Logical File System Kernel Services
The Logical File System services (also known as the fp_services) allow processes running in kernel mode to open and manipulate files in the same way that user-mode processes do. Data access limitations make it unreasonable to accomplish these tasks with system calls, so a subset of the file system calls has been provided with an alternate kernel-only interface.
The Logical File System services are one component of the logical file system, which provides the functions required to map system call requests to virtual file system requests. The logical file system is responsible for resolution of file names and file descriptors. It tracks all open files in the system using the file table. The Logical File System services are lower level entry points into the system call support within the logical file system.
Routines in the kernel that must access data stored in files or that must set up paths to devices are the primary users of these services. This occurs most commonly in device drivers, where a lower level device driver must be accessed or where the device requires microcode to be downloaded. Use of the Logical File System services is not, however, restricted to these cases.
- The fp_open service with a path name to the file or device it must access.
- The fp_opendev service with the device number of a device it must access.
- The fp_getf service with a file descriptor for the file or device. If the process wants to retain access past the duration of the system call, it must then call the fp_hold service to acquire a private file pointer.
These three services return a file pointer that is needed to call the other Logical File System services. The other services provide the functions that are provided by the corresponding system calls.