))
577 err = parse_production(g, head, state);
579 err = "'->' missing in production";
581 } else if (tk.num == TK_mark
582 && text_is(tk.txt, "|")) {
583 // another production for same non-term
585 err = parse_production(g, head, state);
587 err = "First production must have a head";
588 } else if (tk.num == TK_mark
589 && text_is(tk.txt, "$")) {
590 err = dollar_line(state, g);
592 err = "Unrecognised token at start of line.";
600 fprintf(stderr, "Error at line %d: %s\n",
607 ## Analysing the grammar
609 The central task in analysing the grammar is to determine a set of
610 states to drive the parsing process. This will require finding
611 various sets of symbols and of "items". Some of these sets will need
612 to attach information to each element in the set, so they are more
615 Each "item" may have a 'look-ahead' or `LA` set associated with
616 it. Many of these will be the same as each other. To avoid memory
617 wastage, and to simplify some comparisons of sets, these sets will be
618 stored in a list of unique sets, each assigned a number.
620 Once we have the data structures in hand to manage these sets and
621 lists, we can start setting the 'nullable' flag, build the 'FIRST'
622 sets, and then create the item sets which define the various states.
626 Though we don't only store symbols in these sets, they are the main
627 things we store, so they are called symbol sets or "symsets".
629 A symset has a size, an array of shorts, and an optional array of data
630 values, which are also shorts. If the array of data is not present,
631 we store an impossible pointer, as `NULL` is used to indicate that no
632 memory has been allocated yet;
637 unsigned short *syms, *data;
639 #define NO_DATA ((unsigned short *)1)
640 const struct symset INIT_SYMSET = { 0, NULL, NO_DATA };
641 const struct symset INIT_DATASET = { 0, NULL, NULL };
643 The arrays of shorts are allocated in blocks of 8 and are kept sorted
644 by using an insertion sort. We don't explicitly record the amount of
645 allocated space as it can be derived directly from the current `cnt` using
646 `((cnt - 1) | 7) + 1`.
649 static void symset_add(struct symset *s, unsigned short key, unsigned short val)
652 int current = ((s->cnt-1) | 7) + 1;
653 if (current == s->cnt) {
655 s->syms = realloc(s->syms, sizeof(*s->syms) * current);
656 if (s->data != NO_DATA)
657 s->data = realloc(s->data, sizeof(*s->data) * current);
660 while (i > 0 && s->syms[i-1] > key) {
661 s->syms[i] = s->syms[i-1];
662 if (s->data != NO_DATA)
663 s->data[i] = s->data[i-1];
667 if (s->data != NO_DATA)
672 Finding a symbol (or item) in a `symset` uses a simple binary search.
673 We return the index where the value was found (so data can be accessed),
674 or `-1` to indicate failure.
676 static int symset_find(struct symset *ss, unsigned short key)
683 while (lo + 1 < hi) {
684 int mid = (lo + hi) / 2;
685 if (ss->syms[mid] <= key)
690 if (ss->syms[lo] == key)
695 We will often want to form the union of two symsets. When we do, we
696 will often want to know if anything changed (as they might mean there
697 is more work to do). So `symset_union` reports whether anything was
698 added to the first set. We use a slow quadratic approach as these
699 sets don't really get very big. If profiles shows this to be a problem is
700 can be optimised later.
702 static int symset_union(struct symset *a, struct symset *b)
706 for (i = 0; i < b->cnt; i++)
707 if (symset_find(a, b->syms[i]) < 0) {
708 unsigned short data = 0;
709 if (b->data != NO_DATA)
711 symset_add(a, b->syms[i], data);
717 And of course we must be able to free a symset.
719 static void symset_free(struct symset ss)
722 if (ss.data != NO_DATA)
728 Some symsets are simply stored somewhere appropriate in the `grammar`
729 data structure, others needs a bit of indirection. This applies
730 particularly to "LA" sets. These will be paired with "items" in an
731 "itemset". As itemsets will be stored in a symset, the "LA" set needs to be
732 stored in the `data` field, so we need a mapping from a small (short)
733 number to an LA `symset`.
735 As mentioned earlier this involves creating a list of unique symsets.
737 For now, we just use a linear list sorted by insertion. A skiplist
738 would be more efficient and may be added later.
745 struct setlist *next;
748 ###### grammar fields
749 struct setlist *sets;
754 static int ss_cmp(struct symset a, struct symset b)
757 int diff = a.cnt - b.cnt;
760 for (i = 0; i < a.cnt; i++) {
761 diff = (int)a.syms[i] - (int)b.syms[i];
768 static int save_set(struct grammar *g, struct symset ss)
770 struct setlist **sl = &g->sets;
774 while (*sl && (cmp = ss_cmp((*sl)->ss, ss)) < 0)
781 s = malloc(sizeof(*s));
790 Finding a set by number is currently performed by a simple linear search.
791 If this turns out to hurt performance, we can store the sets in a dynamic
792 array like the productions.
794 static struct symset set_find(struct grammar *g, int num)
796 struct setlist *sl = g->sets;
797 while (sl && sl->num != num)
803 ### Setting `nullable`
805 We set `nullable` on the head symbol for any production for which all
806 the body symbols (if any) are nullable. As this is a recursive
807 definition, any change in the `nullable` setting means that we need to
808 re-evaluate where it needs to be set.
810 We simply loop around performing the same calculations until no more
817 static void set_nullable(struct grammar *g)
820 while (check_again) {
823 for (p = 0; p < g->production_count; p++) {
824 struct production *pr = g->productions[p];
827 if (pr->head->nullable)
829 for (s = 0; s < pr->body_size; s++)
830 if (! pr->body[s]->nullable)
832 if (s == pr->body_size) {
833 pr->head->nullable = 1;
840 ### Building the `first` sets
842 When calculating what can follow a particular non-terminal, we will need to
843 know what the "first" terminal in an subsequent non-terminal might be. So
844 we calculate the `first` set for every non-terminal and store them in an
845 array. We don't bother recording the "first" set for terminals as they are
848 As the "first" for one symbol might depend on the "first" of another,
849 we repeat the calculations until no changes happen, just like with
850 `set_nullable`. This makes use of the fact that `symset_union`
851 reports if any change happens.
853 The core of this which finds the "first" of part of a production body
854 will be reused for computing the "follow" sets, so we split it out
855 into a separate function.
