This text concentrates on the programming interface that exists between the UNIX kernel and applications software that runs in the UNIX environment - the UNIX system call interface. The techniques required by systems programmers are developed in depth and illustrated by a wealth of examples. This edition has been restructured in two parts - fundamental system services, and advanced and special features. It has also been updated throughout to reflect the following hierarchy of standards in the UNIX world - the IEEE POSIX standard, AT&T's SVID (System V Interface Definition) Issue 3, corresponding to UNIX System V.4, and X/Open. In an appendix new to this edition, the relationship between UNIX implementations and standards is explored. All examples conform to UNIX System V and the European Standard XOPEN.