If this is for deserialisation then it's okay for x[0] to be signed. You just need to recast the result as int32_t (or simply assign to an int32_t variable with

If this is for deserialisation then it's okay for x[0] to be signed. You just need to recast the result as int32_t (or simply assign to an int32_t variable without any cast) and it is not UB.