857 ###### grammar fields
858 struct symset *first;
862 static int add_first(struct production *pr, int start,
863 struct symset *target, struct grammar *g,
868 for (s = start; s < pr->body_size; s++) {
869 struct symbol *bs = pr->body[s];
870 if (bs->type == Terminal) {
871 if (symset_find(target, bs->num) < 0) {
872 symset_add(target, bs->num, 0);
876 } else if (symset_union(target, &g->first[bs->num]))
882 *to_end = (s == pr->body_size);
886 static void build_first(struct grammar *g)
890 g->first = calloc(g->num_syms, sizeof(g->first[0]));
891 for (s = 0; s < g->num_syms; s++)
892 g->first[s] = INIT_SYMSET;
894 while (check_again) {
897 for (p = 0; p < g->production_count; p++) {
898 struct production *pr = g->productions[p];
899 struct symset *head = &g->first[pr->head->num];
901 if (add_first(pr, 0, head, g, NULL))
907 ### Building the `follow` sets.
909 There are two different situations when we will want to generate "follow"
910 sets. If we are doing an SLR analysis, we want to generate a single
911 "follow" set for each non-terminal in the grammar. That is what is
912 happening here. If we are doing an LALR or LR analysis we will want
913 to generate a separate "LA" set for each item. We do that later
916 There are two parts to generating a "follow" set. Firstly we look at
917 every place that any non-terminal appears in the body of any
918 production, and we find the set of possible "first" symbols after
919 there. This is done using `add_first` above and only needs to be done
920 once as the "first" sets are now stable and will not change.
924 for (p = 0; p < g->production_count; p++) {
925 struct production *pr = g->productions[p];
928 for (b = 0; b < pr->body_size - 1; b++) {
929 struct symbol *bs = pr->body[b];
930 if (bs->type == Terminal)
932 add_first(pr, b+1, &g->follow[bs->num], g, NULL);
936 The second part is to add the "follow" set of the head of a production
937 to the "follow" sets of the final symbol in the production, and any
938 other symbol which is followed only by `nullable` symbols. As this
939 depends on "follow" itself we need to repeatedly perform the process
940 until no further changes happen.
944 for (again = 0, p = 0;
945 p < g->production_count;
946 p < g->production_count-1
947 ? p++ : again ? (again = 0, p = 0)
949 struct production *pr = g->productions[p];
952 for (b = pr->body_size - 1; b >= 0; b--) {
953 struct symbol *bs = pr->body[b];
954 if (bs->type == Terminal)
956 if (symset_union(&g->follow[bs->num],
957 &g->follow[pr->head->num]))
964 We now just need to create and initialise the `follow` list to get a
967 ###### grammar fields
968 struct symset *follow;
971 static void build_follow(struct grammar *g)
974 g->follow = calloc(g->num_syms, sizeof(g->follow[0]));
975 for (s = 0; s < g->num_syms; s++)
976 g->follow[s] = INIT_SYMSET;
980 ### Building itemsets and states
982 There are three different levels of detail that can be chosen for
983 building the itemsets and states for the LR grammar. They are:
985 1. LR(0) or SLR(1), where no look-ahead is considered.
986 2. LALR(1) where we build look-ahead sets with each item and merge
987 the LA sets when we find two paths to the same "kernel" set of items.
988 3. LR(1) where different look-ahead for any item in the set means
989 a different state must be created.
991 ###### forward declarations
992 enum grammar_type { LR0, LR05, SLR, LALR, LR1 };
994 We need to be able to look through existing states to see if a newly
995 generated state already exists. For now we use a simple sorted linked
998 An item is a pair of numbers: the production number and the position of
999 "DOT", which is an index into the body of the production.
1000 As the numbers are not enormous we can combine them into a single "short"
1001 and store them in a `symset` - 4 bits for "DOT" and 12 bits for the
1002 production number (so 4000 productions with maximum of 15 symbols in the
1005 Comparing the itemsets will be a little different to comparing symsets
1006 as we want to do the lookup after generating the "kernel" of an
1007 itemset, so we need to ignore the offset=zero items which are added during
1010 To facilitate this, we modify the "DOT" number so that "0" sorts to
1011 the end of the list in the symset, and then only compare items before
1015 static inline unsigned short item_num(int production, int index)
1017 return production | ((31-index) << 11);
1019 static inline int item_prod(unsigned short item)
1021 return item & 0x7ff;
1023 static inline int item_index(unsigned short item)
1025 return (31-(item >> 11)) & 0x1f;
1028 For LR(1) analysis we need to compare not just the itemset in a state
1029 but also the LA sets. As we assign each unique LA set a number, we
1030 can just compare the symset and the data values together.
1033 static int itemset_cmp(struct symset a, struct symset b,
1034 enum grammar_type type)
1040 i < a.cnt && i < b.cnt &&
1041 item_index(a.syms[i]) > 0 &&
1042 item_index(b.syms[i]) > 0;
1044 int diff = a.syms[i] - b.syms[i];
1048 diff = a.data[i] - b.data[i];
1053 if (i == a.cnt || item_index(a.syms[i]) == 0)
1057 if (i == b.cnt || item_index(b.syms[i]) == 0)
1063 if (type < LR1 || av == -1)
1066 a.data[i] - b.data[i];
1069 And now we can build the list of itemsets. The lookup routine returns
1070 both a success flag and a pointer to where in the list an insert
1071 should happen, so we don't need to search a second time.
1075 struct itemset *next;
1077 struct symset items;
1078 struct symset go_to;
1082 ###### grammar fields
1083 struct itemset *items;
1087 static int itemset_find(struct grammar *g, struct itemset ***where,
1088 struct symset kernel, enum grammar_type type)
1090 struct itemset **ip;
1092 for (ip = &g->items; *ip ; ip = & (*ip)->next) {
1093 struct itemset *i = *ip;
1095 diff = itemset_cmp(i->items, kernel, type);
1108 Adding an itemset may require merging the LA sets if LALR analysis is
1109 happening. If any new LA set add symbol that weren't in the old LA set, we
1110 clear the `completed` flag so that the dependants of this itemset will be
1111 recalculated and their LA sets updated.
1113 `add_itemset` must consume the symsets it is passed, either by adding
1114 them to a data structure, of freeing them.
1116 static int add_itemset(struct grammar *g, struct symset ss,
1117 enum grammar_type type)
1119 struct itemset **where, *is;
1121 int found = itemset_find(g, &where, ss, type);
1123 is = calloc(1, sizeof(*is));
1124 is->state = g->states;
1128 is->go_to = INIT_DATASET;
1137 for (i = 0; i < ss.cnt; i++) {
1138 struct symset temp = INIT_SYMSET, s;
1139 if (ss.data[i] == is->items.data[i])
1141 s = set_find(g, is->items.data[i]);
1142 symset_union(&temp, &s);
1143 s = set_find(g, ss.data[i]);
1144 if (symset_union(&temp, &s)) {
1145 is->items.data[i] = save_set(g, temp);
1156 To build all the itemsets, we first insert the initial itemset made from the
1157 start symbol, complete each itemset, and then generate new itemsets from old
1158 until no new ones can be made.
