An application program interface (API) provides means for dynamic filesystem creation, that allows complex filesystem hierarchies to be created and written on-the-fly without having to generate a complete on-disk representation. The filesystem is laid out in a one-parent many-children hierarchy of file and folder objects. The API recursively traverses the filesystem hierarchy beginning at the root node to determine the filesystem's structure. Callback functions for retrieval of file data are determined for each filesystem object and stored in memory. Output blocks are allocated for file data as needed. During the writing phase, the callback functions are executed to retrieve file data which is then written to the desired computer readable media.