Standards Conformance

Over the years UNIX has undergone many revisions, and on occasion has split into independent developments from the common base line. One of the most fundamental of these historical code forks was between what we call BSD, the academic Berkeley System Distribution of UNIX, and the commercial System V distribution described by the System V Interface Definition (SVID). Each of these developments introduced new features to the C library that were not necessarily mirrored in the other: BSD developed APIs for sockets and signal handling for example; among others SVID introduced APIs for inter-process communication (IPCs), and shared memory management. Luckily, with a few exceptions, the new APIs written by these two developments are not mutually exclusive, and thus modern UNIX C libraries, including the GNU C library, support both.

The task of recombining the BSD and SVID flavours of UNIX into a single specification was done by the International Standards Organisation, when they ratified ISO/IEC 9899:1990 (commonly known as ISO C) from the earlier ANSI C standard of 1989. The GNU C library complies with this standard wherever it is applicable to the standard C library.

More recently, the Portable Operating System Interface (POSIX) was issued as ISO/IEC 9945-1:1996. It builds upon and is a superset of the ISO C standard, and includes a detailed specification of the requirements for a portable C library interface. Most developers who care about the portability of their code, write to this standard. The GNU C library also conforms to this standard.

Also of note is the GNU C library's conformance to the so called ISO C99 standard. This standard has not yet gained large scale acceptance, so although its features are available to you if you use GCC and the GNU C library, you may find that if you do use them, few other environments will be able to compile your code for the time being.

This book does not aim to teach the facilities that are described in these standards documents: Voir Further Reading, for details of some books we recommend if you need to learn about the standard C library APIs.