A new file_operations method

The file_operations structure contains pointers to functions which implement I/O operations on files and char devices. These operations include the usual suspects, such as "open", "read", "write", "llseek", etc., along with some more esoteric ones ("sendfile", "get_unmapped_area"). The file_operations structure tends not to change very often; changes here can force updating a great many filesystems and drivers.

The NFS maintainers recently ran into a problem: it is not possible to simultaneously implement the O_DIRECT and O_APPEND modes over NFS. Rather than silently fail to implement a request to do so, the NFS developers have submitted a patch which adds an fcntl() method to the file_operations structure. Its prototype is:

    long (*fcntl)(unsigned int fd, unsigned int cmd, 
                  unsigned long arg, struct file *filp);

The fd, cmd, and arg parameters come straight from user space. A file descriptor is an unusual argument for a file_operations method, but the generic fcntl() code needs it. filp is, as usual, a pointer to the file structure for the open file.

If a module does not provide a fcntl() method, the call is handled in the usual way. Otherwise, the new fcntl() function should provide a complete implementation of that system call. Typically, the method will perform whatever device- or filesystem-specific work is needed (NFS simply checks for the O_DIRECT|O_APPEND combination and returns a failure code if it's there), then pass all four arguments to generic_file_fcnt(), which is exported to modules.

This patch is currently in the -mm tree; it will likely find its way into the mainline sometime after 2.6.5 comes out.

Index entries for this article
Kernel	struct file_operations

---

to post comments