1160 Completing an itemset means finding all the items where "DOT" is followed by
1161 a nonterminal and adding "DOT=0" items for every production from that
1162 non-terminal - providing each item hasn't already been added.
1164 If LA sets are needed, the LA set for each new item is found using
1165 `add_first` which was developed earlier for `FIRST` and `FOLLOW`. This may
1166 be supplemented by the LA set for the item which produce the new item.
1168 We also collect a set of all symbols which follow "DOT" (in `done`) as this
1169 is used in the next stage.
1171 NOTE: precedence handling should happen here - I haven't written this yet
1174 ###### complete itemset
1175 for (i = 0; i < is->items.cnt; i++) {
1176 int p = item_prod(is->items.syms[i]);
1177 int bs = item_index(is->items.syms[i]);
1178 struct production *pr = g->productions[p];
1181 struct symset LA = INIT_SYMSET;
1182 unsigned short sn = 0;
1184 if (bs == pr->body_size)
1187 if (symset_find(&done, s->num) < 0)
1188 symset_add(&done, s->num, 0);
1189 if (s->type != Nonterminal)
1195 add_first(pr, bs+1, &LA, g, &to_end);
1197 struct symset ss = set_find(g, is->items.data[i]);
1198 symset_union(&LA, &ss);
1200 sn = save_set(g, LA);
1201 LA = set_find(g, sn);
1203 /* Add productions for this symbol */
1204 for (p2 = s->first_production;
1205 p2 < g->production_count &&
1206 g->productions[p2]->head == s;
1208 int itm = item_num(p2, 0);
1209 int pos = symset_find(&is->items, itm);
1211 symset_add(&is->items, itm, sn);
1212 /* Will have re-ordered, so start
1213 * from beginning again */
1215 } else if (type >= LALR) {
1216 struct symset ss = set_find(g, is->items.data[pos]);
1217 struct symset tmp = INIT_SYMSET;
1219 symset_union(&tmp, &ss);
1220 if (symset_union(&tmp, &LA)) {
1221 is->items.data[pos] = save_set(g, tmp);
1229 For each symbol we found (and placed in `done`) we collect all the items for
1230 which this symbol is next, and create a new itemset with "DOT" advanced over
1231 the symbol. This is then added to the collection of itemsets (or merged
1232 with a pre-existing itemset).
1234 ###### derive itemsets
1235 // Now we have a completed itemset, so we need to
1236 // create all the 'next' itemset and create them
1237 // if they don't exist.
1238 for (i = 0; i < done.cnt; i++) {
1240 unsigned short state;
1241 struct symset newitemset = INIT_SYMSET;
1243 newitemset = INIT_DATASET;
1245 for (j = 0; j < is->items.cnt; j++) {
1246 int itm = is->items.syms[j];
1247 int p = item_prod(itm);
1248 int bp = item_index(itm);
1249 struct production *pr = g->productions[p];
1250 unsigned short la = 0;
1253 if (bp == pr->body_size)
1255 if (pr->body[bp]->num != done.syms[i])
1258 la = is->items.data[j];
1259 pos = symset_find(&newitemset, pr->head->num);
1261 symset_add(&newitemset, item_num(p, bp+1), la);
1262 else if (type >= LALR) {
1263 // Need to merge la set.
1264 int la2 = newitemset.data[pos];
1266 struct symset ss = set_find(g, la2);
1267 struct symset LA = INIT_SYMSET;
1268 symset_union(&LA, &ss);
1269 ss = set_find(g, la);
1270 if (symset_union(&LA, &ss))
1271 newitemset.data[pos] = save_set(g, LA);
1277 state = add_itemset(g, newitemset, type);
1278 if (symset_find(&is->go_to, done.syms[i]) < 0)
1279 symset_add(&is->go_to, done.syms[i], state);
1282 All that is left is to crate the initial itemset from production zero, and
1283 with `TK_eof` as the LA set.
1286 static void build_itemsets(struct grammar *g, enum grammar_type type)
1288 struct symset first = INIT_SYMSET;
1291 unsigned short la = 0;
1293 // LA set just has eof
1294 struct symset eof = INIT_SYMSET;
1295 symset_add(&eof, TK_eof, 0);
1296 la = save_set(g, eof);
1297 first = INIT_DATASET;
1299 // production 0, offset 0 (with no data)
1300 symset_add(&first, item_num(0, 0), la);
1301 add_itemset(g, first, type);
1302 for (again = 0, is = g->items;
1304 is = is->next ?: again ? (again = 0, g->items) : NULL) {
1306 struct symset done = INIT_SYMSET;
1317 ### Completing the analysis.
1319 The exact process of analysis depends on which level was requested. For
1320 `LR0` and `LR05` we don't need first and follow sets at all. For `LALR` and
1321 `LR1` we need first, but not follow. For `SLR` we need both.
1323 We don't build the "action" tables that you might expect as the parser
1324 doesn't use them. They will be calculated to some extent if needed for
1327 Once we have built everything we allocate arrays for the two lists:
1328 symbols and itemsets. This allows more efficient access during reporting.
1329 The symbols are grouped as terminals and non-terminals and we record the
1330 changeover point in `first_nonterm`.
1332 ###### grammar fields
1333 struct symbol **symtab;
1334 struct itemset **statetab;
1337 ###### grammar_analyse
1339 static void grammar_analyse(struct grammar *g, enum grammar_type type)
1343 int snum = TK_reserved;
1344 for (s = g->syms; s; s = s->next)
1345 if (s->num < 0 && s->type == Terminal) {
1349 g->first_nonterm = snum;
1350 for (s = g->syms; s; s = s->next)
1356 g->symtab = calloc(g->num_syms, sizeof(g->symtab[0]));
1357 for (s = g->syms; s; s = s->next)
1358 g->symtab[s->num] = s;
1367 build_itemsets(g, type);
1369 g->statetab = calloc(g->states, sizeof(g->statetab[0]));
1370 for (is = g->items; is ; is = is->next)
1371 g->statetab[is->state] = is;
1374 ## Reporting on the Grammar
1376 The purpose of the report is to give the grammar developer insight into
1377 how the grammar parser will work. It is basically a structured dump of
1378 all the tables that have been generated, plus an description of any conflicts.
