Advanced Programming in the UNIX® Environment

ptg10805159

Section 1.12 Summary 23

application

code

Clibrary

functions

system calls

kernel

user process

Figure 1.12 Difference between C library functions and system calls

we comparethe unbuffered I/O functions (Chapter 3) and the standardI/O functions

(Chapter 5).

The process control system calls (fork,exec,andwaitpid)are usually invoked

by the user’s application code directly.(Recall the bare-bones shell in Figure1.7.) But

some library routines exist to simplify certain common cases: thesystemandpopen

library routines, for example. In Section 8.13, we’ll show an implementation of the

systemfunction that invokes the basic process control system calls.We’ll enhance this

example in Section 10.18 to handle signals correctly.

To define the interface to the UNIX System that most programmers use, we have to

describe both the system calls and some of the library functions. If we described only

the sbrksystem call, for example, we would skip the moreprogrammer-friendly

malloclibrary function that many applications use. In this text, we’ll use the term

functionto refer to both system calls and library functions, except when the distinction is

necessary.

1.12 Summary

This chapter has provided a short tour of the UNIX System.We’ve described some of

the fundamental terms that we’ll encounter over and over again.We’ve seen numerous

small examples of UNIX programs to give us a feel for what the remainder of the text

talks about.