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. Finally `calc` is a test grammar for a simple calculator. The
21 `parsergen` program built from the C code in this file can extract
22 that grammar directly from this file and process it.
25 ###### File: parsergen.c
30 ## forward declarations
41 ###### File: libparser.c
48 ###### File: parsergen.mk
51 parsergen.c parsergen.mk libparser.c parser.h : parsergen.mdc
54 ## Reading the grammar
56 The grammar must be stored in a markdown literate code file as parsed
57 by "[mdcode][]". It must have three top level (i.e. unreferenced)
58 sections: `header`, `code`, and `grammar`. The first two will be
59 literally copied into the generated `.c` and `.h`. files. The last
60 contains the grammar. This is tokenised with "[scanner][]".
62 If the `--tag` option is given, then any top level header that doesn't
63 start with the tag is ignored, and the tag is striped from the rest. So
65 means that the three needed sections must be `Foo: header`, `Foo: code`,
69 [scanner]: scanner.html
75 ###### parser includes
79 The grammar contains production sets, precedence/associativity
80 declarations, and data type declarations. These are all parsed with
81 _ad hoc_ parsing as we don't have a parser generator yet.
83 The precedence and associativity can be set for each production, but
84 can be inherited from symbols. The data type (either structure or a
85 reference to a structure) is potentially defined for each non-terminal
86 and describes what C structure is used to store information about each
90 enum assoc {Left, Right, Non};
91 char *assoc_names[] = {"Left","Right","Non"};
94 struct text struct_name;
97 unsigned short precedence;
101 unsigned short precedence;
109 The strings reported by `mdcode` and `scanner` are `struct text` which have
110 length rather than being null terminated. To help with printing and
111 comparing we define `text_is` and `prtxt`, which should possibly go in
112 `mdcode`. `scanner` does provide `text_dump` which is useful for strings
113 which might contain control characters.
115 `strip_tag` is a bit like `strncmp`, but adds a test for a colon,
116 because that is what we need to detect tags.
119 static int text_is(struct text t, char *s)
121 return (strlen(s) == t.len &&
122 strncmp(s, t.txt, t.len) == 0);
124 static void prtxt(struct text t)
126 printf("%.*s", t.len, t.txt);
129 static int strip_tag(struct text *t, char *tag)
131 int skip = strlen(tag) + 1;
132 if (skip >= t->len ||
133 strncmp(t->txt, tag, skip-1) != 0 ||
134 t->txt[skip-1] != ':')
136 while (skip < t->len && t->txt[skip] == ' ')
145 Productions are comprised primarily of symbols - terminal and
146 non-terminal. We do not make a syntactic distinction between the two,
147 though non-terminals must be identifiers. Non-terminal symbols are
148 those which appear in the head of a production, terminal symbols are
149 those which don't. There are also "virtual" symbols used for precedence
150 marking discussed later, and sometimes we won't know what type a symbol
153 ###### forward declarations
154 enum symtype { Unknown, Virtual, Terminal, Nonterminal };
155 char *symtypes = "UVTN";
159 Symbols can be either `TK_ident` or `TK_mark`. They are saved in a
160 table of known symbols and the resulting parser will report them as
161 `TK_reserved + N`. A small set of identifiers are reserved for the
162 different token types that `scanner` can report.
165 static char *reserved_words[] = {
166 [TK_error] = "ERROR",
167 [TK_number] = "NUMBER",
168 [TK_ident] = "IDENTIFIER",
170 [TK_string] = "STRING",
171 [TK_multi_string] = "MULTI_STRING",
174 [TK_newline] = "NEWLINE",
180 Note that `TK_eof` and the two `TK_*_comment` tokens cannot be
181 recognised. The former is automatically expected at the end of the text
182 being parsed. The latter are always ignored by the parser.
184 All of these symbols are stored in a simple symbol table. We use the
185 `struct text` from `mdcode` to store the name and link them together
186 into a sorted list using an insertion sort.
188 We don't have separate `find` and `insert` functions as any symbol we
189 find needs to be remembered. We simply expect `find` to always return a
190 symbol, but its type might be `Unknown`.
199 ###### grammar fields
204 static int text_cmp(struct text a, struct text b)
209 int cmp = strncmp(a.txt, b.txt, len);
213 return a.len - b.len;
216 static struct symbol *sym_find(struct grammar *g, struct text s)
218 struct symbol **l = &g->syms;
223 (cmp = text_cmp((*l)->name, s)) < 0)
227 n = calloc(1, sizeof(*n));
236 static void symbols_init(struct grammar *g)
238 int entries = sizeof(reserved_words)/sizeof(reserved_words[0]);
240 for (i = 0; i < entries; i++) {
243 t.txt = reserved_words[i];
246 t.len = strlen(t.txt);
253 ### Data types and precedence.
255 Data type specification and precedence specification are both
256 introduced by a dollar sign at the start of the line. If the next
257 word is `LEFT`, `RIGHT` or `NON`, then the line specifies a
258 precedence, otherwise it specifies a data type.
260 The data type name is simply stored and applied to the head of all
261 subsequent productions. It must be the name of a structure optionally
262 preceded by an asterisk which means a reference or "pointer". So
263 `$expression` maps to `struct expression` and `$*statement` maps to
264 `struct statement *`.
266 Any productions given before the first data type declaration will have
267 no data type associated with them and can carry no information. In
268 order to allow other non-terminals to have no type, the data type
269 `$void` can be given. This does *not* mean that `struct void` will be
270 used, but rather than no type will be associated with future
273 The precedence line must contain a list of symbols - typically
274 terminal symbols, but not necessarily. It can only contain symbols
275 that have not been seen yet, so precedence declaration must precede
276 the productions that they affect.
278 A precedence line may also contain a "virtual" symbol which is an
279 identifier preceded by `$$`. Virtual terminals carry precedence
280 information but are not included in the grammar. A production can
281 declare that it inherits the precedence of a given virtual symbol.
283 This use for `$$` precludes it from being used as a symbol in the
284 described language. Two other symbols: `${` and `}$` are also
287 Each new precedence line introduces a new precedence level and
288 declares how it associates. This level is stored in each symbol
289 listed and may be inherited by any production which uses the symbol. A
290 production inherits from the last symbol which has a precedence.
292 ###### grammar fields
293 struct text current_type;
298 enum symbols { TK_virtual = TK_reserved, TK_open, TK_close };
299 static const char *known[] = { "$$", "${", "}$" };
302 static char *dollar_line(struct token_state *ts, struct grammar *g, int isref)
304 struct token t = token_next(ts);
309 if (t.num != TK_ident) {
310 err = "type or assoc expected after '$'";
313 if (text_is(t.txt, "LEFT"))
315 else if (text_is(t.txt, "RIGHT"))
317 else if (text_is(t.txt, "NON"))
320 g->current_type = t.txt;
321 g->type_isref = isref;
322 if (text_is(t.txt, "void"))
323 g->current_type.txt = NULL;
325 if (t.num != TK_newline) {
326 err = "Extra tokens after type name";
333 err = "$* cannot be followed by a precedence";
337 // This is a precedence line, need some symbols.
341 while (t.num != TK_newline) {
342 enum symtype type = Terminal;
344 if (t.num == TK_virtual) {
347 if (t.num != TK_ident) {
348 err = "$$ must be followed by a word";
351 } else if (t.num != TK_ident &&
353 err = "Illegal token in precedence line";
356 s = sym_find(g, t.txt);
357 if (s->type != Unknown) {
358 err = "Symbols in precedence line must not already be known.";
362 s->precedence = g->prec_levels;
367 err = "No symbols given on precedence line";
371 while (t.num != TK_newline && t.num != TK_eof)
378 A production either starts with an identifier which is the head
379 non-terminal, or a vertical bar (`|`) in which case this production
380 uses the same head as the previous one. The identifier must be
381 followed by a `->` mark. All productions for a given non-terminal must
382 be grouped together with the `nonterminal ->` given only once.
384 After this a (possibly empty) sequence of identifiers and marks form
385 the body of the production. A virtual symbol may be given after the
386 body by preceding it with `$$`. If a virtual symbol is given, the
387 precedence of the production is that for the virtual symbol. If none
388 is given, the precedence is inherited from the last symbol in the
389 production which has a precedence specified.
391 After the optional precedence may come the `${` mark. This indicates
392 the start of a code fragment. If present, this must be on the same
393 line as the start of the production.
395 All of the text from the `${` through to the matching `}$` is
396 collected and forms the code-fragment for the production. It must all
397 be in one `code_node` of the literate code. The `}$` must be
398 at the end of a line.
400 Text in the code fragment will undergo substitutions where `$N` for
401 some numeric `N` will be replaced with a variable holding the parse
402 information for the particular symbol in the production. `$0` is the
403 head of the production, `$1` is the first symbol of the body, etc.
404 The type of `$N` for a terminal symbol is `struct token`. For
405 a non-terminal, it is whatever has been declared for that symbol.
