Change %d to %llu and add cast to unsigned long long for size_t arguments,
otherwise compilation will fail with errors like:
test1.c:70:15: error: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Werror=format=]
%zu is avoided to stay compatible with old libc versions (like old Visual
Studio).
The user might want to use the userdata for something different, so the
serializer should ignore it by default.
Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.
Matthias Schiffer