The smallest directories (in size) in the Linux source tree are ipc and lib. The former is an implementation of the System V interprocess communication primitives, namely semaphores, message queues, and shared memory; they often get forgotten, but many applications use them (especially shared memory). The latter directory includes generic support functions, similar to the ones available in the standard C library.

The generic library functions are a very small subset of those available in user space, but cover the indispensable things you generally need to write code: string functions (including simple_atol to convert a string to a long integer with error checking) and <ctype.h> functions. The most important file in this directory is vsprintf.c; it implements the function by the same name, which sits at the core of sprintf and printk. Another important file is inflate.c, which includes the decompressing code of gzip.