407 While building productions we will need to add to an array which needs to
411 static void array_add(void *varray, int *cnt, void *new)
413 void ***array = varray;
416 current = ((*cnt-1) | (step-1))+1;
417 if (*cnt == current) {
420 *array = realloc(*array, current * sizeof(void*));
422 (*array)[*cnt] = new;
426 Collecting the code fragment simply involves reading tokens until we
427 hit the end token `}$`, and noting the character position of the start and
431 static struct text collect_code(struct token_state *state,
436 code.txt = start.txt.txt + start.txt.len;
438 t = token_next(state);
439 while (t.node == start.node &&
440 t.num != TK_close && t.num != TK_error &&
442 if (t.num == TK_close && t.node == start.node)
443 code.len = t.txt.txt - code.txt;
449 Now we have all the bit we need to parse a full production.
454 ###### grammar fields
455 struct production **productions;
456 int production_count;
458 ###### production fields
460 struct symbol **body;
465 int first_production;
468 static char *parse_production(struct grammar *g,
470 struct token_state *state)
472 /* Head has already been parsed. */
475 struct production p, *pp;
477 memset(&p, 0, sizeof(p));
479 tk = token_next(state);
480 while (tk.num == TK_ident || tk.num == TK_mark) {
481 struct symbol *bs = sym_find(g, tk.txt);
482 if (bs->type == Unknown)
484 if (bs->type == Virtual) {
485 err = "Virtual symbol not permitted in production";
488 if (bs->precedence) {
489 p.precedence = bs->precedence;
492 array_add(&p.body, &p.body_size, bs);
493 tk = token_next(state);
495 if (tk.num == TK_virtual) {
497 tk = token_next(state);
498 if (tk.num != TK_ident) {
499 err = "word required after $$";
502 vs = sym_find(g, tk.txt);
503 if (vs->type != Virtual) {
504 err = "symbol after $$ must be virtual";
507 p.precedence = vs->precedence;
509 tk = token_next(state);
511 if (tk.num == TK_open) {
512 p.code = collect_code(state, tk);
513 if (p.code.txt == NULL) {
514 err = "code fragment not closed properly";
517 tk = token_next(state);
519 if (tk.num != TK_newline && tk.num != TK_eof) {
520 err = "stray tokens at end of line";
523 pp = malloc(sizeof(*pp));
525 array_add(&g->productions, &g->production_count, pp);
528 while (tk.num != TK_newline && tk.num != TK_eof)
529 tk = token_next(state);
533 With the ability to parse production and dollar-lines, we have nearly all
534 that we need to parse a grammar from a `code_node`.
536 The head of the first production will effectively be the `start` symbol of
537 the grammar. However it won't _actually_ be so. Processing the grammar is
538 greatly simplified if the real start symbol only has a single production,
539 and expects `$eof` as the final terminal. So when we find the first
540 explicit production we insert an extra production as production zero which
543 ###### Example: production 0
546 where `START` is the first non-terminal given.
548 ###### create production zero
549 struct production *p = calloc(1,sizeof(*p));
550 struct text start = {"$start",6};
551 struct text eof = {"$eof",4};
552 p->head = sym_find(g, start);
553 p->head->type = Nonterminal;
554 array_add(&p->body, &p->body_size, head);
555 array_add(&p->body, &p->body_size, sym_find(g, eof));
556 p->head->first_production = g->production_count;
557 array_add(&g->productions, &g->production_count, p);
559 Now we are ready to read in the grammar.
562 static struct grammar *grammar_read(struct code_node *code)
564 struct token_config conf = {
567 .words_marks = known,
568 .known_count = sizeof(known)/sizeof(known[0]),
570 .ignored = (1 << TK_line_comment)
571 | (1 << TK_block_comment)
574 | (1 << TK_multi_string)
579 struct token_state *state = token_open(code, &conf);
581 struct symbol *head = NULL;
585 g = calloc(1, sizeof(*g));
588 for (tk = token_next(state); tk.num != TK_eof;
589 tk = token_next(state)) {
590 if (tk.num == TK_newline)
592 if (tk.num == TK_ident) {
594 head = sym_find(g, tk.txt);
595 if (head->type == Nonterminal)
596 err = "This non-terminal has already be used.";
597 else if (head->type == Virtual)
598 err = "Virtual symbol not permitted in head of production";
600 head->type = Nonterminal;
601 head->struct_name = g->current_type;
602 head->isref = g->type_isref;
603 if (g->production_count == 0) {
604 ## create production zero
606 head->first_production = g->production_count;
607 tk = token_next(state);
608 if (tk.num == TK_mark &&
609 text_is(tk.txt, "->"))
610 err = parse_production(g, head, state);
612 err = "'->' missing in production";
614 } else if (tk.num == TK_mark
615 && text_is(tk.txt, "|")) {
616 // another production for same non-term
618 err = parse_production(g, head, state);
620 err = "First production must have a head";
621 } else if (tk.num == TK_mark
622 && text_is(tk.txt, "$")) {
623 err = dollar_line(state, g, 0);
624 } else if (tk.num == TK_mark
625 && text_is(tk.txt, "$*")) {
626 err = dollar_line(state, g, 1);
628 err = "Unrecognised token at start of line.";
636 fprintf(stderr, "Error at line %d: %s\n",
643 ## Analysing the grammar
645 The central task in analysing the grammar is to determine a set of
646 states to drive the parsing process. This will require finding
647 various sets of symbols and of "items". Some of these sets will need
648 to attach information to each element in the set, so they are more
651 Each "item" may have a 'look-ahead' or `LA` set associated with
652 it. Many of these will be the same as each other. To avoid memory
653 wastage, and to simplify some comparisons of sets, these sets will be
654 stored in a list of unique sets, each assigned a number.
656 Once we have the data structures in hand to manage these sets and
657 lists, we can start setting the 'nullable' flag, build the 'FIRST'
658 sets, and then create the item sets which define the various states.
662 Though we don't only store symbols in these sets, they are the main
663 things we store, so they are called symbol sets or "symsets".
665 A symset has a size, an array of shorts, and an optional array of data
666 values, which are also shorts. If the array of data is not present,
667 we store an impossible pointer, as `NULL` is used to indicate that no
668 memory has been allocated yet;
673 unsigned short *syms, *data;
675 #define NO_DATA ((unsigned short *)1)
676 const struct symset INIT_SYMSET = { 0, NULL, NO_DATA };
677 const struct symset INIT_DATASET = { 0, NULL, NULL };
679 The arrays of shorts are allocated in blocks of 8 and are kept sorted
680 by using an insertion sort. We don't explicitly record the amount of
681 allocated space as it can be derived directly from the current `cnt` using
682 `((cnt - 1) | 7) + 1`.
685 static void symset_add(struct symset *s, unsigned short key, unsigned short val)
688 int current = ((s->cnt-1) | 7) + 1;
689 if (current == s->cnt) {
691 s->syms = realloc(s->syms, sizeof(*s->syms) * current);
692 if (s->data != NO_DATA)
693 s->data = realloc(s->data, sizeof(*s->data) * current);
696 while (i > 0 && s->syms[i-1] > key) {
697 s->syms[i] = s->syms[i-1];
698 if (s->data != NO_DATA)
699 s->data[i] = s->data[i-1];
703 if (s->data != NO_DATA)
708 Finding a symbol (or item) in a `symset` uses a simple binary search.
709 We return the index where the value was found (so data can be accessed),
710 or `-1` to indicate failure.
712 static int symset_find(struct symset *ss, unsigned short key)
719 while (lo + 1 < hi) {
720 int mid = (lo + hi) / 2;
721 if (ss->syms[mid] <= key)
726 if (ss->syms[lo] == key)
731 We will often want to form the union of two symsets. When we do, we
732 will often want to know if anything changed (as they might mean there
733 is more work to do). So `symset_union` reports whether anything was
734 added to the first set. We use a slow quadratic approach as these
735 sets don't really get very big. If profiles shows this to be a problem is
736 can be optimised later.
738 static int symset_union(struct symset *a, struct symset *b)
742 for (i = 0; i < b->cnt; i++)
743 if (symset_find(a, b->syms[i]) < 0) {
744 unsigned short data = 0;
745 if (b->data != NO_DATA)
747 symset_add(a, b->syms[i], data);
753 And of course we must be able to free a symset.
755 static void symset_free(struct symset ss)
758 if (ss.data != NO_DATA)
764 Some symsets are simply stored somewhere appropriate in the `grammar`
765 data structure, others needs a bit of indirection. This applies
766 particularly to "LA" sets. These will be paired with "items" in an
767 "itemset". As itemsets will be stored in a symset, the "LA" set needs to be
768 stored in the `data` field, so we need a mapping from a small (short)
769 number to an LA `symset`.
771 As mentioned earlier this involves creating a list of unique symsets.
773 For now, we just use a linear list sorted by insertion. A skiplist
774 would be more efficient and may be added later.
