## 16 Compatibility with other versions of `m4` ¶

This chapter describes the many of the differences between this implementation of `m4`, and of other implementations found under UNIX, such as System V Release 4, Solaris, and BSD flavors. In particular, it lists the known differences and extensions to POSIX. However, the list is not necessarily comprehensive.

At the time of this writing, POSIX 2001 (also known as IEEE Std 1003.1-2001) is the latest standard, although a new version of POSIX is under development and includes several proposals for modifying what `m4` is required to do. The requirements for `m4` are shared between SUSv3 and POSIX, and can be viewed at https://www.opengroup.org/onlinepubs/000095399/utilities/m4.html.