Add a json_c_set_serialization_double_format() function to set the *library-wide* format for how doubles are written to a serialized JSON output.

configure.ac

@@ -41,6 +41,18 @@ AC_CHECK_HEADER(inttypes.h,[AC_DEFINE([JSON_C_HAVE_INTTYPES_H],[1],[Public defin
 AC_C_CONST
 AC_TYPE_SIZE_T
 
+AC_CACHE_CHECK([for __thread support], ac_cv___thread, [dnl
+AC_LINK_IFELSE([dnl
+AC_LANG_PROGRAM([[#undef __thread
+static __thread int a; int foo (int b) { return a + b; }]],
+		[[exit (foo (0));]])],
+	       ac_cv___thread=yes, ac_cv___thread=no)
+])
+AS_IF([test "x$ac_cv___thread" != xno],
+[AC_DEFINE(HAVE___THREAD, 1, [Have __thread]),
+ AC_DEFINE(SPEC___THREAD, [__thread], [Specifier for __thread])]
+)
+
 # Checks for library functions.
 AC_FUNC_VPRINTF
 AC_FUNC_MEMCMP

json_object.c

@@ -692,6 +692,50 @@ int json_object_set_int64(struct json_object *jso,int64_t new_value){
 
 /* json_object_double */
 
+#ifdef HAVE___THREAD
+// i.e. __thread or __declspec(thread)
+static SPEC___THREAD char *tls_serialization_float_format = NULL;
+#endif
+static char *global_serialization_float_format = NULL;
+
+int json_c_set_serialization_double_format(const char *double_format, int global_or_thread)
+{
+	if (global_or_thread == JSON_C_OPTION_GLOBAL)
+	{
+#ifdef HAVE___THREAD
+		if (tls_serialization_float_format)
+		{
+			free(tls_serialization_float_format);
+			tls_serialization_float_format = NULL;
+		}
+#endif
+		if (global_serialization_float_format)
+			free(global_serialization_float_format);
+		global_serialization_float_format = double_format ? strdup(double_format) : NULL;
+	}
+	else if (global_or_thread == JSON_C_OPTION_THREAD)
+	{
+#ifdef HAVE___THREAD
+		if (tls_serialization_float_format)
+		{
+			free(tls_serialization_float_format);
+			tls_serialization_float_format = NULL;
+		}
+		tls_serialization_float_format = double_format ? strdup(double_format) : NULL;
+#else
+		_set_last_err("json_c_set_option: not compiled with __thread support\n");
+		return -1;
+#endif
+	}
+	else
+	{
+		_set_last_err("json_c_set_option: invalid global_or_thread value: %d\n", global_or_thread);
+		return -1;
+	}
+	return 0;
+}
+
+
 static int json_object_double_to_json_string_format(struct json_object* jso,
 						    struct printbuf *pb,
 						    int level,
@@ -712,13 +756,31 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
       size = snprintf(buf, sizeof(buf), "Infinity");
     else
       size = snprintf(buf, sizeof(buf), "-Infinity");
-  else
-    size = snprintf(buf, sizeof(buf),
-        format ? format : 
-          (modf(jso->o.c_double, &dummy) == 0) ? "%.17g.0" : "%.17g",
-          jso->o.c_double);
-  if(size < 0 || size >= (int)sizeof(buf))
-    size = (int)sizeof(buf);
+	else
+	{
+		const char *std_format = "%.17g";
+
+#ifdef HAVE___THREAD
+		if (tls_serialization_float_format)
+			std_format = tls_serialization_float_format;
+		else
+#endif
+		if (global_serialization_float_format)
+			std_format = global_serialization_float_format;
+		if (!format)
+			format = std_format;
+		size = snprintf(buf, sizeof(buf), format, jso->o.c_double);
+		if (modf(jso->o.c_double, &dummy) == 0)
+		{
+			// Ensure it looks like a float, even if snprintf didn't.
+			strncat(buf, ".0", sizeof(buf) - 1);
+			if (size >= 0)
+				size += 2; // yes, even if strncat ran out of room
+		}
+	}
+	// although unlikely, snprintf can fail
+	if (size < 0)
+		return -1;
 
   p = strchr(buf, ',');
   if (p) {
@@ -736,8 +798,13 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
     *(++p) = 0;
     size = p-buf;
   }
-  printbuf_memappend(pb, buf, size);
-  return size;
+
+	if (size >= (int)sizeof(buf))
+		// The standard formats are guaranteed not to overrun the buffer,
+		// but if a custom one happens to do so, just silently truncate.
+		size = sizeof(buf) - 1;
+	printbuf_memappend(pb, buf, size);
+	return size;
 }
 
 static int json_object_double_to_json_string_default(struct json_object* jso,

json_object.h

@@ -105,6 +105,22 @@ extern "C" {
 #undef TRUE
 #define TRUE ((json_bool)1)
 