781 struct setlist *next;
784 ###### grammar fields
785 struct setlist *sets;
790 static int ss_cmp(struct symset a, struct symset b)
793 int diff = a.cnt - b.cnt;
796 for (i = 0; i < a.cnt; i++) {
797 diff = (int)a.syms[i] - (int)b.syms[i];
804 static int save_set(struct grammar *g, struct symset ss)
806 struct setlist **sl = &g->sets;
810 while (*sl && (cmp = ss_cmp((*sl)->ss, ss)) < 0)
817 s = malloc(sizeof(*s));
826 Finding a set by number is currently performed by a simple linear search.
827 If this turns out to hurt performance, we can store the sets in a dynamic
828 array like the productions.
830 static struct symset set_find(struct grammar *g, int num)
832 struct setlist *sl = g->sets;
833 while (sl && sl->num != num)
839 ### Setting `nullable`
841 We set `nullable` on the head symbol for any production for which all
842 the body symbols (if any) are nullable. As this is a recursive
843 definition, any change in the `nullable` setting means that we need to
844 re-evaluate where it needs to be set.
846 We simply loop around performing the same calculations until no more
853 static void set_nullable(struct grammar *g)
856 while (check_again) {
859 for (p = 0; p < g->production_count; p++) {
860 struct production *pr = g->productions[p];
863 if (pr->head->nullable)
865 for (s = 0; s < pr->body_size; s++)
866 if (! pr->body[s]->nullable)
868 if (s == pr->body_size) {
869 pr->head->nullable = 1;
876 ### Setting `can_eol`
878 In order to be able to ignore newline tokens when not relevant, but
879 still include them in the parse when needed, we will need to know
880 which states can start a "line-like" section of code. We ignore
881 newlines when there is an indent since the most recent start of a
884 To know what is line-like, we first need to know which symbols can end
885 a line-like section, which is precisely those which can end with a
886 newline token. These symbols don't necessarily alway end with a
887 newline, but they can. Hence they are not described as "lines" but
890 Clearly the `TK_newline` token can end with a newline. Any symbol
891 which is the head of a production that contains a line-ending symbol
892 followed only by nullable symbols is also a line-ending symbol. We
893 use a new field `can_eol` to record this attribute of symbols, and
894 compute it in a repetitive manner similar to `set_nullable`.
900 static void set_can_eol(struct grammar *g)
903 g->symtab[TK_newline]->can_eol = 1;
904 while (check_again) {
907 for (p = 0; p < g->production_count; p++) {
908 struct production *pr = g->productions[p];
911 if (pr->head->can_eol)
914 for (s = pr->body_size - 1; s >= 0; s--) {
915 if (pr->body[s]->can_eol) {
916 pr->head->can_eol = 1;
920 if (!pr->body[s]->nullable)
927 ### Building the `first` sets
929 When calculating what can follow a particular non-terminal, we will need to
930 know what the "first" terminal in any subsequent non-terminal might be. So
931 we calculate the `first` set for every non-terminal and store them in an
932 array. We don't bother recording the "first" set for terminals as they are
935 As the "first" for one symbol might depend on the "first" of another,
936 we repeat the calculations until no changes happen, just like with
937 `set_nullable`. This makes use of the fact that `symset_union`
938 reports if any change happens.
940 The core of this which finds the "first" of part of a production body
941 will be reused for computing the "follow" sets, so we split it out
942 into a separate function.
944 ###### grammar fields
945 struct symset *first;
949 static int add_first(struct production *pr, int start,
950 struct symset *target, struct grammar *g,
955 for (s = start; s < pr->body_size; s++) {
956 struct symbol *bs = pr->body[s];
957 if (bs->type == Terminal) {
958 if (symset_find(target, bs->num) < 0) {
959 symset_add(target, bs->num, 0);
963 } else if (symset_union(target, &g->first[bs->num]))
969 *to_end = (s == pr->body_size);
973 static void build_first(struct grammar *g)
977 g->first = calloc(g->num_syms, sizeof(g->first[0]));
978 for (s = 0; s < g->num_syms; s++)
979 g->first[s] = INIT_SYMSET;
981 while (check_again) {
984 for (p = 0; p < g->production_count; p++) {
985 struct production *pr = g->productions[p];
986 struct symset *head = &g->first[pr->head->num];
988 if (add_first(pr, 0, head, g, NULL))
994 ### Building the `follow` sets.
996 There are two different situations when we will want to generate "follow"
997 sets. If we are doing an SLR analysis, we want to generate a single
998 "follow" set for each non-terminal in the grammar. That is what is
999 happening here. If we are doing an LALR or LR analysis we will want
1000 to generate a separate "LA" set for each item. We do that later
1001 in state generation.
1003 There are two parts to generating a "follow" set. Firstly we look at
1004 every place that any non-terminal appears in the body of any
1005 production, and we find the set of possible "first" symbols after
1006 there. This is done using `add_first` above and only needs to be done
1007 once as the "first" sets are now stable and will not change.
1011 for (p = 0; p < g->production_count; p++) {
1012 struct production *pr = g->productions[p];
1015 for (b = 0; b < pr->body_size - 1; b++) {
1016 struct symbol *bs = pr->body[b];
1017 if (bs->type == Terminal)
1019 add_first(pr, b+1, &g->follow[bs->num], g, NULL);
1023 The second part is to add the "follow" set of the head of a production
1024 to the "follow" sets of the final symbol in the production, and any
1025 other symbol which is followed only by `nullable` symbols. As this
1026 depends on "follow" itself we need to repeatedly perform the process
1027 until no further changes happen.
1031 for (again = 0, p = 0;
1032 p < g->production_count;
1033 p < g->production_count-1
1034 ? p++ : again ? (again = 0, p = 0)
1036 struct production *pr = g->productions[p];
1039 for (b = pr->body_size - 1; b >= 0; b--) {
1040 struct symbol *bs = pr->body[b];
1041 if (bs->type == Terminal)
1043 if (symset_union(&g->follow[bs->num],
1044 &g->follow[pr->head->num]))
1051 We now just need to create and initialise the `follow` list to get a
1054 ###### grammar fields
1055 struct symset *follow;
1058 static void build_follow(struct grammar *g)
1061 g->follow = calloc(g->num_syms, sizeof(g->follow[0]));
1062 for (s = 0; s < g->num_syms; s++)
1063 g->follow[s] = INIT_SYMSET;
1067 ### Building itemsets and states
1069 There are three different levels of detail that can be chosen for
1070 building the itemsets and states for the LR grammar. They are:
1072 1. LR(0) or SLR(1), where no look-ahead is considered.
1073 2. LALR(1) where we build look-ahead sets with each item and merge
1074 the LA sets when we find two paths to the same "kernel" set of items.
1075 3. LR(1) where different look-ahead for any item in the set means
1076 a different state must be created.
1078 ###### forward declarations
1079 enum grammar_type { LR0, LR05, SLR, LALR, LR1 };
1081 We need to be able to look through existing states to see if a newly
1082 generated state already exists. For now we use a simple sorted linked
1085 An item is a pair of numbers: the production number and the position of
1086 "DOT", which is an index into the body of the production.
1087 As the numbers are not enormous we can combine them into a single "short"
1088 and store them in a `symset` - 4 bits for "DOT" and 12 bits for the
1089 production number (so 4000 productions with maximum of 15 symbols in the
1092 Comparing the itemsets will be a little different to comparing symsets
1093 as we want to do the lookup after generating the "kernel" of an
1094 itemset, so we need to ignore the offset=zero items which are added during
1097 To facilitate this, we modify the "DOT" number so that "0" sorts to
1098 the end of the list in the symset, and then only compare items before
1102 static inline unsigned short item_num(int production, int index)
1104 return production | ((31-index) << 11);
1106 static inline int item_prod(unsigned short item)
1108 return item & 0x7ff;
1110 static inline int item_index(unsigned short item)
1112 return (31-(item >> 11)) & 0x1f;
1115 For LR(1) analysis we need to compare not just the itemset in a state
1116 but also the LA sets. As we assign each unique LA set a number, we
1117 can just compare the symset and the data values together.
1120 static int itemset_cmp(struct symset a, struct symset b,
1121 enum grammar_type type)
1127 i < a.cnt && i < b.cnt &&
1128 item_index(a.syms[i]) > 0 &&
1129 item_index(b.syms[i]) > 0;
1131 int diff = a.syms[i] - b.syms[i];
1135 diff = a.data[i] - b.data[i];
1140 if (i == a.cnt || item_index(a.syms[i]) == 0)
1144 if (i == b.cnt || item_index(b.syms[i]) == 0)
1150 if (type < LR1 || av == -1)
1153 a.data[i] - b.data[i];
1156 And now we can build the list of itemsets. The lookup routine returns
1157 both a success flag and a pointer to where in the list an insert
1158 should happen, so we don't need to search a second time.
