Summary

We have introduced some of the APIs dealing with UNIX process creation: fork(), exec(), and wait(). However, we have just skimmed the surface. For more detail, read Stevens and Rago“Advanced Programming in the UNIX Environment” by W. Richard Stevens, Stephen A. Rago. Addison-Wesley, 2005. All nuances and subtleties of using UNIX APIs are found herein. Buy this book! Read it! And most importantly, live it., of course, particularly the chapters on Process Control, Process Relationships, and Signals; there is much to extract from the wisdom therein. While our passion for the UNIX process API remains strong, we should also note that such positivity is not uniform. For example, a recent paper by systems researchers from Microsoft, Boston University, and ETH in Switzerland details some problems with fork(), and advocates for other, simpler process creation APIs such as spawn()“A fork() in the road” by Andrew Baumann, Jonathan Appavoo, Orran Krieger, Timothy Roscoe. HotOS ’19, Bertinoro, Italy. A fun paper full of fork()ing rage. Read it to get an opposing viewpoint on the UNIX process API. Presented at the always lively HotOS workshop, where systems researchers go to present extreme opinions in the hopes of pushing the community in new directions.. Read it, and the related work it refers to, to understand this different vantage point. While it’s generally good to trust this course, remember too that the authors have opinions; those opinions may not (always) be as widely shared as you might think.

ASIDE: KEY PROCESS API TERMS

• Each process has a name; in most systems, that name is a number known as a process ID (PID).

• The fork() system call is used in UNIX systems to create a new process. The creator is called the parent; the newly created process is called the child. As sometimes occurs in real life“They could be twins!” by Phoebe Jackson-Edwards. The Daily Mail. March 1, 2016. Available: www.dailymail.co.uk/femail/article-3469189/Photos-children-look- IDENTICAL-parents-age-sweep-web.html. This hard-hitting piece of journalism shows a bunch of weirdly similar child/parent photos and is frankly kind of mesmerizing. Go ahead, waste two minutes of your life and check it out. But don’t forget to come back here! This, in a microcosm, is the danger of surfing the web., the child process is a nearly identical copy of the parent.

• The wait() system call allows a parent to wait for its child to complete execution.

• The exec() family of system calls allows a child to break free from its similarity to its parent and execute an entirely new program.

• A UNIX shell commonly uses fork(), wait(), and exec() to launch user commands; the separation of fork and exec enables features like input/output redirection, pipes, and other cool features, all without changing anything about the programs being run.

• Process control is available in the form of signals, which can cause jobs to stop, continue, or even terminate.

• Which processes can be controlled by a particular person is encapsulated in the notion of a user; the operating system allows multiple users onto the system, and ensures users can only control their own processes.

• A superuser can control all processes (and indeed do many other things); this role should be assumed infrequently and with caution for security reasons.