1380 ###### grammar_report
1381 static int grammar_report(struct grammar *g, enum grammar_type type)
1387 return report_conflicts(g, type);
1390 Firstly we have the complete list of symbols, together with the "FIRST"
1391 set if that was generated.
1395 static void report_symbols(struct grammar *g)
1399 printf("SYMBOLS + FIRST:\n");
1401 printf("SYMBOLS:\n");
1403 for (n = 0; n < g->num_syms; n++) {
1404 struct symbol *s = g->symtab[n];
1408 printf(" %c%3d%c: ",
1409 s->nullable ? '*':' ',
1410 s->num, symtypes[s->type]);
1413 printf(" (%d%s)", s->precedence,
1414 assoc_names[s->assoc]);
1416 if (g->first && s->type == Nonterminal) {
1419 for (j = 0; j < g->first[n].cnt; j++) {
1422 prtxt(g->symtab[g->first[n].syms[j]]->name);
1429 Then we have to follow sets if they were computed.
1431 static void report_follow(struct grammar *g)
1434 printf("FOLLOW:\n");
1435 for (n = 0; n < g->num_syms; n++)
1436 if (g->follow[n].cnt) {
1440 prtxt(g->symtab[n]->name);
1441 for (j = 0; j < g->follow[n].cnt; j++) {
1444 prtxt(g->symtab[g->follow[n].syms[j]]->name);
1450 And finally the item sets. These include the GO TO tables and, for
1451 LALR and LR1, the LA set for each item. Lots of stuff, so we break
1452 it up a bit. First the items, with production number and associativity.
1454 static void report_item(struct grammar *g, int itm)
1456 int p = item_prod(itm);
1457 int dot = item_index(itm);
1458 struct production *pr = g->productions[p];
1462 prtxt(pr->head->name);
1464 for (i = 0; i < pr->body_size; i++) {
1465 printf(" %s", (dot == i ? ". ": ""));
1466 prtxt(pr->body[i]->name);
1468 if (dot == pr->body_size)
1472 printf(" (%d%s)", pr->precedence,
1473 assoc_names[pr->assoc]);
1477 The LA sets which are (possibly) reported with each item:
1479 static void report_la(struct grammar *g, int lanum)
1481 struct symset la = set_find(g, lanum);
1485 printf(" LOOK AHEAD(%d)", lanum);
1486 for (i = 0; i < la.cnt; i++) {
1489 prtxt(g->symtab[la.syms[i]]->name);
1494 Then the go to sets:
1497 static void report_goto(struct grammar *g, struct symset gt)
1502 for (i = 0; i < gt.cnt; i++) {
1504 prtxt(g->symtab[gt.syms[i]]->name);
1505 printf(" -> %d\n", gt.data[i]);
1509 Now we can report all the item sets complete with items, LA sets, and GO TO.
1511 static void report_itemsets(struct grammar *g)
1514 printf("ITEM SETS(%d)\n", g->states);
1515 for (s = 0; s < g->states; s++) {
1517 struct itemset *is = g->statetab[s];
1518 printf(" Itemset %d:\n", s);
1519 for (j = 0; j < is->items.cnt; j++) {
1520 report_item(g, is->items.syms[j]);
1521 if (is->items.data != NO_DATA)
1522 report_la(g, is->items.data[j]);
1524 report_goto(g, is->go_to);
1528 ### Reporting conflicts
1530 Conflict detection varies a lot among different analysis levels. However
1531 LR0 and LR0.5 are quite similar - having no follow sets, and SLR, LALR and
1532 LR1 are also similar as they have FOLLOW or LA sets.
1536 ## conflict functions
1538 static int report_conflicts(struct grammar *g, enum grammar_type type)
1541 printf("Conflicts:\n");
1543 cnt = conflicts_lr0(g, type);
1545 cnt = conflicts_slr(g, type);
1547 printf(" - no conflicts\n");
1551 LR0 conflicts are any state which have both a reducible item and
1554 LR05 conflicts only occurs if two possibly reductions exist,
1555 as shifts always over-ride reductions.
1557 ###### conflict functions
1558 static int conflicts_lr0(struct grammar *g, enum grammar_type type)
1562 for (i = 0; i < g->states; i++) {
1563 struct itemset *is = g->statetab[i];
1564 int last_reduce = -1;
1565 int prev_reduce = -1;
1566 int last_shift = -1;
1570 for (j = 0; j < is->items.cnt; j++) {
1571 int itm = is->items.syms[j];
1572 int p = item_prod(itm);
1573 int bp = item_index(itm);
1574 struct production *pr = g->productions[p];
1576 if (bp == pr->body_size) {
1577 prev_reduce = last_reduce;
1581 if (pr->body[bp]->type == Terminal)
1584 if (type == LR0 && last_reduce >= 0 && last_shift >= 0) {
1585 printf(" State %d has both SHIFT and REDUCE:\n", i);
1586 report_item(g, is->items.syms[last_shift]);
1587 report_item(g, is->items.syms[last_reduce]);
1590 if (prev_reduce >= 0) {
1591 printf(" State %d has 2 (or more) reducible items\n", i);
1592 report_item(g, is->items.syms[prev_reduce]);
1593 report_item(g, is->items.syms[last_reduce]);
1600 SLR, LALR, and LR1 conflicts happen if two reducible items have over-lapping
1601 look ahead, or if a symbol in a look-ahead can be shifted. The differ only
1602 in the source of the look ahead set.
