There are a few other differences between 2.0 and 2.1.43. They don’t deserve special treatment in my opinion, so I’ll summarize them quickly.

The proc_register_dynamic function disappeared in 2.1.29. Recent kernels use the proc_register interface for every /proc file; if the low_ino field in struct proc_dir_entry is 0, then a dynamic inode number is assigned. The header sysdep-2.1.h defines proc_register_dynamic as proc_register when compiling for 2.1.29 or newer; this works as long as the proc_dir_entry structure being registered features 0 as the inode number.

In the field of network interface drivers, the rebuild_header device method has a new prototype from 2.1.15 onwards. You won’t be concerned with this difference as long as you develop Ethernet drivers, as Ethernet drivers don’t implement their own method; they fall back on the general-purpose Ethernet implementation. The sysdep-2.1 header defines the macro __USE_OLD_REBUILD_HEADER__ when the old implementation is needed. The sample module snull shows how to use the macro, but it’s not worth showing here.

Another change in network code affects struct enet_statistics, which doesn’t exist any more since 2.1.25. There is a new structure, struct net_device_stats, in its place, which is declared in <linux/netdevice.h> instead of <linux/if_ether.h>. The new structure is just like the old one, with the addition of two fields to store byte counters: unsigned long rx_bytes, tx_bytes;. A full-featured network interface driver should increment these counters along with rx_packets and tx_packets, athough a quick project might disregard the counters. The kernel headers define enet_statistics (the name of the old structure) to net_device_stats (the name of the new structure) to ease portability of existing drivers.

As a final note, I’d like to point out that the current pointer is no longer a global variable; the x86, Alpha, and Sparc kernel ports use smart tricks to store current in the processor itself. The kernel developers thus managed to squeeze out a few CPU cycles more. This trick avoids a lot of memory references, and sometimes frees a general-purpose register; the compiler often allocates processor registers to cache a few frequently used memory locations, and current is frequently used. Different tricks are used in the different ports to optimize access. The Alpha and Sparc versions use a processor register (one not used by the compiler’s optimization) to store current. The Intel processor, on the other hand, has a limited number of registers, and the compiler uses all of them; the trick in this case consists in storing struct task_struct and the kernel stack page in consecutive virtual-memory pages. This allows the current pointer to be ``encoded'' in the stack pointer. For every platform supported by Linux the header, <asm/current.h> shows the actual implementation chosen.

Like any vital piece of software, Linux continues to change. If you want to write drivers for the latest and greatest kernel, you’ll need to keep up to date with kernel development. Although dealing with incompatibilities might look like hard work, two observations are due. First, the major programming techniques are in place and are unlikely to change (at least not often). Second, each change makes things better, and usually leaves you less work to do on future development.