The paradigm of Internet of Vehicles (IoV) as an extension to the IoT concept can assist the development of smart cities. IoV relies on vehicular communications to allow vehicles to communicate in real time with other vehicles, roadside infrastructure and pedestrians. The main goal is to enable not only road safety services but also time-constrained IoT applications. In this context, Cellular-Vehicle-to-Everything communication (C-V2X) is a prominent technology to accomplish IoV goals. Currently there are two versions of C-V2X available, LTE-V2X and NR-V2X, and in both, mechanisms of resource allocation play an important role in their performance. Therefore, our goal in this paper is to present an extensive state-of-the-art of resource allocation on C-V2X technology. Thus, we first present the main technical aspect related to LTE-V2X and NR-V2X technologies. Afterwords, we present and categorize the related works for both technologies, pointing out their main differences, advantages and drawbacks. Finally, we present the challenges, open issues and future technical trends related to resource allocation in C-V2X.