POSIX Compliance allows developers the maximum flexibility in creating application software that can be port on to the other execution environments. Before POSIX, application written on one version can not be used on other version. Solution to this problem, POSIX was introduced. It is an interface standard defined by IEEE. It is an Open API, for this we need to implement header files and methods related to the header files. It includes a porting of function from FreeBSD as well as NetBSD for various file system including NFS and FAT file System. It pertains the OS, not the file system specifically, but a part of this API is used to access file system.