1604 We build a dataset mapping terminal to item for possible SHIFTs and then
1605 another for possible REDUCE operations. We report when we get conflicts
1608 static int conflicts_slr(struct grammar *g, enum grammar_type type)
1613 for (i = 0; i < g->states; i++) {
1614 struct itemset *is = g->statetab[i];
1615 struct symset shifts = INIT_DATASET;
1616 struct symset reduce = INIT_DATASET;
1620 /* First collect the shifts */
1621 for (j = 0; j < is->items.cnt; j++) {
1622 unsigned short itm = is->items.syms[j];
1623 int p = item_prod(itm);
1624 int bp = item_index(itm);
1625 struct production *pr = g->productions[p];
1627 if (bp < pr->body_size &&
1628 pr->body[bp]->type == Terminal) {
1630 int sym = pr->body[bp]->num;
1631 if (symset_find(&shifts, sym) < 0)
1632 symset_add(&shifts, sym, itm);
1635 /* Now look for reduction and conflicts */
1636 for (j = 0; j < is->items.cnt; j++) {
1637 unsigned short itm = is->items.syms[j];
1638 int p = item_prod(itm);
1639 int bp = item_index(itm);
1640 struct production *pr = g->productions[p];
1642 if (bp < pr->body_size)
1647 la = g->follow[pr->head->num];
1649 la = set_find(g, is->items.data[j]);
1651 for (k = 0; k < la.cnt; k++) {
1652 int pos = symset_find(&shifts, la.syms[k]);
1654 printf(" State %d has SHIFT/REDUCE conflict on ", i);
1655 prtxt(g->symtab[la.syms[k]]->name);
1657 report_item(g, shifts.data[pos]);
1658 report_item(g, itm);
1661 pos = symset_find(&reduce, la.syms[k]);
1663 symset_add(&reduce, la.syms[k], itm);
1666 printf(" State %d has REDUCE/REDUCE conflict on ", i);
1667 prtxt(g->symtab[la.syms[k]]->name);
1669 report_item(g, itm);
1670 report_item(g, reduce.data[pos]);
1674 symset_free(shifts);
1675 symset_free(reduce);
1681 ## Generating the parser
1683 The export part of the parser is the `parse_XX` function, where the name
1684 `XX` is based on the name of the parser files.
1686 This takes a `code_node`, a partially initialized `token_config`, and an
1687 optional `FILE` to send tracing to. The `token_config` gets the list of
1688 known words added and then is used with the `code_node` to initialize the
1691 `parse_XX` then call the library function `parser_run` to actually complete
1692 the parse. This needs the `states` table and function to call the various
1693 pieces of code provided in the grammar file, so they are generated first.
1695 ###### parser_generate
1697 static void gen_parser(FILE *f, struct grammar *g, char *file, char *name)
1703 gen_reduce(f, g, file);
1706 fprintf(f, "#line 0 \"gen_parser\"\n");
1707 fprintf(f, "void *parse_%s(struct code_node *code, struct token_config *config, FILE *trace)\n",
1710 fprintf(f, "\tstruct token_state *tokens;\n");
1711 fprintf(f, "\tconfig->words_marks = known;\n");
1712 fprintf(f, "\tconfig->known_count = sizeof(known)/sizeof(known[0]);\n");
1713 fprintf(f, "\tconfig->ignored |= (1 << TK_line_comment) | (1 << TK_block_comment);\n");
1714 fprintf(f, "\ttokens = token_open(code, config);\n");
1715 fprintf(f, "\tvoid *rv = parser_run(tokens, states, do_reduce, do_free, trace, non_term, config->known_count);\n");
1716 fprintf(f, "\ttoken_close(tokens);\n");
1717 fprintf(f, "\treturn rv;\n");
1718 fprintf(f, "}\n\n");
1721 ### Table words table
1723 The know words is simply an array of terminal symbols.
1724 The table of nonterminals used for tracing is a similar array.
1728 static void gen_known(FILE *f, struct grammar *g)
1731 fprintf(f, "#line 0 \"gen_known\"\n");
1732 fprintf(f, "static const char *known[] = {\n");
1733 for (i = TK_reserved;
1734 i < g->num_syms && g->symtab[i]->type == Terminal;
1736 fprintf(f, "\t\"%.*s\",\n", g->symtab[i]->name.len,
1737 g->symtab[i]->name.txt);
1738 fprintf(f, "};\n\n");
1741 static void gen_non_term(FILE *f, struct grammar *g)
1744 fprintf(f, "#line 0 \"gen_non_term\"\n");
1745 fprintf(f, "static const char *non_term[] = {\n");
1746 for (i = TK_reserved;
1749 if (g->symtab[i]->type == Nonterminal)
1750 fprintf(f, "\t\"%.*s\",\n", g->symtab[i]->name.len,
1751 g->symtab[i]->name.txt);
1752 fprintf(f, "};\n\n");
1755 ### States and the goto tables.
1757 For each state we record the goto table and the reducible production if
1759 Some of the details of the reducible production are stored in the
1760 `do_reduce` function to come later. Here we store the production number,
1761 the body size (useful for stack management) and the resulting symbol (useful
1762 for knowing how to free data later).
1764 The go to table is stored in a simple array of `sym` and corresponding
1767 ###### exported types
1775 const struct lookup * go_to;
1784 static void gen_goto(FILE *f, struct grammar *g)
1787 fprintf(f, "#line 0 \"gen_goto\"\n");
1788 for (i = 0; i < g->states; i++) {
1790 fprintf(f, "static const struct lookup goto_%d[] = {\n",
1792 struct symset gt = g->statetab[i]->go_to;
1793 for (j = 0; j < gt.cnt; j++)
1794 fprintf(f, "\t{ %d, %d },\n",
1795 gt.syms[j], gt.data[j]);
1802 static void gen_states(FILE *f, struct grammar *g)
1805 fprintf(f, "#line 0 \"gen_states\"\n");
1806 fprintf(f, "static const struct state states[] = {\n");
1807 for (i = 0; i < g->states; i++) {
1808 struct itemset *is = g->statetab[i];
1810 for (j = 0; j < is->items.cnt; j++) {
1811 int itm = is->items.syms[j];
1812 int p = item_prod(itm);
1813 int bp = item_index(itm);
1814 struct production *pr = g->productions[p];
1816 if (bp < pr->body_size)
1818 /* This is what we reduce */
1823 fprintf(f, "\t[%d] = { %d, goto_%d, %d, %d, %d },\n",
1824 i, is->go_to.cnt, i, prod,
1825 prod < 0 ? -1 : g->productions[prod]->body_size,
1826 prod < 0 ? -1 : g->productions[prod]->head->num);
1828 fprintf(f, "};\n\n");
1831 ### The `do_reduce` function and the code
1833 When the parser engine decides to reduce a production, it calls `do_reduce`.
1834 This has two functions.
1836 Firstly, if a non-NULL `trace` file is passed, it prints out details of the
1837 production being reduced. Secondly it runs the code that was included with
1838 the production if any.
1840 This code needs to be able to store data somewhere. Rather than requiring
1841 `do_reduce` to `malloc` that "somewhere", we pass in a large buffer and have
1842 `do_reduce` return the size to be saved.