1162 struct itemset *next;
1164 struct symset items;
1165 struct symset go_to;
1170 ###### grammar fields
1171 struct itemset *items;
1175 static int itemset_find(struct grammar *g, struct itemset ***where,
1176 struct symset kernel, enum grammar_type type)
1178 struct itemset **ip;
1180 for (ip = &g->items; *ip ; ip = & (*ip)->next) {
1181 struct itemset *i = *ip;
1183 diff = itemset_cmp(i->items, kernel, type);
1196 Adding an itemset may require merging the LA sets if LALR analysis is
1197 happening. If any new LA set add symbol that weren't in the old LA set, we
1198 clear the `completed` flag so that the dependants of this itemset will be
1199 recalculated and their LA sets updated.
1201 `add_itemset` must consume the symsets it is passed, either by adding
1202 them to a data structure, of freeing them.
1204 static int add_itemset(struct grammar *g, struct symset ss,
1205 enum grammar_type type, int starts_line)
1207 struct itemset **where, *is;
1209 int found = itemset_find(g, &where, ss, type);
1211 is = calloc(1, sizeof(*is));
1212 is->state = g->states;
1216 is->go_to = INIT_DATASET;
1217 is->starts_line = starts_line;
1226 for (i = 0; i < ss.cnt; i++) {
1227 struct symset temp = INIT_SYMSET, s;
1228 if (ss.data[i] == is->items.data[i])
1230 s = set_find(g, is->items.data[i]);
1231 symset_union(&temp, &s);
1232 s = set_find(g, ss.data[i]);
1233 if (symset_union(&temp, &s)) {
1234 is->items.data[i] = save_set(g, temp);
1245 To build all the itemsets, we first insert the initial itemset made from the
1246 start symbol, complete each itemset, and then generate new itemsets from old
1247 until no new ones can be made.
1249 Completing an itemset means finding all the items where "DOT" is followed by
1250 a nonterminal and adding "DOT=0" items for every production from that
1251 non-terminal - providing each item hasn't already been added.
1253 If LA sets are needed, the LA set for each new item is found using
1254 `add_first` which was developed earlier for `FIRST` and `FOLLOW`. This may
1255 be supplemented by the LA set for the item which produce the new item.
1257 We also collect a set of all symbols which follow "DOT" (in `done`) as this
1258 is used in the next stage.
1260 NOTE: precedence handling should happen here - I haven't written this yet
1263 ###### complete itemset
1264 for (i = 0; i < is->items.cnt; i++) {
1265 int p = item_prod(is->items.syms[i]);
1266 int bs = item_index(is->items.syms[i]);
1267 struct production *pr = g->productions[p];
1270 struct symset LA = INIT_SYMSET;
1271 unsigned short sn = 0;
1273 if (bs == pr->body_size)
1276 if (symset_find(&done, s->num) < 0)
1277 symset_add(&done, s->num, 0);
1278 if (s->type != Nonterminal)
1284 add_first(pr, bs+1, &LA, g, &to_end);
1286 struct symset ss = set_find(g, is->items.data[i]);
1287 symset_union(&LA, &ss);
1289 sn = save_set(g, LA);
1290 LA = set_find(g, sn);
1293 /* Add productions for this symbol */
1294 for (p2 = s->first_production;
1295 p2 < g->production_count &&
1296 g->productions[p2]->head == s;
1298 int itm = item_num(p2, 0);
1299 int pos = symset_find(&is->items, itm);
1301 symset_add(&is->items, itm, sn);
1302 /* Will have re-ordered, so start
1303 * from beginning again */
1305 } else if (type >= LALR) {
1306 struct symset ss = set_find(g, is->items.data[pos]);
1307 struct symset tmp = INIT_SYMSET;
1309 symset_union(&tmp, &ss);
1310 if (symset_union(&tmp, &LA)) {
1311 is->items.data[pos] = save_set(g, tmp);
1319 For each symbol we found (and placed in `done`) we collect all the items for
1320 which this symbol is next, and create a new itemset with "DOT" advanced over
1321 the symbol. This is then added to the collection of itemsets (or merged
1322 with a pre-existing itemset).
1324 ###### derive itemsets
1325 // Now we have a completed itemset, so we need to
1326 // compute all the 'next' itemsets and create them
1327 // if they don't exist.
1328 for (i = 0; i < done.cnt; i++) {
1330 unsigned short state;
1331 int starts_line = 0;
1332 struct symbol *sym = g->symtab[done.syms[i]];
1333 struct symset newitemset = INIT_SYMSET;
1335 newitemset = INIT_DATASET;
1338 (sym->nullable && is->starts_line))
1340 for (j = 0; j < is->items.cnt; j++) {
1341 int itm = is->items.syms[j];
1342 int p = item_prod(itm);
1343 int bp = item_index(itm);
1344 struct production *pr = g->productions[p];
1345 unsigned short la = 0;
1348 if (bp == pr->body_size)
1350 if (pr->body[bp] != sym)
1353 la = is->items.data[j];
1354 pos = symset_find(&newitemset, pr->head->num);
1356 symset_add(&newitemset, item_num(p, bp+1), la);
1357 else if (type >= LALR) {
1358 // Need to merge la set.
1359 int la2 = newitemset.data[pos];
1361 struct symset ss = set_find(g, la2);
1362 struct symset LA = INIT_SYMSET;
1363 symset_union(&LA, &ss);
1364 ss = set_find(g, la);
1365 if (symset_union(&LA, &ss))
1366 newitemset.data[pos] = save_set(g, LA);
1372 state = add_itemset(g, newitemset, type, starts_line);
1373 if (symset_find(&is->go_to, done.syms[i]) < 0)
1374 symset_add(&is->go_to, done.syms[i], state);
1377 All that is left is to crate the initial itemset from production zero, and
1378 with `TK_eof` as the LA set.
1381 static void build_itemsets(struct grammar *g, enum grammar_type type)
1383 struct symset first = INIT_SYMSET;
1386 unsigned short la = 0;
1388 // LA set just has eof
1389 struct symset eof = INIT_SYMSET;
1390 symset_add(&eof, TK_eof, 0);
1391 la = save_set(g, eof);
1392 first = INIT_DATASET;
1394 // production 0, offset 0 (with no data)
1395 symset_add(&first, item_num(0, 0), la);
1396 add_itemset(g, first, type, g->productions[0]->body[0]->can_eol);
1397 for (again = 0, is = g->items;
1399 is = is->next ?: again ? (again = 0, g->items) : NULL) {
1401 struct symset done = INIT_SYMSET;
1412 ### Completing the analysis.
1414 The exact process of analysis depends on which level was requested. For
1415 `LR0` and `LR05` we don't need first and follow sets at all. For `LALR` and
1416 `LR1` we need first, but not follow. For `SLR` we need both.
1418 We don't build the "action" tables that you might expect as the parser
1419 doesn't use them. They will be calculated to some extent if needed for
1422 Once we have built everything we allocate arrays for the two lists:
1423 symbols and itemsets. This allows more efficient access during reporting.
1424 The symbols are grouped as terminals and non-terminals and we record the
1425 changeover point in `first_nonterm`.
1427 ###### grammar fields
1428 struct symbol **symtab;
1429 struct itemset **statetab;
1432 ###### grammar_analyse
1434 static void grammar_analyse(struct grammar *g, enum grammar_type type)
1438 int snum = TK_reserved;
1439 for (s = g->syms; s; s = s->next)
1440 if (s->num < 0 && s->type == Terminal) {
1444 g->first_nonterm = snum;
1445 for (s = g->syms; s; s = s->next)
1451 g->symtab = calloc(g->num_syms, sizeof(g->symtab[0]));
1452 for (s = g->syms; s; s = s->next)
1453 g->symtab[s->num] = s;
1463 build_itemsets(g, type);
1465 g->statetab = calloc(g->states, sizeof(g->statetab[0]));
1466 for (is = g->items; is ; is = is->next)
1467 g->statetab[is->state] = is;
1470 ## Reporting on the Grammar
1472 The purpose of the report is to give the grammar developer insight into
1473 how the grammar parser will work. It is basically a structured dump of
1474 all the tables that have been generated, plus an description of any conflicts.
1476 ###### grammar_report
1477 static int grammar_report(struct grammar *g, enum grammar_type type)
1483 return report_conflicts(g, type);
1486 Firstly we have the complete list of symbols, together with the "FIRST"
1487 set if that was generated.
1491 static void report_symbols(struct grammar *g)
1495 printf("SYMBOLS + FIRST:\n");
1497 printf("SYMBOLS:\n");
1499 for (n = 0; n < g->num_syms; n++) {
1500 struct symbol *s = g->symtab[n];
1504 printf(" %c%c%3d%c: ",
1505 s->nullable ? '.':' ',
1506 s->can_eol ? '>':' ',
1507 s->num, symtypes[s->type]);
1510 printf(" (%d%s)", s->precedence,
1511 assoc_names[s->assoc]);
1513 if (g->first && s->type == Nonterminal) {
1516 for (j = 0; j < g->first[n].cnt; j++) {
1519 prtxt(g->symtab[g->first[n].syms[j]]->name);
1526 Then we have to follow sets if they were computed.
1528 static void report_follow(struct grammar *g)
1531 printf("FOLLOW:\n");
1532 for (n = 0; n < g->num_syms; n++)
1533 if (g->follow[n].cnt) {
1537 prtxt(g->symtab[n]->name);
1538 for (j = 0; j < g->follow[n].cnt; j++) {
1541 prtxt(g->symtab[g->follow[n].syms[j]]->name);
1547 And finally the item sets. These include the GO TO tables and, for
1548 LALR and LR1, the LA set for each item. Lots of stuff, so we break
1549 it up a bit. First the items, with production number and associativity.
