Add a json_set_serializer() function to allow the string output of a json_object to be customized.

tests/test_set_serializer.c

@@ -0,0 +1,71 @@
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "json.h"
+#include "printbuf.h"
+
+struct myinfo {
+	int value;
+};
+
+static int freeit_was_called = 0;
+static void freeit(json_object *jso, void *userdata)
+{
+	struct myinfo *info = userdata;
+	printf("freeit, value=%d\n", info->value);
+	// Don't actually free anything here, the userdata is stack allocated.
+	freeit_was_called = 1;
+}
+static int custom_serializer(struct json_object *o,
+					struct printbuf *pb,
+					int level,
+					int flags)
+{
+	sprintbuf(pb, "Custom Output");
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	json_object *my_object;
+
+	MC_SET_DEBUG(1);
+
+	printf("Test setting, then resetting a custom serializer:\n");
+	my_object = json_object_new_object();
+	json_object_object_add(my_object, "abc", json_object_new_int(12));
+	json_object_object_add(my_object, "foo", json_object_new_string("bar"));
+
+	printf("my_object.to_string(standard)=%s\n", json_object_to_json_string(my_object));
+
+	struct myinfo userdata = { .value = 123 };
+	json_object_set_serializer(my_object, custom_serializer, &userdata, freeit);
+
+	printf("my_object.to_string(custom serializer)=%s\n", json_object_to_json_string(my_object));
+
+	printf("Next line of output should be from the custom freeit function:\n");
+	freeit_was_called = 0;
+	json_object_set_serializer(my_object, NULL, NULL, NULL);
+	assert(freeit_was_called);
+
+	printf("my_object.to_string(standard)=%s\n", json_object_to_json_string(my_object));
+
+	json_object_put(my_object);
+
+	// ============================================
+
+	my_object = json_object_new_object();
+	printf("Check that the custom serializer isn't free'd until the last json_object_put:\n");
+	json_object_set_serializer(my_object, custom_serializer, &userdata, freeit);
+	json_object_get(my_object);
+	json_object_put(my_object);
+	printf("my_object.to_string(custom serializer)=%s\n", json_object_to_json_string(my_object));
+	printf("Next line of output should be from the custom freeit function:\n");
+
+	freeit_was_called = 0;	
+	json_object_put(my_object);
+	assert(freeit_was_called);
+
+	return 0;
+}