Advanced Programming in the UNIX® Environment

ptg10805159

6

System Data Files and

Infor mation

6.1 Introduction

AUNIX system requires numerous data files for normal operation: the passwordfile

/etc/passwdand the group file/etc/grouparetwo files that arefrequently used by

various programs. For example, the passwordfile is used every time a user logs in to a

UNIX system and every time someone executes anls -lcommand.

Historically,these data files have been ASCII text files and wereread with the

standardI/O library.But for larger systems, a sequential scan through the password

file becomes time consuming. We want to be able to storethese data files in a format

other than ASCII text, but still provide an interface for an application program that

works with any file format. The portable interfaces to these data files arethe subject of

this chapter.Wealso cover the system identification functions and the time and date

functions.

6.2 Password File

The UNIX System’s passwordfile, called the user database by POSIX.1, contains the

fields shown in Figure6.1. These fields arecontained in apasswdstructurethat is

defined in<pwd.h>.

Note that POSIX.1 specifies only 5 of the 10 fields in thepasswdstructure. Most platforms

support at least 7 of the fields. The BSD-derived platforms support all 10.

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