1551 static void report_item(struct grammar *g, int itm)
1553 int p = item_prod(itm);
1554 int dot = item_index(itm);
1555 struct production *pr = g->productions[p];
1559 prtxt(pr->head->name);
1561 for (i = 0; i < pr->body_size; i++) {
1562 printf(" %s", (dot == i ? ". ": ""));
1563 prtxt(pr->body[i]->name);
1565 if (dot == pr->body_size)
1569 printf(" (%d%s)", pr->precedence,
1570 assoc_names[pr->assoc]);
1574 The LA sets which are (possibly) reported with each item:
1576 static void report_la(struct grammar *g, int lanum)
1578 struct symset la = set_find(g, lanum);
1582 printf(" LOOK AHEAD(%d)", lanum);
1583 for (i = 0; i < la.cnt; i++) {
1586 prtxt(g->symtab[la.syms[i]]->name);
1591 Then the go to sets:
1594 static void report_goto(struct grammar *g, struct symset gt)
1599 for (i = 0; i < gt.cnt; i++) {
1601 prtxt(g->symtab[gt.syms[i]]->name);
1602 printf(" -> %d\n", gt.data[i]);
1606 Now we can report all the item sets complete with items, LA sets, and GO TO.
1608 static void report_itemsets(struct grammar *g)
1611 printf("ITEM SETS(%d)\n", g->states);
1612 for (s = 0; s < g->states; s++) {
1614 struct itemset *is = g->statetab[s];
1615 printf(" Itemset %d:%s\n", s, is->starts_line?" (startsline)":"");
1616 for (j = 0; j < is->items.cnt; j++) {
1617 report_item(g, is->items.syms[j]);
1618 if (is->items.data != NO_DATA)
1619 report_la(g, is->items.data[j]);
1621 report_goto(g, is->go_to);
1625 ### Reporting conflicts
1627 Conflict detection varies a lot among different analysis levels. However
1628 LR0 and LR0.5 are quite similar - having no follow sets, and SLR, LALR and
1629 LR1 are also similar as they have FOLLOW or LA sets.
1633 ## conflict functions
1635 static int report_conflicts(struct grammar *g, enum grammar_type type)
1638 printf("Conflicts:\n");
1640 cnt = conflicts_lr0(g, type);
1642 cnt = conflicts_slr(g, type);
1644 printf(" - no conflicts\n");
1648 LR0 conflicts are any state which have both a reducible item and
1651 LR05 conflicts only occurs if two possibly reductions exist,
1652 as shifts always over-ride reductions.
1654 ###### conflict functions
1655 static int conflicts_lr0(struct grammar *g, enum grammar_type type)
1659 for (i = 0; i < g->states; i++) {
1660 struct itemset *is = g->statetab[i];
1661 int last_reduce = -1;
1662 int prev_reduce = -1;
1663 int last_shift = -1;
1667 for (j = 0; j < is->items.cnt; j++) {
1668 int itm = is->items.syms[j];
1669 int p = item_prod(itm);
1670 int bp = item_index(itm);
1671 struct production *pr = g->productions[p];
1673 if (bp == pr->body_size) {
1674 prev_reduce = last_reduce;
1678 if (pr->body[bp]->type == Terminal)
1681 if (type == LR0 && last_reduce >= 0 && last_shift >= 0) {
1682 printf(" State %d has both SHIFT and REDUCE:\n", i);
1683 report_item(g, is->items.syms[last_shift]);
1684 report_item(g, is->items.syms[last_reduce]);
1687 if (prev_reduce >= 0) {
1688 printf(" State %d has 2 (or more) reducible items\n", i);
1689 report_item(g, is->items.syms[prev_reduce]);
1690 report_item(g, is->items.syms[last_reduce]);
1697 SLR, LALR, and LR1 conflicts happen if two reducible items have over-lapping
1698 look ahead, or if a symbol in a look-ahead can be shifted. The differ only
1699 in the source of the look ahead set.
1701 We build a dataset mapping terminal to item for possible SHIFTs and then
1702 another for possible REDUCE operations. We report when we get conflicts
1705 static int conflicts_slr(struct grammar *g, enum grammar_type type)
1710 for (i = 0; i < g->states; i++) {
1711 struct itemset *is = g->statetab[i];
1712 struct symset shifts = INIT_DATASET;
1713 struct symset reduce = INIT_DATASET;
1717 /* First collect the shifts */
1718 for (j = 0; j < is->items.cnt; j++) {
1719 unsigned short itm = is->items.syms[j];
1720 int p = item_prod(itm);
1721 int bp = item_index(itm);
1722 struct production *pr = g->productions[p];
1724 if (bp < pr->body_size &&
1725 pr->body[bp]->type == Terminal) {
1727 int sym = pr->body[bp]->num;
1728 if (symset_find(&shifts, sym) < 0)
1729 symset_add(&shifts, sym, itm);
1732 /* Now look for reduction and conflicts */
1733 for (j = 0; j < is->items.cnt; j++) {
1734 unsigned short itm = is->items.syms[j];
1735 int p = item_prod(itm);
1736 int bp = item_index(itm);
1737 struct production *pr = g->productions[p];
1739 if (bp < pr->body_size)
1744 la = g->follow[pr->head->num];
1746 la = set_find(g, is->items.data[j]);
1748 for (k = 0; k < la.cnt; k++) {
1749 int pos = symset_find(&shifts, la.syms[k]);
1751 printf(" State %d has SHIFT/REDUCE conflict on ", i);
1752 prtxt(g->symtab[la.syms[k]]->name);
1754 report_item(g, shifts.data[pos]);
1755 report_item(g, itm);
1758 pos = symset_find(&reduce, la.syms[k]);
1760 symset_add(&reduce, la.syms[k], itm);
1763 printf(" State %d has REDUCE/REDUCE conflict on ", i);
1764 prtxt(g->symtab[la.syms[k]]->name);
1766 report_item(g, itm);
1767 report_item(g, reduce.data[pos]);
1771 symset_free(shifts);
1772 symset_free(reduce);
1778 ## Generating the parser
1780 The export part of the parser is the `parse_XX` function, where the name
1781 `XX` is based on the name of the parser files.
1783 This takes a `code_node`, a partially initialized `token_config`, and an
1784 optional `FILE` to send tracing to. The `token_config` gets the list of
1785 known words added and then is used with the `code_node` to initialize the
1788 `parse_XX` then call the library function `parser_run` to actually complete
1789 the parse. This needs the `states` table and function to call the various
1790 pieces of code provided in the grammar file, so they are generated first.
1792 ###### parser_generate
1794 static void gen_parser(FILE *f, struct grammar *g, char *file, char *name)
1800 gen_reduce(f, g, file);
1803 fprintf(f, "#line 0 \"gen_parser\"\n");
1804 fprintf(f, "void *parse_%s(struct code_node *code, struct token_config *config, FILE *trace)\n",
1807 fprintf(f, "\tstruct token_state *tokens;\n");
1808 fprintf(f, "\tconfig->words_marks = known;\n");
1809 fprintf(f, "\tconfig->known_count = sizeof(known)/sizeof(known[0]);\n");
1810 fprintf(f, "\tconfig->ignored |= (1 << TK_line_comment) | (1 << TK_block_comment);\n");
1811 fprintf(f, "\ttokens = token_open(code, config);\n");
1812 fprintf(f, "\tvoid *rv = parser_run(tokens, states, do_reduce, do_free, trace, non_term, config->known_count);\n");
1813 fprintf(f, "\ttoken_close(tokens);\n");
1814 fprintf(f, "\treturn rv;\n");
1815 fprintf(f, "}\n\n");
1818 ### Table words table
1820 The know words is simply an array of terminal symbols.
1821 The table of nonterminals used for tracing is a similar array.
1825 static void gen_known(FILE *f, struct grammar *g)
1828 fprintf(f, "#line 0 \"gen_known\"\n");
1829 fprintf(f, "static const char *known[] = {\n");
1830 for (i = TK_reserved;
1831 i < g->num_syms && g->symtab[i]->type == Terminal;
1833 fprintf(f, "\t\"%.*s\",\n", g->symtab[i]->name.len,
1834 g->symtab[i]->name.txt);
1835 fprintf(f, "};\n\n");
1838 static void gen_non_term(FILE *f, struct grammar *g)
1841 fprintf(f, "#line 0 \"gen_non_term\"\n");
1842 fprintf(f, "static const char *non_term[] = {\n");
1843 for (i = TK_reserved;
1846 if (g->symtab[i]->type == Nonterminal)
1847 fprintf(f, "\t\"%.*s\",\n", g->symtab[i]->name.len,
1848 g->symtab[i]->name.txt);
1849 fprintf(f, "};\n\n");
1852 ### States and the goto tables.