1844 The code fragment requires translation when written out. Any `$N` needs to
1845 be converted to a reference either to that buffer (if `$0`) or to the
1846 structure returned by a previous reduction. These pointer need to be cast
1847 to the appropriate type for each access. All this is handling in
1853 static void gen_code(struct production *p, FILE *f, struct grammar *g)
1856 fprintf(f, "\t\t\t");
1857 for (c = p->code.txt; c < p->code.txt + p->code.len; c++) {
1866 if (*c < '0' || *c > '9') {
1871 while (c[1] >= '0' && c[1] <= '9') {
1873 n = n * 10 + *c - '0';
1876 fprintf(f, "(*(struct %.*s*)ret)",
1877 p->head->struct_name.len,
1878 p->head->struct_name.txt);
1879 else if (n > p->body_size)
1880 fprintf(f, "$%d", n);
1881 else if (p->body[n-1]->type == Terminal)
1882 fprintf(f, "(*(struct token *)body[%d])",
1884 else if (p->body[n-1]->struct_name.txt == NULL)
1885 fprintf(f, "$%d", n);
1887 fprintf(f, "(*(struct %.*s*)body[%d])",
1888 p->body[n-1]->struct_name.len,
1889 p->body[n-1]->struct_name.txt, n-1);
1896 static void gen_reduce(FILE *f, struct grammar *g, char *file)
1899 fprintf(f, "#line 0 \"gen_reduce\"\n");
1900 fprintf(f, "static int do_reduce(int prod, void **body, void *ret)\n");
1902 fprintf(f, "\tint ret_size = 0;\n");
1904 fprintf(f, "\tswitch(prod) {\n");
1905 for (i = 0; i < g->production_count; i++) {
1906 struct production *p = g->productions[i];
1907 fprintf(f, "\tcase %d:\n", i);
1912 if (p->head->struct_name.txt)
1913 fprintf(f, "\t\tret_size = sizeof(struct %.*s);\n",
1914 p->head->struct_name.len,
1915 p->head->struct_name.txt);
1917 fprintf(f, "\t\tbreak;\n");
1919 fprintf(f, "\t}\n\treturn ret_size;\n}\n\n");
1924 As each non-terminal can potentially cause a different type of data
1925 structure to be allocated and filled in, we need to be able to free it when
1928 It is particularly important to have fine control over freeing during error
1929 recovery where individual stack frames might need to be freed.
1931 For this, the grammar author required to defined a `free_XX` function for
1932 each structure that is used by a non-terminal. `do_free` all call whichever
1933 is appropriate given a symbol number, and will call `free` (as is
1934 appropriate for tokens` on any terminal symbol.
1938 static void gen_free(FILE *f, struct grammar *g)
1942 fprintf(f, "#line 0 \"gen_free\"\n");
1943 fprintf(f, "static void do_free(short sym, void *asn)\n");
1945 fprintf(f, "\tif (!asn) return;\n");
1946 fprintf(f, "\tif (sym < %d) {\n", g->first_nonterm);
1947 fprintf(f, "\t\tfree(asn);\n\t\treturn;\n\t}\n");
1948 fprintf(f, "\tswitch(sym) {\n");
1950 for (i = 0; i < g->num_syms; i++) {
1951 struct symbol *s = g->symtab[i];
1953 s->type != Nonterminal ||
1954 s->struct_name.txt == NULL)
1957 fprintf(f, "\tcase %d:\n", s->num);
1958 fprintf(f, "\t\tfree_%.*s(asn);\n",
1960 s->struct_name.txt);
1961 fprintf(f, "\t\tbreak;\n");
1963 fprintf(f, "\t}\n}\n\n");
1966 ## The main routine.
1968 There are three key parts to the "main" function of parsergen: processing
1969 the arguments, loading the grammar file, and dealing with the grammar.
1971 ### Argument processing.
1973 Command line options allow the selection of analysis mode, name of output
1974 file, and whether or not a report should be generated. By default we create
1975 a report only if no code output was requested.
1977 The `parse_XX` function name uses the basename of the output file which
1978 should not have a suffix (`.c`). `.c` is added to the given name for the
1979 code, and `.h` is added for the header (if header text is specifed in the
1986 static const struct option long_options[] = {
1987 { "LR0", 0, NULL, '0' },
1988 { "LR05", 0, NULL, '5' },
1989 { "SLR", 0, NULL, 'S' },
1990 { "LALR", 0, NULL, 'L' },
1991 { "LR1", 0, NULL, '1' },
1992 { "report", 0, NULL, 'R' },
1993 { "output", 1, NULL, 'o' },
1994 { NULL, 0, NULL, 0 }
1996 const char *options = "05SL1Ro:";
1998 ###### process arguments
2000 char *outfile = NULL;
2004 enum grammar_type type = LR05;
2005 while ((opt = getopt_long(argc, argv, options,
2006 long_options, NULL)) != -1) {
2021 outfile = optarg; break;
2023 fprintf(stderr, "Usage: parsergen ...\n");
2028 infile = argv[optind++];
2030 fprintf(stderr, "No input file given\n");
2033 if (outfile && report == 1)
2036 if (name && strchr(name, '/'))
2037 name = strrchr(name, '/')+1;
2039 if (optind < argc) {
2040 fprintf(stderr, "Excess command line arguments\n");
2044 ### Loading the grammar file
2046 To be able to run `mdcode` and `scanner` on the grammar we need to memory
2049 One we have extracted the code (with `mdcode`) we expect to file three
2050 sections: header, code, and grammar. Anything else is an error.
2052 "header" and "code" are optional, though it is hard to build a working
2053 parser with neither. "grammar" must be provided.
2057 #include <sys/mman.h>
2062 static void pr_err(char *msg)
2065 fprintf(stderr, "%s\n", msg);
2069 struct section *table;
2073 fd = open(infile, O_RDONLY);
2075 fprintf(stderr, "parsergen: cannot open %s: %s\n",
2076 infile, strerror(errno));
2079 len = lseek(fd, 0, 2);
2080 file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
2081 table = code_extract(file, file+len, pr_err);
2083 struct code_node *hdr = NULL;
2084 struct code_node *code = NULL;
2085 struct code_node *gram = NULL;
2086 for (s = table; s; s = s->next) {
2087 if (text_is(s->section, "header"))
2089 else if (text_is(s->section, "code"))
2091 else if (text_is(s->section, "grammar"))
2094 fprintf(stderr, "Unknown content section: %.*s\n",
2095 s->section.len, s->section.txt);
2100 ### Processing the input
2102 As we need to append an extention to a filename and then open it for
2103 writing, and we need to do this twice, it helps to have a separate function.
2107 static FILE *open_ext(char *base, char *ext)
2109 char *fn = malloc(strlen(base) + strlen(ext) + 1);
2111 strcat(strcpy(fn, base), ext);
2117 If we can read the grammar, then we analyse and optionally report on it. If
2118 the report finds conflicts we will exit with an error status.
