examples: add an example with a reentrant parser in Flex+Bison

Suggested by Eric S. Raymond. https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00066.html * examples/c/reentrant-calc/Makefile, examples/c/reentrant-calc/README.md, * examples/c/reentrant-calc/parse.y, examples/c/reentrant-calc/scan.l * examples/c/reentrant-calc/lexcalc.test, * examples/c/reentrant-calc/local.mk: New.
2026-03-09 12:23:04 +00:00 · 2019-02-16 13:13:30 +01:00
parent 0adda755a2
commit 6289d673a0
15 changed files with 845 additions and 3 deletions
--- a/examples/c/reccalc/scan.l
+++ b/examples/c/reccalc/scan.l
@@ -0,0 +1,85 @@
+/* Prologue (directives).   -*- C++ -*- */
+
+/* Disable Flex features we don't need, to avoid warnings. */
+%option nodefault noinput nounput noyywrap
+
+%option reentrant
+
+%{
+#include <assert.h>
+#include <limits.h> /* INT_MIN */
+#include <stdlib.h> /* strtol */
+
+#include "parse.h"
+%}
+
+%x SC_STRING
+
+%%
+%{
+  int nesting = 0;
+  char *str = NULL;
+  int size = 0;
+  int capacity = 0;
+#define STR_APPEND()                                    \
+  do {                                                  \
+    if (capacity < size + yyleng + 1)                   \
+      {                                                 \
+        do                                              \
+          capacity = capacity ? 2 * capacity : 128;     \
+        while (capacity < size + yyleng + 1);           \
+        str = realloc (str, capacity);                  \
+      }                                                 \
+    strncpy (str + size, yytext, yyleng);               \
+    size += yyleng;                                     \
+    assert (size < capacity);                           \
+  } while (0)
+%}
+
+ // Rules.
+
+"+"      return TOK_PLUS;
+"-"      return TOK_MINUS;
+"*"      return TOK_STAR;
+"/"      return TOK_SLASH;
+
+"("      nesting += 1; BEGIN SC_STRING;
+
+ /* Scan an integer.  */
+[0-9]+   {
+  errno = 0;
+  long n = strtol (yytext, NULL, 10);
+  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
+    yyerror (yyscanner, res, "integer is out of range");
+  yylval->TOK_NUM = (int) n;
+  return TOK_NUM;
+}
+
+ /* Ignore white spaces. */
+[ \t]+   continue;
+
+"\n"     return TOK_EOL;
+
+.        yyerror (yyscanner, res, "syntax error, invalid character: %c", yytext[0]);
+
+<SC_STRING>
+{
+  "("+   nesting += yyleng; STR_APPEND ();
+  ")"    {
+    if (!--nesting)
+      {
+        BEGIN INITIAL;
+        if (str)
+          str[size] = 0;
+        yylval->TOK_STR = str;
+        return TOK_STR;
+      }
+    else
+      STR_APPEND ();
+  }
+  [^()]+  STR_APPEND ();
+}
+
+<<EOF>>  return TOK_EOF;
+%%
+/* Epilogue (C code). */