1854 For each state we record the goto table, the reducible production if
1855 there is one, or a symbol to shift for error recovery.
1856 Some of the details of the reducible production are stored in the
1857 `do_reduce` function to come later. Here we store the production number,
1858 the body size (useful for stack management) and the resulting symbol (useful
1859 for knowing how to free data later).
1861 The go to table is stored in a simple array of `sym` and corresponding
1864 ###### exported types
1872 const struct lookup * go_to;
1883 static void gen_goto(FILE *f, struct grammar *g)
1886 fprintf(f, "#line 0 \"gen_goto\"\n");
1887 for (i = 0; i < g->states; i++) {
1889 fprintf(f, "static const struct lookup goto_%d[] = {\n",
1891 struct symset gt = g->statetab[i]->go_to;
1892 for (j = 0; j < gt.cnt; j++)
1893 fprintf(f, "\t{ %d, %d },\n",
1894 gt.syms[j], gt.data[j]);
1901 static void gen_states(FILE *f, struct grammar *g)
1904 fprintf(f, "#line 0 \"gen_states\"\n");
1905 fprintf(f, "static const struct state states[] = {\n");
1906 for (i = 0; i < g->states; i++) {
1907 struct itemset *is = g->statetab[i];
1908 int j, prod = -1, prod_len;
1910 int shift_len = 0, shift_remain = 0;
1911 for (j = 0; j < is->items.cnt; j++) {
1912 int itm = is->items.syms[j];
1913 int p = item_prod(itm);
1914 int bp = item_index(itm);
1915 struct production *pr = g->productions[p];
1917 if (bp < pr->body_size) {
1918 if (shift_sym < 0 ||
1919 (shift_len == bp && shift_remain > pr->body_size - bp)) {
1920 shift_sym = pr->body[bp]->num;
1922 shift_remain = pr->body_size - bp;
1926 /* This is what we reduce */
1927 if (prod < 0 || prod_len < pr->body_size) {
1929 prod_len = pr->body_size;
1934 fprintf(f, "\t[%d] = { %d, goto_%d, %d, %d, %d, 0, %d },\n",
1935 i, is->go_to.cnt, i, prod,
1936 g->productions[prod]->body_size,
1937 g->productions[prod]->head->num,
1940 fprintf(f, "\t[%d] = { %d, goto_%d, -1, -1, -1, %d, %d },\n",
1941 i, is->go_to.cnt, i, shift_sym,
1944 fprintf(f, "};\n\n");
1947 ### The `do_reduce` function and the code
1949 When the parser engine decides to reduce a production, it calls `do_reduce`.
1950 This has two functions.
1952 Firstly, if a non-NULL `trace` file is passed, it prints out details of the
1953 production being reduced. Secondly it runs the code that was included with
1954 the production if any.
1956 This code needs to be able to store data somewhere. Rather than requiring
1957 `do_reduce` to `malloc` that "somewhere", we pass in a large buffer and have
1958 `do_reduce` return the size to be saved.
1960 The code fragment requires translation when written out. Any `$N` needs to
1961 be converted to a reference either to that buffer (if `$0`) or to the
1962 structure returned by a previous reduction. These pointer need to be cast
1963 to the appropriate type for each access. All this is handling in
1969 static void gen_code(struct production *p, FILE *f, struct grammar *g)
1972 fprintf(f, "\t\t\t");
1973 for (c = p->code.txt; c < p->code.txt + p->code.len; c++) {
1982 if (*c < '0' || *c > '9') {
1987 while (c[1] >= '0' && c[1] <= '9') {
1989 n = n * 10 + *c - '0';
1992 fprintf(f, "(*(struct %.*s*%s)ret)",
1993 p->head->struct_name.len,
1994 p->head->struct_name.txt,
1995 p->head->isref ? "*":"");
1996 else if (n > p->body_size)
1997 fprintf(f, "$%d", n);
1998 else if (p->body[n-1]->type == Terminal)
1999 fprintf(f, "(*(struct token *)body[%d])",
2001 else if (p->body[n-1]->struct_name.txt == NULL)
2002 fprintf(f, "$%d", n);
2004 fprintf(f, "(*(struct %.*s*%s)body[%d])",
2005 p->body[n-1]->struct_name.len,
2006 p->body[n-1]->struct_name.txt,
2007 p->body[n-1]->isref ? "*":"", n-1);
2014 static void gen_reduce(FILE *f, struct grammar *g, char *file)
2017 fprintf(f, "#line 0 \"gen_reduce\"\n");
2018 fprintf(f, "static int do_reduce(int prod, void **body, void *ret)\n");
2020 fprintf(f, "\tint ret_size = 0;\n");
2022 fprintf(f, "\tswitch(prod) {\n");
2023 for (i = 0; i < g->production_count; i++) {
2024 struct production *p = g->productions[i];
2025 fprintf(f, "\tcase %d:\n", i);
2030 if (p->head->struct_name.txt)
2031 fprintf(f, "\t\tret_size = sizeof(struct %.*s%s);\n",
2032 p->head->struct_name.len,
2033 p->head->struct_name.txt,
2034 p->head->isref ? "*":"");
2036 fprintf(f, "\t\tbreak;\n");
2038 fprintf(f, "\t}\n\treturn ret_size;\n}\n\n");
2043 As each non-terminal can potentially cause a different type of data
2044 structure to be allocated and filled in, we need to be able to free it when
2047 It is particularly important to have fine control over freeing during error
2048 recovery where individual stack frames might need to be freed.
2050 For this, the grammar author required to defined a `free_XX` function for
2051 each structure that is used by a non-terminal. `do_free` all call whichever
2052 is appropriate given a symbol number, and will call `free` (as is
2053 appropriate for tokens` on any terminal symbol.
2057 static void gen_free(FILE *f, struct grammar *g)
2061 fprintf(f, "#line 0 \"gen_free\"\n");
2062 fprintf(f, "static void do_free(short sym, void *asn)\n");
2064 fprintf(f, "\tif (!asn) return;\n");
2065 fprintf(f, "\tif (sym < %d) {\n", g->first_nonterm);
2066 fprintf(f, "\t\tfree(asn);\n\t\treturn;\n\t}\n");
2067 fprintf(f, "\tswitch(sym) {\n");
2069 for (i = 0; i < g->num_syms; i++) {
2070 struct symbol *s = g->symtab[i];
2072 s->type != Nonterminal ||
2073 s->struct_name.txt == NULL)
2076 fprintf(f, "\tcase %d:\n", s->num);
2078 fprintf(f, "\t\tfree_%.*s(*(void**)asn);\n",
2080 s->struct_name.txt);
2082 fprintf(f, "\t\tfree_%.*s(asn);\n",
2084 s->struct_name.txt);
2085 fprintf(f, "\t\tbreak;\n");
2087 fprintf(f, "\t}\n}\n\n");
2090 ## The main routine.
2092 There are three key parts to the "main" function of parsergen: processing
2093 the arguments, loading the grammar file, and dealing with the grammar.
2095 ### Argument processing.
2097 Command line options allow the selection of analysis mode, name of output
2098 file, and whether or not a report should be generated. By default we create
2099 a report only if no code output was requested.
2101 The `parse_XX` function name uses the basename of the output file which
2102 should not have a suffix (`.c`). `.c` is added to the given name for the
2103 code, and `.h` is added for the header (if header text is specifed in the
2110 static const struct option long_options[] = {
2111 { "LR0", 0, NULL, '0' },
2112 { "LR05", 0, NULL, '5' },
2113 { "SLR", 0, NULL, 'S' },
2114 { "LALR", 0, NULL, 'L' },
2115 { "LR1", 0, NULL, '1' },
2116 { "tag", 1, NULL, 't' },
2117 { "report", 0, NULL, 'R' },
2118 { "output", 1, NULL, 'o' },
2119 { NULL, 0, NULL, 0 }
2121 const char *options = "05SL1t:Ro:";
2123 ###### process arguments
2125 char *outfile = NULL;
2130 enum grammar_type type = LR05;
2131 while ((opt = getopt_long(argc, argv, options,
2132 long_options, NULL)) != -1) {
2147 outfile = optarg; break;
2149 tag = optarg; break;
2151 fprintf(stderr, "Usage: parsergen ...\n");
2156 infile = argv[optind++];
2158 fprintf(stderr, "No input file given\n");
2161 if (outfile && report == 1)
2164 if (name && strchr(name, '/'))
2165 name = strrchr(name, '/')+1;
2167 if (optind < argc) {
2168 fprintf(stderr, "Excess command line arguments\n");
2172 ### Loading the grammar file
2174 To be able to run `mdcode` and `scanner` on the grammar we need to memory
2177 One we have extracted the code (with `mdcode`) we expect to file three
2178 sections: header, code, and grammar. Anything else is an error.