2120 ###### process input
2121 struct grammar *g = NULL;
2123 fprintf(stderr, "No grammar section provided\n");
2126 g = grammar_read(gram);
2128 fprintf(stderr, "Failure to parse grammar\n");
2133 grammar_analyse(g, type);
2135 if (grammar_report(g, type))
2139 If a headers section is defined, we write it out and include a declaration
2140 for the `parse_XX` function so it can be used from separate code.
2142 if (rv == 0 && hdr && outfile) {
2143 FILE *f = open_ext(outfile, ".h");
2145 code_node_print(f, hdr, infile);
2146 fprintf(f, "void *parse_%s(struct code_node *code, struct token_config *config, FILE *trace);\n",
2150 fprintf(stderr, "Cannot create %s.h\n",
2156 And if all goes well and an output file was provided, we create the `.c`
2157 file with the code section (if any) and the parser tables and function.
2159 if (rv == 0 && outfile) {
2160 FILE *f = open_ext(outfile, ".c");
2163 code_node_print(f, code, infile);
2164 gen_parser(f, g, infile, name);
2167 fprintf(stderr, "Cannot create %s.c\n",
2173 And that about wraps it up. We need to set the locale so that UTF-8 is
2174 recognised properly, and link with `libicuuc` is `libmdcode` requires that.
2176 ###### File: parsergen.mk
2177 parsergen : parsergen.o libscanner.o libmdcode.o
2178 $(CC) $(CFLAGS) -o parsergen parsergen.o libscanner.o libmdcode.o -licuuc
2185 int main(int argc, char *argv[])
2190 setlocale(LC_ALL,"");
2192 ## process arguments
2199 ## The SHIFT/REDUCE parser
2201 Having analysed the grammar and generated all the table, we only need the
2202 shift/reduce engine to bring it all together.
2204 ### Goto table lookup
2206 The parser generator has nicely provided us with goto tables sorted by
2207 symbol number. We need a binary search function to find a symbol in the
2210 ###### parser functions
2212 static int search(const struct state *l, int sym)
2215 int hi = l->go_to_cnt;
2219 while (lo + 1 < hi) {
2220 int mid = (lo + hi) / 2;
2221 if (l->go_to[mid].sym <= sym)
2226 if (l->go_to[lo].sym == sym)
2227 return l->go_to[lo].state;
2232 ### The state stack.
2234 The core data structure for the parser is the stack. This tracks all the
2235 symbols that have been recognised or partially recognised.
2237 The stack usually won't grow very large - maybe a few tens of entries. So
2238 we dynamically resize and array as required but never bother to shrink it
2241 We keep the stack as two separate allocations. One, `asn_stack` stores the
2242 "abstract syntax nodes" which are created by each reduction. When we call
2243 `do_reduce` we need to pass an array of the `asn`s of the body of the
2244 production, and by keeping a separate `asn` stack, we can just pass a
2245 pointer into this stack.
2247 The other allocation stores all other stack fields of which there are two.
2248 The `state` is the most important one and guides the parsing process. The
2249 `sym` is nearly unnecessary. However when we want to free entries from the
2250 `asn_stack`, it helps to know what type they are so we can call the right
2251 freeing function. The symbol leads us to the right free function through
2254 As well as the stack of frames we have a `next` frame which is
2255 assembled from the incoming token and other information prior to
2256 pushing it onto the stack.
2258 ###### parser functions
2272 Two operations are needed on the stack - shift (which is like push) and pop.
2274 Shift applies not only to terminals but also to non-terminals. When we
2275 reduce a production we will pop off entries corresponding to the body
2276 symbols, then push on an item for the head of the production. This last is
2277 exactly the same process as shifting in a terminal so we use the same
2280 To simplify other code we arrange for `shift` to fail if there is no `goto`
2281 state for the symbol. This is useful in basic parsing due to our design
2282 that we shift when we can, and reduce when we cannot. So the `shift`
2283 function reports if it could.
2285 So `shift` finds the next state. If that succeed it extends the allocations
2286 if needed and pushed all the information onto the stacks.
2288 ###### parser functions
2290 static int shift(struct parser *p,
2292 const struct state states[])
2294 // Push an entry onto the stack
2295 int newstate = search(&states[p->next.state], p->next.sym);
2298 if (p->tos >= p->stack_size) {
2299 p->stack_size += 10;
2300 p->stack = realloc(p->stack, p->stack_size
2301 * sizeof(p->stack[0]));
2302 p->asn_stack = realloc(p->asn_stack, p->stack_size
2303 * sizeof(p->asn_stack[0]));
2305 p->stack[p->tos] = p->next;
2306 p->asn_stack[p->tos] = asn;
2308 p->next.state = newstate;
2312 `pop` simply moves the top of stack (`tos`) back down the required amount
2313 and frees any `asn` entries that need to be freed. It is called _after_ we
2314 reduce a production, just before we `shift` the nonterminal in.
2316 ###### parser functions
2318 static void pop(struct parser *p, int num,
2319 void(*do_free)(short sym, void *asn))
2323 for (i = 0; i < num; i++)
2324 do_free(p->stack[p->tos+i].sym,
2325 p->asn_stack[p->tos+i]);
2328 p->next.state = p->stack[p->tos].state;
2331 ### Memory allocation
2333 The `scanner` returns tokens in a local variable - we want them in allocated
2334 memory so they can live in the `asn_stack`. Similarly the `asn` produced by
2335 a reduce is in a large buffer. Both of these require some allocation and
2336 copying, hence `memdup` and `tokcopy`.
2338 ###### parser includes
2341 ###### parser functions
2343 void *memdup(void *m, int len)
2349 memcpy(ret, m, len);
2353 static struct token *tok_copy(struct token tk)
2355 struct token *new = malloc(sizeof(*new));
2360 ### The heart of the parser.
2362 Now we have the parser. If we can shift, we do. If not and we can reduce
2363 we do. If the production we reduced was production zero, then we have
2364 accepted the input and can finish.
2366 We return whatever `asn` was returned by reducing production zero.
2368 If we can neither shift nor reduce we have an error to handle. We pop
2369 single entries off the stack until we can shift the `TK_error` symbol, then
2370 drop input tokens until we find one we can shift into the new error state.
