The first major program written in C was the UNIX operating System, and for many years C was considered to be inextricably linked with UNIX. Now, however, C is an important language independent of UNIX.
Although it is a high-level language, C is much closer to assembly language than are most other high-level languages. This closeness to the underlying machine language allows C programmers to write very efficient code. The low-level nature of C, however, can make the language difficult to use for some types of applications.