2180 "header" and "code" are optional, though it is hard to build a working
2181 parser with neither. "grammar" must be provided.
2185 #include <sys/mman.h>
2190 static void pr_err(char *msg)
2193 fprintf(stderr, "%s\n", msg);
2197 struct section *table;
2201 fd = open(infile, O_RDONLY);
2203 fprintf(stderr, "parsergen: cannot open %s: %s\n",
2204 infile, strerror(errno));
2207 len = lseek(fd, 0, 2);
2208 file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
2209 table = code_extract(file, file+len, pr_err);
2211 struct code_node *hdr = NULL;
2212 struct code_node *code = NULL;
2213 struct code_node *gram = NULL;
2214 for (s = table; s; s = s->next) {
2215 struct text sec = s->section;
2216 if (tag && !strip_tag(&sec, tag))
2218 if (text_is(sec, "header"))
2220 else if (text_is(sec, "code"))
2222 else if (text_is(sec, "grammar"))
2225 fprintf(stderr, "Unknown content section: %.*s\n",
2226 s->section.len, s->section.txt);
2231 ### Processing the input
2233 As we need to append an extention to a filename and then open it for
2234 writing, and we need to do this twice, it helps to have a separate function.
2238 static FILE *open_ext(char *base, char *ext)
2240 char *fn = malloc(strlen(base) + strlen(ext) + 1);
2242 strcat(strcpy(fn, base), ext);
2248 If we can read the grammar, then we analyse and optionally report on it. If
2249 the report finds conflicts we will exit with an error status.
2251 ###### process input
2252 struct grammar *g = NULL;
2254 fprintf(stderr, "No grammar section provided\n");
2257 g = grammar_read(gram);
2259 fprintf(stderr, "Failure to parse grammar\n");
2264 grammar_analyse(g, type);
2266 if (grammar_report(g, type))
2270 If a headers section is defined, we write it out and include a declaration
2271 for the `parse_XX` function so it can be used from separate code.
2273 if (rv == 0 && hdr && outfile) {
2274 FILE *f = open_ext(outfile, ".h");
2276 code_node_print(f, hdr, infile);
2277 fprintf(f, "void *parse_%s(struct code_node *code, struct token_config *config, FILE *trace);\n",
2281 fprintf(stderr, "Cannot create %s.h\n",
2287 And if all goes well and an output file was provided, we create the `.c`
2288 file with the code section (if any) and the parser tables and function.
2290 if (rv == 0 && outfile) {
2291 FILE *f = open_ext(outfile, ".c");
2294 code_node_print(f, code, infile);
2295 gen_parser(f, g, infile, name);
2298 fprintf(stderr, "Cannot create %s.c\n",
2304 And that about wraps it up. We need to set the locale so that UTF-8 is
2305 recognised properly, and link with `libicuuc` is `libmdcode` requires that.
2307 ###### File: parsergen.mk
2308 parsergen : parsergen.o libscanner.o libmdcode.o
2309 $(CC) $(CFLAGS) -o parsergen parsergen.o libscanner.o libmdcode.o -licuuc
2316 int main(int argc, char *argv[])
2321 setlocale(LC_ALL,"");
2323 ## process arguments
2330 ## The SHIFT/REDUCE parser
2332 Having analysed the grammar and generated all the table, we only need the
2333 shift/reduce engine to bring it all together.
2335 ### Goto table lookup
2337 The parser generator has nicely provided us with goto tables sorted by
2338 symbol number. We need a binary search function to find a symbol in the
2341 ###### parser functions
2343 static int search(const struct state *l, int sym)
2346 int hi = l->go_to_cnt;
2350 while (lo + 1 < hi) {
2351 int mid = (lo + hi) / 2;
2352 if (l->go_to[mid].sym <= sym)
2357 if (l->go_to[lo].sym == sym)
2358 return l->go_to[lo].state;
2363 ### The state stack.
2365 The core data structure for the parser is the stack. This tracks all the
2366 symbols that have been recognised or partially recognised.
2368 The stack usually won't grow very large - maybe a few tens of entries. So
2369 we dynamically resize and array as required but never bother to shrink it
2372 We keep the stack as two separate allocations. One, `asn_stack` stores the
2373 "abstract syntax nodes" which are created by each reduction. When we call
2374 `do_reduce` we need to pass an array of the `asn`s of the body of the
2375 production, and by keeping a separate `asn` stack, we can just pass a
2376 pointer into this stack.
2378 The other allocation stores all other stack fields of which there are four.
2379 The `state` is the most important one and guides the parsing process. The
2380 `sym` is nearly unnecessary. However when we want to free entries from the
2381 `asn_stack`, it helps to know what type they are so we can call the right
2382 freeing function. The symbol leads us to the right free function through
2385 The `indents` count and the `starts_indented` flag track the line
2386 indents in the symbol. These are used to allow indent information to
2387 guide parsing and error recovery.
2389 As well as the stack of frames we have a `next` frame which is
2390 assembled from the incoming token and other information prior to
2391 pushing it onto the stack.
2393 ###### parser functions
2399 short starts_indented;
2401 short newline_permitted;
2410 Two operations are needed on the stack - shift (which is like push) and pop.
2412 Shift applies not only to terminals but also to non-terminals. When we
2413 reduce a production we will pop off entries corresponding to the body
2414 symbols, then push on an item for the head of the production. This last is
2415 exactly the same process as shifting in a terminal so we use the same
2418 To simplify other code we arrange for `shift` to fail if there is no `goto`
2419 state for the symbol. This is useful in basic parsing due to our design
2420 that we shift when we can, and reduce when we cannot. So the `shift`
2421 function reports if it could.
2423 So `shift` finds the next state. If that succeed it extends the allocations
2424 if needed and pushes all the information onto the stacks.
2426 ###### parser functions
2428 static int shift(struct parser *p,
2430 const struct state states[])
2432 // Push an entry onto the stack
2433 int newstate = search(&states[p->next.state], p->next.sym);
2436 if (p->tos >= p->stack_size) {
2437 p->stack_size += 10;
2438 p->stack = realloc(p->stack, p->stack_size
2439 * sizeof(p->stack[0]));
2440 p->asn_stack = realloc(p->asn_stack, p->stack_size
2441 * sizeof(p->asn_stack[0]));
2443 p->stack[p->tos] = p->next;
2444 p->asn_stack[p->tos] = asn;
2446 p->next.state = newstate;
2447 p->next.indents = 0;
2448 p->next.starts_indented = 0;
2449 // if new state doesn't start a line, we inherit newline_permitted status
2450 if (states[newstate].starts_line)
2451 p->next.newline_permitted = 1;
2455 `pop` simply moves the top of stack (`tos`) back down the required amount
2456 and frees any `asn` entries that need to be freed. It is called _after_ we
2457 reduce a production, just before we `shift` the nonterminal in.
2459 ###### parser functions
2461 static void pop(struct parser *p, int num,
2462 void(*do_free)(short sym, void *asn))
2466 for (i = 0; i < num; i++) {
2467 p->next.indents += p->stack[p->tos+i].indents;
2468 do_free(p->stack[p->tos+i].sym,
2469 p->asn_stack[p->tos+i]);
2473 p->next.state = p->stack[p->tos].state;
2474 p->next.starts_indented = p->stack[p->tos].starts_indented;
2475 p->next.newline_permitted = p->stack[p->tos].newline_permitted;
2476 if (p->next.indents > p->next.starts_indented)
2477 p->next.newline_permitted = 0;
2481 ### Memory allocation
2483 The `scanner` returns tokens in a local variable - we want them in allocated
2484 memory so they can live in the `asn_stack`. Similarly the `asn` produced by
2485 a reduce is in a large buffer. Both of these require some allocation and
2486 copying, hence `memdup` and `tokcopy`.
2488 ###### parser includes
2491 ###### parser functions
2493 void *memdup(void *m, int len)
2499 memcpy(ret, m, len);
2503 static struct token *tok_copy(struct token tk)
2505 struct token *new = malloc(sizeof(*new));
2510 ### The heart of the parser.
2512 Now we have the parser. If we can shift, we do. If not and we can reduce
2513 we do. If the production we reduced was production zero, then we have
2514 accepted the input and can finish.
2516 We return whatever `asn` was returned by reducing production zero.
2518 If we can neither shift nor reduce we have an error to handle. We pop
2519 single entries off the stack until we can shift the `TK_error` symbol, then
2520 drop input tokens until we find one we can shift into the new error state.
2522 When we find `TK_in` and `TK_out` tokens which report indents we need
2523 to handle them directly as the grammar cannot express what we want to
2526 `TK_in` tokens are easy: we simply update the `next` stack frame to
2527 record how many indents there are and that the next token started with
2530 `TK_out` tokens must either be counted off against any pending indent,
2531 or must force reductions until there is a pending indent which isn't
2532 at the start of a production.
