Linux Device Drivers, Second Edition

Backward Compatibility

This chapter, so far, has described the kernel programming interface for version 2.4 of the Linux kernel. Unfortunately, this interface has changed significantly over the course of kernel development. These changes represent improvements in how things are done, but, once again, they also pose a challenge for those who wish to write drivers that are compatible across multiple versions of the kernel.

Insofar as this chapter is concerned, there are few noticeable differences between versions 2.4 and 2.2. Version 2.2, however, changed many of the prototypes of the file_operations methods from what 2.0 had; access to user space was greatly modified (and simplified) as well. The semaphore mechanism was not as well developed in Linux 2.0. And, finally, the 2.1 development series introduced the directory entry (dentry) cache.

Changes in the File Operations Structure

A number of factors drove the changes in the file_operations methods. The longstanding 2 GB file-size limit caused problems even in the Linux 2.0 days. As a result, the 2.1 development series started using the loff_t type, a 64-bit value, to represent file positions and lengths. Large file support was not completely integrated until version 2.4 of the kernel, but much of the groundwork was done earlier and had to be accommodated by driver writers.

Another change introduced during 2.1 development was the addition of the f_pos pointer argument to the read and write methods. This change was made ...