Some Things Are Very Different
If all the basics are the same, what has changed? Well, these things:
Number of system calls
Languages we use
Subsystems we program
Need for portability
Relevance of UNIX standards
More System Calls
The number of system calls has quadrupled, more or less, depending on what you mean by "system call." The first edition of Advanced UNIX Programming focused on only about 70 genuine kernel system callsfor example, open, read, and write; but not library calls like fopen, fread, and fwrite. The second edition includes about 300. (There are about 1,100 standard function calls in all, but many of those are part of the Standard C Library or are obviously not kernel facilities.) Today's UNIX has threads, real-time signals, asynchronous I/O, and new interprocess-communication features (POSIX IPC), none of which existed 20 years ago. This has caused, or been caused by, the evolution of UNIX from an educational and research system to a universal operating system. It shows up in embedded systems (parking meters, digital video recorders); inside Macintoshes; on a few million web servers; and is even becoming a desktop system for the masses. All of these uses were unanticipated in 1984.
Even if you're working in one of these modern languages, though, you still need to know what going on "down below," because UNIX still definesand, to a degree, limitswhat the higher-level languages can do. This is a challenge for many students who want to learn UNIX, but don't want to learn C. And for their teachers, who tire of debugging memory problems and explaining the distinction between declarations and definitions.
To enable students to learn UNIX without first learning C, I developed a Java-to-UNIX system-call interface that I call Jtux. It allows almost all of the UNIX system calls to be executed from Java, using the same arguments and datatypes as the official C calls. You can find out more about Jtux and download its source code from http://basepath.com/aup/.
The third area of change is that UNIX is both more visible than ever (sold by Wal-Mart!) and more hidden, underneath subsystems like J2EE and web servers, Apache, Oracle, and desktops such as KDE or GNOME. Many application programmers are programming for these subsystems, rather than for UNIX directly. What's more, the subsystems themselves are usually insulated from UNIX by a thin portability layer that has different implementations for different operating systems. Thus, many UNIX system programmers these days are working on middleware, rather than on the end-user applications that are several layers higher up.
The fourth change is the requirement for portability between UNIX systems, including Linux and the BSD-derivatives, one of which is the Macintosh OS X kernel (Darwin). Portability was of some interest in 1984, but today it's essential. No developer wants to be locked into a commercial version of UNIX without the possibility of moving to Linux or BSD, and no Linux developer wants to be locked into only one distribution. Platforms like Java help a lot, but only serious attention to the kernel APIs, along with careful testing, will ensure that the code is really portable. Indeed, you almost never hear a developer say that he or she is writing for XYZ's UNIX. It's much more common to hear "UNIX and Linux," implying that the vendor choice will be made later. (The three biggest proprietary UNIX hardware companiesSun, HP, and IBMare all strong supporters of Linux.)
More Complete Standards
The requirement for portability is connected with the fifth area of change, the role of standards. In 1984, a UNIX standards effort was just starting. The IEEE's POSIX group hadn't yet been formed. Its first standard, which emerged in 1988, was a tremendous effort of exceptional quality and rigor, but it was of very little use to real-world developers because it left out too many APIs, such as those for interprocess communication and networking. That minimalist approach to standards changed dramatically when The Open Group was formed from the merger of X/Open and the Open Software Foundation in 1996. Its objective was to include all the APIs that the important applications were using, and to specify them as well as time allowedwhich meant less precisely than POSIX did. They even named one of their standards Spec 1170, the number being the total of 926 APIs, 70 headers, and 174 commands. Quantity over quality, maybe, but the result meant that for the first time programmers would find in the standard the APIs they really needed. Today, The Open Group's Single UNIX Specification is the best guide for UNIX programmers who need to write portably.