2534 ###### parser includes
2537 void *parser_run(struct token_state *tokens,
2538 const struct state states[],
2539 int (*do_reduce)(int, void**, void*),
2540 void (*do_free)(short, void*),
2541 FILE *trace, const char *non_term[], int knowns)
2543 struct parser p = { 0 };
2544 struct token *tk = NULL;
2548 p.next.newline_permitted = states[0].starts_line;
2550 struct token *err_tk;
2552 tk = tok_copy(token_next(tokens));
2553 p.next.sym = tk->num;
2555 parser_trace(trace, &p, tk, states, non_term, knowns);
2557 if (p.next.sym == TK_in) {
2558 p.next.starts_indented = 1;
2564 if (p.next.sym == TK_out) {
2565 if (p.stack[p.tos-1].indents > p.stack[p.tos-1].starts_indented ||
2566 (p.stack[p.tos-1].indents == 1 &&
2567 states[p.next.state].reduce_size > 1)) {
2568 p.stack[p.tos-1].indents -= 1;
2569 if (p.stack[p.tos-1].indents == p.stack[p.tos-1].starts_indented) {
2570 // no internal indent any more, reassess 'newline_permitted'
2571 if (states[p.stack[p.tos-1].state].starts_line)
2572 p.stack[p.tos-1].newline_permitted = 1;
2574 p.stack[p.tos-1].newline_permitted = p.stack[p.tos-2].newline_permitted;
2580 // fall through and force a REDUCE (as 'shift'
2583 if (p.next.sym == TK_newline) {
2584 if (!p.tos || ! p.stack[p.tos-1].newline_permitted) {
2590 if (shift(&p, tk, states)) {
2594 if (states[p.next.state].reduce_prod >= 0) {
2596 int prod = states[p.next.state].reduce_prod;
2597 int size = states[p.next.state].reduce_size;
2599 static char buf[16*1024];
2600 p.next.sym = states[p.next.state].reduce_sym;
2602 body = p.asn_stack +
2603 (p.tos - states[p.next.state].reduce_size);
2605 bufsize = do_reduce(prod, body, buf);
2607 pop(&p, size, do_free);
2608 shift(&p, memdup(buf, bufsize), states);
2613 if (tk->num == TK_out) {
2614 // Indent problem - synthesise tokens to get us
2616 fprintf(stderr, "Synthesize %d to handle indent problem\n", states[p.next.state].shift_sym);
2617 p.next.sym = states[p.next.state].shift_sym;
2618 shift(&p, tok_copy(*tk), states);
2619 // FIXME need to report this error somehow
2622 /* Error. We walk up the stack until we
2623 * find a state which will accept TK_error.
2624 * We then shift in TK_error and see what state
2625 * that takes us too.
2626 * Then we discard input tokens until
2627 * we find one that is acceptable.
2630 err_tk = tok_copy(*tk);
2631 p.next.sym = TK_error;
2632 while (shift(&p, err_tk, states) == 0
2634 // discard this state
2635 pop(&p, 1, do_free);
2638 // no state accepted TK_error
2641 while (search(&states[p.next.state], tk->num) < 0 &&
2642 tk->num != TK_eof) {
2644 tk = tok_copy(token_next(tokens));
2645 if (tk->num == TK_in)
2646 p.next.indents += 1;
2647 if (tk->num == TK_out) {
2648 if (p.next.indents == 0)
2650 p.next.indents -= 1;
2653 if (p.tos == 0 && tk->num == TK_eof)
2658 ret = p.asn_stack[0];
2660 pop(&p, p.tos, do_free);
2666 ###### exported functions
2667 void *parser_run(struct token_state *tokens,
2668 const struct state states[],
2669 int (*do_reduce)(int, void**, void*),
2670 void (*do_free)(short, void*),
2671 FILE *trace, const char *non_term[], int knowns);
2675 Being able to visualize the parser in action can be invaluable when
2676 debugging the parser code, or trying to understand how the parse of a
2677 particular grammar progresses. The stack contains all the important
2678 state, so just printing out the stack every time around the parse loop
2679 can make it possible to see exactly what is happening.
2681 This doesn't explicitly show each SHIFT and REDUCE action. However they
2682 are easily deduced from the change between consecutive lines, and the
2683 details of each state can be found by cross referencing the states list
2684 in the stack with the "report" that parsergen can generate.
2686 For terminal symbols, we just dump the token. For non-terminals we
2687 print the name of the symbol. The look ahead token is reported at the
2688 end inside square brackets.
2690 ###### parser functions
2692 static char *reserved_words[] = {
2693 [TK_error] = "ERROR",
2696 [TK_newline] = "NEWLINE",
2699 static void parser_trace_state(FILE *trace, struct frame *f, const struct state states[])
2701 fprintf(trace, "(%d", f->state);
2703 fprintf(trace, "%c%d", f->starts_indented?':':'.',
2705 if (states[f->state].starts_line)
2706 fprintf(trace, "s");
2707 if (f->newline_permitted)
2708 fprintf(trace, "n");
2709 fprintf(trace, ") ");
2712 void parser_trace(FILE *trace, struct parser *p,
2713 struct token *tk, const struct state states[],
2714 const char *non_term[], int knowns)
2717 for (i = 0; i < p->tos; i++) {
2718 int sym = p->stack[i].sym;
2719 parser_trace_state(trace, &p->stack[i], states);
2720 if (sym < TK_reserved &&
2721 reserved_words[sym] != NULL)
2722 fputs(reserved_words[sym], trace);
2723 else if (sym < TK_reserved + knowns) {
2724 struct token *t = p->asn_stack[i];
2725 text_dump(trace, t->txt, 20);
2727 fputs(non_term[sym - TK_reserved - knowns],
2731 parser_trace_state(trace, &p->next, states);
2732 fprintf(trace, " [");
2733 if (tk->num < TK_reserved &&
2734 reserved_words[tk->num] != NULL)
2735 fputs(reserved_words[tk->num], trace);
2737 text_dump(trace, tk->txt, 20);
2738 fputs("]\n", trace);
2743 The obvious example for a parser is a calculator.
2745 As `scanner` provides number parsing function using `libgmp` is it not much
2746 work to perform arbitrary rational number calculations.
2748 This calculator takes one expression, or an equality test per line. The
2749 results are printed and in any equality test fails, the program exits with
2752 Embedding mdcode inside mdcode is rather horrible. I'd like to find a
2753 better approach, but as the grammar file must have 3 components I need
2754 something like this.
2756 ###### File: parsergen.mk
2757 calc.c calc.h : parsergen parsergen.mdc
2758 ./parsergen --tag calc -o calc parsergen.mdc
2759 calc : calc.o libparser.o libscanner.o libmdcode.o libnumber.o
2760 $(CC) $(CFLAGS) -o calc calc.o libparser.o libscanner.o libmdcode.o libnumber.o -licuuc -lgmp
2765 // what do we use for a demo-grammar? A calculator of course.
2777 #include <sys/mman.h>
2782 #include "scanner.h"
2788 static void free_number(struct number *n)
2794 int main(int argc, char *argv[])
2796 int fd = open(argv[1], O_RDONLY);
2797 int len = lseek(fd, 0, 2);
2798 char *file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
2799 struct section *s = code_extract(file, file+len, NULL);
2800 struct token_config config = {
2801 .ignored = (1 << TK_line_comment)
2802 | (1 << TK_block_comment)
2805 .number_chars = ".,_+-",
2809 parse_calc(s->code, &config, argc > 2 ? stderr : NULL);
2815 Session -> Session Line
2818 Line -> Expression NEWLINE ${ gmp_printf("Answer = %Qd\n", $1.val);
2819 { mpf_t fl; mpf_init2(fl, 20); mpf_set_q(fl, $1.val);
2820 gmp_printf(" or as a decimal: %Fg\n", fl);
2824 | Expression = Expression NEWLINE ${
2825 if (mpq_equal($1.val, $3.val))
2826 gmp_printf("Both equal %Qd\n", $1.val);
2828 gmp_printf("NOT EQUAL: %Qd\n != : %Qd\n",
2833 | NEWLINE ${ printf("Blank line\n"); }$
2834 | ERROR NEWLINE ${ printf("Skipped a bad line\n"); }$
2837 Expression -> Expression + Term ${ mpq_init($0.val); mpq_add($0.val, $1.val, $3.val); }$
2838 | Expression - Term ${ mpq_init($0.val); mpq_sub($0.val, $1.val, $3.val); }$
2839 | Term ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
2841 Term -> Term * Factor ${ mpq_init($0.val); mpq_mul($0.val, $1.val, $3.val); }$
2842 | Term / Factor ${ mpq_init($0.val); mpq_div($0.val, $1.val, $3.val); }$
2843 | Factor ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
2845 Factor -> NUMBER ${ if (number_parse($0.val, $0.tail, $1.txt) == 0) mpq_init($0.val); }$
2846 | ( Expression ) ${ mpq_init($0.val); mpq_set($0.val, $2.val); }$