added a working thread-safe lemon example

lemon_example/.gitignore

@@ -0,0 +1,5 @@
+flex_scanner.h
+flex_scanner.c
+lemon_parser.h
+lemon_parser.c
+lemon_test

lemon_example/build_and_run.sh

@@ -0,0 +1,9 @@
+rm -f lemon_test lemon_parser.c flex_scanner.c
+echo "Lemon"
+lemon lemon_parser.y
+echo "Flex"
+# flex lexer.l
+flex --outfile=flex_scanner.c --header-file=flex_scanner.h flex_scanner.l
+echo "Compile"
+g++ main.cpp lemon_parser.c flex_scanner.c -o lemon_test -std=c++11 -pthread
+./lemon_test

lemon_example/flex_scanner.l

@@ -0,0 +1,17 @@
+%{
+    #include "lemon_parser.h"
+    #include <stdlib.h>
+%}
+
+
+%option reentrant
+%option noyywrap
+
+
+%%
+[0-9]+|[0-9]+.[0-9]+      return NUMBER;
+[+]         return PLUS;
+[\n]        return NL;
+[ \t]       ;       /* skip whitespace */
+.           {printf("Unknown character '%c'\n", yytext[0]); return 0;}
+%%

lemon_example/lemon_parser.y

@@ -0,0 +1,23 @@
+%include {
+	#include <assert.h>
+	#include <cstdlib>
+}
+%syntax_error { printf("Lemon syntax error\n"); }
+
+%token_type {const char*}
+%type expr {float}
+
+%left PLUS MINUS .
+
+%extra_argument { float* result }
+
+start   ::= prog.
+
+prog    ::= prog print NL .
+prog    ::= prog print .
+prog    ::= .
+
+print   ::= expr(a) .         { *result = a; }
+
+expr(a) ::= NUMBER(b) .                { a = atof(b); }
+expr(a) ::= expr(b) PLUS expr(c) .      { a = b + c; }

lemon_example/main.cpp

@@ -0,0 +1,65 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <thread>
+#include "flex_scanner.h"
+#include "lemon_parser.h"
+
+// Based on https://github.com/theory/flex-lemon-example
+// and http://stackoverflow.com/questions/24833465/bison-yacc-vs-lemon-vs-standard-input
+typedef float ResultType;
+
+void *ParseAlloc(void *(*mallocProc)(size_t));
+void ParseFree(void *p, void (*freeProc)(void*));
+void Parse(void *yyp, int yymajor, const char* text, ResultType*);
+
+int yylex(void);
+int yylval;
+
+
+float parseString(const char* string) {
+	yyscan_t scanner;
+	yylex_init(&scanner);
+	
+	// Scan the provided string
+	YY_BUFFER_STATE state = yy_scan_string(string, scanner);
+
+	void* lemonParser = ParseAlloc(malloc);
+	int tokenCode;
+	ResultType result;
+	do {
+		tokenCode = yylex(scanner);
+		Parse(lemonParser, tokenCode, yyget_text(scanner), &result);
+		// printf("Token %d\n", tokenCode);
+	} while (tokenCode > 0);
+
+	return result;
+}
+
+
+void multithreadTest(int numOfParses, int id) {
+	for (int n = 0; n < numOfParses; ++n) {
+		int a = rand() % 1000 + 1;
+		int b = rand() % 1000 + 1;
+		int c = a + b;
+		char string[32];
+		sprintf(string, "%d + %d", a, b);
+
+		int result = parseString(string);
+		if (result != c) printf("Error[%d]! %s != %d\n", id, string, result);
+	}
+}
+
+int main(void) {
+	const int numThreads = 10;
+	int numRuns = 5000;
+
+	std::thread threads[numThreads];
+	for (int n = 0; n < numThreads; ++n) {
+		threads[n] = std::thread(multithreadTest, numRuns, n);
+	}
+
+	for (int n = 0; n < numThreads; ++n) {
+		threads[n].join();
+	}
+	return 0;
+}