2373 ###### parser includes
2376 void *parser_run(struct token_state *tokens,
2377 const struct state states[],
2378 int (*do_reduce)(int, void**, void*),
2379 void (*do_free)(short, void*),
2380 FILE *trace, const char *non_term[], int knowns)
2382 struct parser p = { 0 };
2387 tk = tok_copy(token_next(tokens));
2389 p.next.sym = tk->num;
2391 parser_trace(trace, &p, tk, states, non_term, knowns);
2393 if (shift(&p, tk, states)) {
2394 tk = tok_copy(token_next(tokens));
2397 if (states[p.next.state].reduce_prod >= 0) {
2399 int prod = states[p.next.state].reduce_prod;
2400 int size = states[p.next.state].reduce_size;
2402 static char buf[16*1024];
2403 p.next.sym = states[p.next.state].reduce_sym;
2405 body = p.asn_stack +
2406 (p.tos - states[p.next.state].reduce_size);
2408 bufsize = do_reduce(prod, body, buf);
2410 pop(&p, size, do_free);
2411 shift(&p, memdup(buf, bufsize), states);
2416 /* Error. We walk up the stack until we
2417 * find a state which will accept TK_error.
2418 * We then shift in TK_error and see what state
2419 * that takes us too.
2420 * Then we discard input tokens until
2421 * we find one that is acceptable.
2423 p.next.sym = TK_error;
2424 while (shift(&p, tk, states) == 0
2426 // discard this state
2427 pop(&p, 1, do_free);
2429 while (search(&states[p.next.state], tk->num) < 0 &&
2430 tk->num != TK_eof) {
2432 tk = tok_copy(token_next(tokens));
2434 if (p.tos == 0 && tk->num == TK_eof)
2439 ret = p.asn_stack[0];
2441 pop(&p, p.tos, do_free);
2447 ###### exported functions
2448 void *parser_run(struct token_state *tokens,
2449 const struct state states[],
2450 int (*do_reduce)(int, void**, void*),
2451 void (*do_free)(short, void*),
2452 FILE *trace, const char *non_term[], int knowns);
2456 Being able to visualize the parser in action can be invaluable when
2457 debugging the parser code, or trying to understand how the parse of a
2458 particular grammar progresses. The stack contains all the important
2459 state, so just printing out the stack every time around the parse loop
2460 can make it possible to see exactly what is happening.
2462 This doesn't explicitly show each SHIFT and REDUCE action. However they
2463 are easily deduced from the change between consecutive lines, and the
2464 details of each state can be found by cross referencing the states list
2465 in the stack with the "report" that parsergen can generate.
2467 For terminal symbols, we just dump the token. For non-terminals we
2468 print the name of the symbol. The look ahead token is reported at the
2469 end inside square brackets.
2471 ###### parser functions
2473 static char *reserved_words[] = {
2474 [TK_error] = "ERROR",
2477 [TK_newline] = "NEWLINE",
2480 void parser_trace(FILE *trace, struct parser *p,
2481 struct token *tk, const struct state states[],
2482 const char *non_term[], int knowns)
2485 for (i = 0; i < p->tos; i++) {
2486 int sym = p->stack[i].sym;
2487 fprintf(trace, "(%d) ", p->stack[i].state);
2488 if (sym < TK_reserved &&
2489 reserved_words[sym] != NULL)
2490 fputs(reserved_words[sym], trace);
2491 else if (sym < TK_reserved + knowns) {
2492 struct token *t = p->asn_stack[i];
2493 text_dump(trace, t->txt, 20);
2495 fputs(non_term[sym - TK_reserved - knowns],
2499 fprintf(trace, "(%d) [", p->next.state);
2500 if (tk->num < TK_reserved &&
2501 reserved_words[tk->num] != NULL)
2502 fputs(reserved_words[tk->num], trace);
2504 text_dump(trace, tk->txt, 20);
2505 fputs("]\n", trace);
2510 The obvious example for a parser is a calculator.
2512 As `scanner` provides number parsing function using `libgmp` is it not much
2513 work to perform arbitrary rational number calculations.
2515 This calculator takes one expression, or an equality test per line. The
2516 results are printed and in any equality test fails, the program exits with
2519 Embedding mdcode inside mdcode is rather horrible. I'd like to find a
2520 better approach, but as the grammar file must have 3 components I need
2521 something like this.
2523 ###### File: parsergen.mk
2524 calc.c : parsergen calc.cgm
2525 ./parsergen -o calc calc.cgm
2526 calc : calc.o libparser.o libscanner.o libmdcode.o libnumber.o
2527 $(CC) $(CFLAGS) -o calc calc.o libparser.o libscanner.o libmdcode.o libnumber.o -licuuc -lgmp
2535 // what do we use for a demo-grammar? A calculator of course.
2549 #include <sys/mman.h>
2554 #include "scanner.h"
2560 static void free_number(struct number *n)
2566 int main(int argc, char *argv[])
2568 int fd = open(argv[1], O_RDONLY);
2569 int len = lseek(fd, 0, 2);
2570 char *file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
2571 struct section *s = code_extract(file, file+len, NULL);
2572 struct token_config config = {
2573 .ignored = (1 << TK_line_comment)
2574 | (1 << TK_block_comment)
2577 .number_chars = ".,_+-",
2581 parse_calc(s->code, &config, argc > 2 ? stderr : NULL);
2589 Session -> Session Line
2592 Line -> Expression NEWLINE ${ gmp_printf("Answer = %Qd\n", $1.val);
2593 { mpf_t fl; mpf_init2(fl, 20); mpf_set_q(fl, $1.val);
2594 gmp_printf(" or as a decimal: %Fg\n", fl);
2598 | Expression = Expression NEWLINE ${
2599 if (mpq_equal($1.val, $3.val))
2600 gmp_printf("Both equal %Qd\n", $1.val);
2602 gmp_printf("NOT EQUAL: %Qd\n != : %Qd\n",
2607 | NEWLINE ${ printf("Blank line\n"); }$
2608 | ERROR NEWLINE ${ printf("Skipped a bad line\n"); }$
2611 Expression -> Expression + Term ${ mpq_init($0.val); mpq_add($0.val, $1.val, $3.val); }$
2612 | Expression - Term ${ mpq_init($0.val); mpq_sub($0.val, $1.val, $3.val); }$
2613 | Term ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
2615 Term -> Term * Factor ${ mpq_init($0.val); mpq_mul($0.val, $1.val, $3.val); }$
2616 | Term / Factor ${ mpq_init($0.val); mpq_div($0.val, $1.val, $3.val); }$
2617 | Factor ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
2619 Factor -> NUMBER ${ if (number_parse($0.val, $0.tail, $1.txt) == 0) mpq_init($0.val); }$
2620 | ( Expression ) ${ mpq_init($0.val); mpq_set($0.val, $2.val); }$
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