+/**
+ * Set the global value of an option, which will apply to all
+ * current and future threads that have not set a thread-local value.
+ *
+ * @see json_c_set_serialization_double_format
+ */
+#define JSON_C_OPTION_GLOBAL (0)
+/**
+ * Set a thread-local value of an option, overriding the global value.
+ * This will fail if json-c is not compiled with threading enabled, and
+ * with the __thread specifier (or equivalent) available.
+ *
+ * @see json_c_set_serialization_double_format
+ */
+#define JSON_C_OPTION_THREAD (1)
+
 extern const char *json_number_chars;
 extern const char *json_hex_chars;
 
@@ -748,6 +764,21 @@ extern struct json_object* json_object_new_double(double d);
  */
 extern struct json_object* json_object_new_double_s(double d, const char *ds);
 
+/**
+ * Set a global or thread-local json-c option, depending on whether
+ *  JSON_C_OPTION_GLOBAL or JSON_C_OPTION_THREAD is passed.
+ * Thread-local options default to undefined, and inherit from the global
+ *  value, even if the global value is changed after the thread is created.
+ * Attempting to set thread-local options when threading is not compiled in
+ *  will result in an error.  Be sure to check the return value.
+ *
+ * double_format is a "%g" printf format, such as "%.20g"
+ *
+ * @return -1 on errors, 0 on success.
+ */
+int json_c_set_serialization_double_format(const char *double_format, int global_or_thread);
+
+
 
 /** Serialize a json_object of type json_type_double to a string.
  *

tests/test_double_serializer.c

@@ -27,5 +27,31 @@ int main()
 	json_object_set_serializer(obj, NULL, NULL, NULL);
 	printf("obj.to_string(reset)=%s\n", json_object_to_json_string(obj));
 
+	json_object_put(obj);
+	obj = json_object_new_double(0.52381);
+
+	printf("obj.to_string(default format)=%s\n", json_object_to_json_string(obj));
+	if (json_c_set_serialization_double_format("x%0.3fy", JSON_C_OPTION_GLOBAL) < 0)
+		printf("ERROR: json_c_set_serialization_double_format() failed");
+	printf("obj.to_string(with global format)=%s\n", json_object_to_json_string(obj));
+#ifdef HAVE___THREAD
+	if (json_c_set_serialization_double_format("T%0.2fX", JSON_C_OPTION_THREAD) < 0)
+		printf("ERROR: json_c_set_serialization_double_format() failed");
+	printf("obj.to_string(with thread format)=%s\n", json_object_to_json_string(obj));
+	if (json_c_set_serialization_double_format("Ttttttttttttt%0.2fxxxxxxxxxxxxxxxxxxX", JSON_C_OPTION_THREAD) < 0)
+		printf("ERROR: json_c_set_serialization_double_format() failed");
+	printf("obj.to_string(long thread format)=%s\n", json_object_to_json_string(obj));
+	if (json_c_set_serialization_double_format(NULL, JSON_C_OPTION_THREAD) < 0)
+		printf("ERROR: json_c_set_serialization_double_format() failed");
+	printf("obj.to_string(back to global format)=%s\n", json_object_to_json_string(obj));
+#else
+	// Just fake it up, so the output matches.
+	printf("obj.to_string(with thread format)=%s\n", "T0.52X");
+	printf("obj.to_string(back to global format)=%s\n", "x0.524y");
+#endif
+	if (json_c_set_serialization_double_format(NULL, JSON_C_OPTION_GLOBAL) < 0)
+		printf("ERROR: json_c_set_serialization_double_format() failed");
+	printf("obj.to_string(back to default format)=%s\n", json_object_to_json_string(obj));
+
 	json_object_put(obj);
 }

tests/test_double_serializer.expected

@@ -6,3 +6,6 @@ Test explicit serializer with custom userdata:
 obj.to_string(custom)=test
 Test reset serializer:
 obj.to_string(reset)=0.5
+obj.to_string(default format)=0.52381
+obj.to_string(with global format)=x0.524y
+obj.to_string(with thread format)=T0.52X